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Preparation of natural diamond targets for tandem experiments
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Part 1". h#vest&,atiotl o1 target properties PREPARATION OF NATURAL DIAMOND TARGETS FOR T A N D E M EXPERIMENTS M. REBAK, J. P. F. SELLSCIIOP, T. E. DERRY and R. W. FEARICK
Nm'l¢,ar Phv~ic~ Re~c,arH7 L'nit. L"Hivcrsitv (!1 the H'itwater.sraml. JM~anneshur,~, South ..l/)'ica
Charged particle beams have recently proved lo be ~ery useful for the analysis of lighter element impurities in diamond. In this paper ~e describe the successful development of sav, ing, polishing and cleaning techniques, as well as the diagnostic methods used to establish the suitabilit.~ of the diamonds so prepared for tandem beam experiments.
1. hltroduction The dominant emphasis in a target-making conference is unquestionably on the production of very thin, highly uniform composition, self-supporting if possible, targets frequently of separated isotopic material. This paper is concerned with a highly specialized target which is usually infinitely thick to the beam and over the microscopic and sub-microscopic composition of which one has no control. Hence target preparation takes on a completely different set of approaches: these include selection of suitable specimens, classification of such specimens, orientation of specimens, cutting, polishing and cleaning, to name a few. The specimen in question is diamond, and the tandem accelerator has a very special role to play in diamond studies. Diamond is perhaps the most remarkable mate-. rial known to man. The impressive and long list of physical properties, for which diamond represents an extreme form, has long presented the physicist with a challenge. But it presents to the geo-scientist equally an outstanding source of information on the genesis of the crystal itself as well as on the conditions in and composition of the upper mantle itself~). For over a century research in diamond physics has relied on optical probes, but more recently many other diagnostic techniques arising from physics have delved into the electrical and atomic properties. Most recently nuclear probes have been added to this armamentarium and have already made a meaningful and extensive contribution~). The first of these to be used was neutron activation analysis which with its sensitivity going down to the parts per billion level could be exploited only by the development of effective cleaning methods to remove surface contaminants at the trace level3).
This work revealed a vast body of uniquely valuable data but could not cover the lighter elements and particularly the light volatiles which we believe to play an important role in relationship to the physical properties of diamond. Naturally, neutron techniques can say nothing on questions such as lattice location of defects. Charged particle beams lend themselves admirably to specific rather than multi-elemental analysis of the lighter elements and present in addition the possibility of three-dimensional spatial determination. Well-collimated beams can be used in channeling and de-channeling studies which provide unique information. These prospects can be realized however only if the surface of the diamond can be prepared not only to be meticulously clean but also to have the absolute m i n i m u m of crystal disorder. 2. Properties of diamond Diamond is usually appreciated by the technologist for its supreme hardness and by the female of the species for its high refractive index. However, ~50 o
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TARGET PROPERFIES
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M. REBAK et al.
these are but two of its remarkable physical properties. If we bear in mind that carbon crystallizes in two main allotropic forms, diamond and graphite, then the uniqueness of the properties of diamond as compared against those of graphite become even more striking. Only some of these can be considered here. Diamond hardness is best appreciated not by just quoting its index on the Mohs scale (viz. 10) but by considering that in relation to the bond energy/ unit volume (fig. 1) from which it is seen that diamond is an order of magnitude harder than the nearest next materials (Carborundum and A1203) on the Mohs scale. The optical properties of diamond place it in a special position both in terms of its gem qualities and in terms of its value in science and technology. An elegant example of the latter point is that the space probe now on its way to Venus, is equipped with diamond windows with their wide-band transmission characteristics, resistance to chemical attack, hardness and high temperature capabilities. Diamond can be transparent from 2 2 0 0 A to 2.5/xm, and then again from 6/~m u p w a r d s - a vast range. It has a refractive index up to 2.72. Virtually all diamonds have very high electrical resistance, greater than 1016 ohm cm, but by illuminating them with light of the correct wavelength they become excellent (photo)-conductors. Another outstanding property is the thermal conductivity: at 20°C the value for diamond can be 26 W/°C cm, as compared with copper at the same temperature which is only 4 W / ° C c m . At -190°C, diamond is at least 20 times better than copper as a thermal conductor (fig. 2) There are many more physical properties for which diamond is distinguished which we cannot 400
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hope to cover. However we may note that it has the highest known bulk modulus (5.4 × 10 ~2 d y n / cm2), the highest known critical tensile strength for cleavage. In specific directions however, it is very easy to cleave. Diamond has an extremely low coefficient of friction, and very high surface energy. It is evident that the set of physical properties that are embraced in diamond represent many extreme values and these warrant explanation. However the diamond researcher very soon discovers that natural diamonds are extremely variable and one must be very careful to ensure that what we investigate is specific to diamond and not to its impurities. There are three types of defect in diamond. The first of these is that of lattice defects (e.g. twinning, dislocations, stacking faults). The second is the inclusion in the bulk diamond of a well-defined, but not necessarily crystallographic, foreign body. Such inclusions vary from cavitites with fluid in them, to chunks of graphite, to perfectly shaped crystals of cogenetic minerals such as garnet, ilmenite, olivine and even diamond. The third type of defect can again be considered as an inclusion but not in the form of a specific entity: this therefore includes all the dispersed impurity elements, substitutional or interstitial in the lattice, which may also form aggregates of sub-microscopic size. The evidence from nuclear analysis is that no less than 58 impurity elements are commonly found in even the purest gem-quality diamonds. A dominant component of this impurity picture is always the set of right volatiles (H, N, O). Frequently the remainder occur on trace level only, emphasizing therefore the importance of nuclear analysis. Generally it is thought that nuclear analysis gives elemental information only. However if quantitative analysis is carried out, then inter-element correlations can be established and thereby chemical and mineralogical facts unravelled. In the case of diamond, the trace element composition has been unambiguously shown to have a well-defined chemistry. Since this is c o m m o n to all stones studied, it is now accepted that the trace element chemistry is representative of the magma of the upper mantel of the earth where diamonds had their genesis, and that some of this magma is always incorporated into the stone, probably in the form of minute droplets which are fairly uniformly dispersed. In this way the earth scientist has been able to use, arising from nuclear analysis, the
NATURAL
DIAMOND
diamond as a window into the otherwise inaccessible upper mantle of the earth. But it is in the area of the physical properties of diamond where most open questions still remain, and the relationship to the impurity picture remains to be established. It is particularly here that tandem research is essential, since the range of beams in terms of species and energy, and the ease of changing in both cases, lends itself directly to studying particularly the lighter impurities and also the distribution of these both macroscopically and microscopically. 3. C l a s s i f i c a t i o n of d i a m o n d s
Since the properties of diamond are known to cover a range of values, considerable effort has been put into establishing whether groupings of these exist. This has resulted in a classification scheme based on optical properties. The ultraviolet absorption edge distinguishes diamonds into Type I and Type II (fig. 3). A small proportion of Type I stones has an electron spin resonance s i g n a l - these are termed Type Ib and the larger remainder Type Ia. Among Type II stones a small proportion is semi-conducting (resistivity 10-10 3 o h m cm) and are termed lib, with the remainder termed Ila.
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require substantially more than this. For example, nuclear analysis is always accompanied by radiation damage which blackens increasingly the stone making optical measurements of the intrinsic properties less feasible. Hence it is our practice to remove from each stone a piece for optical experiments which in their turn preferably require two parallel fiat faces. Another situation is the need to mount the diamond target, and hence the preparation of suitable surfaces. The most demanding case is that where the best crystal properties are required with the absolute m i n i m u m of disorder at the surf a c e - t h i s is the case where ion channeling and dechanneling are studied. An extension of this is the case of transmission channeling where high quality thin crystal sections (less than 10/~m thick) are needed with faces closely parallel. Yet another situation is where the surface physics and surface chemistry are under study and hence faces of specific crystal orientation for presentation to the tandem beam must be prepared. To satisfy all these demands we are confronted with the need to cut, polish and clean, optimizing crystal integrity through to the outermost monolayer. 5. S a w i n g of d i a m o n d s
4. Criteria for d i a m o n d s as targets in t a n d e m e x p e r i m e n t s
From its birth in the upper mantle, through its rapid passage upwards and finally through the recovery plants, diamond has had a tortuous history. lts transit to the surface is a race against time as it is rapidly dissolving since the pressure and temperature conditions are no longer in the diamond stable region of the phase diagram. The surface of the diamond has therefore been in intimate contact with many materials that bear no relevance to the diamond per se. Cleaning is thus always a prime requirement, whatever the experiment. However many if not most experiments
5.1.
DESCRIPTION OF THE SAW
The saw consists of a rigid cast iron base, pivoted swinging arm and mandrel, The diamond is mounted in the yoke assembly at the end of the swinging arm. The arm may be loaded by an adjustable counterweight, pivotted by an antiback-
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lash threaded shaft (1 m m pitch) through precision sealed bearings. The mandrel rotates horizontally at 12 000 rpm, driven by a flat belt and pulley and a 1/4 H.P. electric motor. The swinging arm can be tilted and shifted, and the yoke assembly can be rotated in order to engage the diamond always on the top of the saw blade in the centre-line of the shaft. Finally the swinging arm is provided with a stopping screw (fig. 4). 5.2. MANDREL ASSEMBLY
Diamond sawing requires several mandrels with various flange sizes. A mandrel is made out of hardened and ground steel alloy, dynamically balanced. For bearing material we found after a long search, carbon blocks to be the most suitable, lubricated by ordinary candle wax. Load on the bearings is considerable. The play on the shaft (longitudinally) is taken up by two adjusting screws which in turn press against two oilsoaked hide strips against the end of the shaft. The cutting blades have to be dressed in position after mounting and quite frequently during sawing especially if sawing is not performed in an " e a s y " direction. Dressing the blade can be performed by gently pressing a block of alumina (A1203), carbon-tungsten, or a highspeed lathe tool against the rotating blade rested on the bracket. The choice of dressing tool depends on the diamond concentration of the blade. Always the final dressing was carried out by a high speed tool ground for parting-off purposes, in order to achieve a fiat top on the blade. Burs have to be removed by emery stick.
5.3. CONDITIONING OF SAW BLADE Experience has taught us that diamond powder of 4 - 8 / x m size gives the best sawing speed and finish. Coarse grit sizes of 2 0 - 4 0 ,tm cut well but one loses a lot from the blade by centrifugal force and at the exit side o f the cut, chipping o f the diamond occurs. Fine diamond sizes such as 3 1!2 p m cut smoothly but very slowly. We found the most effective way of conditioning the blade is to mix the diamond powder with olive oil into a thick pastelike consistency and to smear the paste evenly on an extremely hard steel roller (diam. 24 m m × × 3 0 ram) which rotates freely in a fork provided with a handle. With extreme care the rotating blade is approached with the stationary roller until contact is made and then with an oscillating m o v e m e n t the powder is pressed into the top of the blade. The -
roller must not be allowed to speed up to the same speed as the blade. It has been proved that diamond powder transfer occurs only when the angular velocities are unequal. Two sizes of blade are used. A thick blade is appropriate for " n o t c h i n g " to start a cut. It is soon found that to penetrate the surface of a diamond is usually the most difficult operation in the whole sawing process. This fact underlines the need of choosing the " e a s y " crystallographic direction to saw. The "trial and error" method is best avoided if possible. After notching, which can vary from 0.2 mm to 2 mm in depth, depending on the size and shape of the stone, the sawing blade has to be changed in order to saw the diamond off. The notching blade is 0.09 m m thick, 90 mm in diameter while for sawing we use a blade 0,06 m m thick, 76 m m in diameter. These sizes are starting sizes, because during the process of impregnating and sawing, blades have to be dressed frequently, especially if one requires a cut in a " h a r d " direction. Blades are made out of hardened b e r y l l i u m - copper which, in our opinion, is much superior to phosphor-bronze. 5.4. LOAD The right amount of loading is definitely an important factor for a successful cut. With the adjustable loading weight the load on the saw can be varied between 0 - 4 0 0 g. In the case of notching (0.09 mm) our best results were obtained with a load of 5 0 - 6 0 g. To achieve optimum cutting speed with a thin blade (0.06 mm) we found 5 - 2 0 g load to be the best. An important feature is to damp arm movements by placing a piece of neoprene sheet, 3 mm thick, between the stopping (limiting) screw and the base. Furthermore forming a 1.5 mm radius sphere on the end of the stopping screw is necessary. The result is no bouncing of the arm, no smearing of the blade and the diamond charge lasts. Our average rate of cut has been measured in the " s o f t " direction and found in a width of a cut of 4 m m wide, to be 1 m m per hour. 5.5. DIAMONDMOUNTING The diamond is mounted in a " d o p " or holder for sawing purposes. A great variety of " d o p s " have to be used, basically every diamond requires its own " d o p " . Only the shank size is the same. Depths and diameters vary according to the size and shape of each diamond. The function of the
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" d o p " is to hold a diamond rigidly in position during the sawing process. It may sound simple but one should appreciate that the diamonds used are usually small in size and rather awkward in shape for mounting. Furthermore the fact is that whilst sawing the temperature rises up to 2 0 0 - 3 0 0 ° C , so that with the effect of vibration as well, the " d o p " has to support the diamond very well. The best results have been achieved with three types of mounting: 1) Cementing 2) Staking 3) Vacuum chucking. 1) After searching widely we found " S y n a b o n d 5000" 4) (alkyl :z-cyanoacrylate) to be the most suitable cement for diamond. The use of a counter support is necessary, in order to hold the diamond securely and at the final stage of sawing it will hold the off-cut in position. One must emphasize the fact that the diamond must be supported firmly in position, because during the sawing operation the smallest m o v e m e n t can result in the specimen shattering, and the destruction of the blade and mandrel. 2) The diamond is ~ s t a k e d " into a metal " d o p ' " tailored specifically for each stone. We usually machine in this " d o p " a taper or hole for the purpose of " n e s t i n g " the stone. The diamond is pressed in and finally located by a cap shrunk or pressfitted on. 3) Vacuum chucking proved to be the most effective way to hold a diamond. This technique has only one limitation, namely, the diamond has to have one facet or window polished on already. In most of our work this method was used, because we already had one or two parallel windows polished on the diamond before we sawed a thin slice off (usually 0.4-0.5 mm thick). After measuring the minimum diameter needed to sink the diamond into the vacuum holder the recess is turned on a lathe. The depth has proved not to be very important as long as it is not less than 0.5 m m in order to give adequate physical strength. A number of pumping holes were drilled according to the size of the window and the bottom of the recess is carefully deburred and lapped. After a thin layer of " S y n a b o n d " cement has been applied on the window, the stone is pressed into position. The setting is about 45 s . The role of the Synabond is to establish the stone square in position but it does not have the strength to hold the stone in the dop during the sawing. The dop is placed in the yoke
assembly of the saw and the vacuum connection coupled on the shank. A rotary backing pump is used, and within ten minutes we registered 10 2 torr vacuum or better.
6. Polishing of diamond 6.1. DESCRIPTION OF TIlE POLISHING MA(ItlNE This method uses a rotating cast iron wheel commonly known as a "scaife". The wheel is 300 mm in diameter and rotates about a vertical axis. The running speed is designed at a nominal 2750 rpm from a 3-phase 380 V supply. The spindle is fitted with the rotor, instead of a pulley. As the polishing wheel is driven by electrical induction the usual belt drive and motor bearings are completely eliminated. The bottom point of the spindle runs in a bronze bearing with oil bath and the top point runs in a self-adjusting lubricated bearing. The system is smooth, silent, rigid and trouble-free due to its simplicity. Since the wheel has a spindle, conventional scouring and balancing methods may be applied (fig. 5).
Fig. 5. The diamond polishing machine. V. T A R G E T P R O P E R T I E S
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Wheels are changed easily and quickly by merely releasing the top bearing and slipping the wheel out. The spirit level setting is always maintained and a simple height adjustment allows each wheel to be set exactly to the correct height in seconds, thus compensating for varying thicknesses of wheel or varying lengths of spindle. The work top is designed to accommodate six diamond holders known as " t a n g s " , with only one in use at a time, as well as a " b l o c k i n g " attachment. 6.2. CONDITIONING THE SCAIFE The usual method of conditioning the scaife is to charge the wheel with a mixture of diampnd powder and olive oil. It works, but we found this method has its shortcomings. The most successful method developed is as follows: The cast iron scaife is dressed and scoured, electronically balanced, placed in position and cleaned, and finally the surface is degreased scrupulously. The motor is switched on for a few seconds, and the active area of the scaife marked with a felt pen approximately in three concentric zones. The diamond powders are mixed with sodium silicate and the mixture is spread thinly and evenly onto the surface with a perspex strip within the marks. Three different grit sizes of diamond powder are used: on the outside area 0 - 1 / z m , in the middle area 0 - 2 / l m and finally 4 - 8 / z m for the innermost zone. The scaife is heated with a hair dryer for about 15-20 rain. In order to work the diamond powder into the scouring grooves and into the pores of the scaife, a fairly large piece of boart diamond, held in the tang, is used. The scaife is run at half speed, and using firstly light pressure combined with an oscillating m o v e m e n t the diamond powder is worked in. Soon the surface of the scaife runs smoothly, and one can switch on to full speed and increase the load. In about three minutes the scaife is fully impregnated. Using this method the powder loss is negligible. The scaife now has three different grit sizes impregnated. The coarse ( 4 - 8 / x m ) inner ring is used for stock-removing, and only in the same area we keep on adding diamond powder mixed with olive oil. This is necessary for lubrication purposes in order to avoid burn marks. We have shown that diamonds polished on the 4 - 8 / ~ m area give ½-1 # m finish, which we establish by using comparative measurements. One theory that has been put forward in explanation of this observation is that
the diamond grit breaks up into smaller pieces during polishing. As the stone polishes down to near size, we oscillate the diamond on an everincreasing radius of the whole scaife in order to eliminate the scaife marks and to get a fine finish. All our specimens polished by this technique show 1/4 ~ m or better surface finish. 6.3.
POLISHING THIN PARALLEL DIAMONDS FOR SIZE
We have produced many varieties of parallel polished thin diamond slices. For example we prepared a suite of eight pieces of 0.35 mm thickness for ion dechanneling experiments, and a batch of twenty-four pieces 0.2 m m thick for the purpose of nitrogen analysis and associated infrared measurements. The requirements are demanding. The surface finish must be as fine as possible, on both sides, while parallelity and the size have to be accurate. Difficulties in polishing thin diamonds for size have arisen from four important factors, i.e. to hold a diamond securely in position during polishing, to find the best load per cutting speed ratio, vibrations, and sizing. We have resolved these in the following way. Firstly we saw a piece off from the bulk of the diamond, about (0.5_+0.1)mm thick (2 cuts), and then mount it in a dop with the second cut facing outwards and then polished to finish. The dop is mounted in the tang which is hand held. For mounting, we use "'Miracle" s) stone in an open dop. Miracle stone comes in fine powder form and is mixed with water prior to application to a thick paste-like consistency. We fill up the dop with the paste to form a half sphere and then place the diamond slice in the middle of the pool. With a small size vibrator we press gently and vibrate at the same time until it sinks in. It is dried in front of an infrared heat source or in a drying box, for about thirty minutes. The artificial stone has by now set but has no adhesive strength. We then scrape off from the top of the diamond the excess artificial stone, and with a sharp pair of harispring tweezers ease out the diamond, taking great care not to crumble the edge of the imprint. The diamond slice is cleaned in acetone ultrasonically "Secotine ''6) adhesive is then applied into the diamond imprint and the cleaned and dried diamond pressed home gently, taking care to maintain the same orientation. The assembly is dried in the same manner as above for 1-2 h. Diamonds pre-
NAIURAL
DIAMOND
pared in this manner usually hold in position, if the e n g a g i n g - d i s e n g a g i n g for polishing is done extremely carefully. R a m m i n g the diamond on the surface of the scaiFe will usually cause breaking (shattering) the specimen into pieces, and deep dugin marks in the scailE surface. The polishing direction is established before lowering the diamond, but often slight adjustment is necessary to find the optimal polishing direction, by rotating the dop. To adjust the specimen to the right plane, the stem of the tang (3/16" dia copper rod/ has to be bent accordingly. This operation has to be carried out carefully with slow rotating scaiFe speed lapprox. 3 0 0 - 5 0 0 rpm). In our experience loading the diamond during polishing needs no more than 2 0 0 - 5 0 0 g. Set rules cannot be applied here: the skilled hand and sensitive ears of the polisher are essential. Polishing of diamond is probably a combination of mechanical abrasion and thermo-chemical processes. After several experiments we became convinced that the load is not the important factor, but rather the speed to achieve high cutting ratio. It also has been proved that during polishing a diamond heats up to between 250~C and 300°C if it is polished in an " ' e a s y " direction, otherwise the diamond can heat up ir~ a matter of minutes to become red hot. We have no figures of our own to substantiate this claim but the temperature in the contact area must be ver~ high. The biggest problem has been to find an effective adhesive. Most of the commercially available adhesives are thermo-plastic at polishing temperature. We have tried to cool the diamond in order to make the adhesive hold, but the cutting speed dropped by almost an order of magnitude. In an extreme test case we cooled the diamond down to LN2 temperature, and using the same parameters found the cutting ratio bad reduced approximately eightl~ld. After the diamond face is flat, all saw marks removed and the surface finish satisfactory, the polishing is completed. To take the diamond out of the artificial stone holder the best method is to submerge the dop in water and with a sharp object, like an old-fashioned g r a m o p h o n e needle held in a pinvice, scratch the stone away all around the diamond. The stone is quite brittle and one can easily dislodge the diamond. In order to polish the second side, the diamond has to be placed in a v a c u u m chuck (described in sect. 5). Preparation of the v a c u u m chuck is the
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same as described before except the depth of the recess has to be accurately machined in. The depth in the dop will be the thickness of the final diamond. Now the v a c u u m chuck should be placed in the carefully aligned blocking attachment (parallel phme to scaife), which consists of a rigid cast carrier and slide to sweep in and out over the surface of the scaiFe, and the vertical m o v e m e n t is quided by two rows of precision ballbearings. The vacuum p u m p is coupled up and when pressure reaches 10 m torr, polishing can be carried out. An electric multimeter is hooked up on the 10 k£2 scale between the scaiFe and the blocking attachment for thickness monitoring purposes. The blocking attachment is electrically insulated from the scaiFe. As polishing the diamond advances, the meter starts indicating until at Full scale deflection the specimen is the same size as the depth of the recess in the dop, because the dop now itself makes contact electrically with the scaife. With this method of gauging our average variation of thickness is +0.001 nm or better, Parallelity one can clearly see depends on the alignment of the attachment versus scaife, plus accuracy of the slides. With the blocking attachment to achieve 0.5 m m parallel pieces represents no difficulty in our present set-up, but to produce 0.35 and 0.2 m m thick pieces our failure rate increased rapidly. On investigating the cause of this failure we have come to the conclusion that vibrations cause the diamond to shatter and break. It has also been observed that breaking appears to occur at the m o m e n t of rebounce. Several types of damping device have been considered. Designs have been drawn up which proved to be very complicated and costly. We are facing a challenge here of competing with the skilled hand which has all the sensors and feedback systems built in. We turned back to the hand operated tang. In our considered opinion, the scaiFe should be standing on a platform separately from the building on a deep foundation, far away even from traffic vibrations. The scaiFe should be dressed and balanced in position after diamond charging and conditioni n g - this is not a trivial requirement. Using a precision tang with a sensitive spirit level built in, we adjusted it carefully with an empty dop in position for level without the diamond mounted in. Handling it with extreme care so as not to upset the adjustments, the diamond is mounted in. Vacuum chucking and the electric metering technique are employed. Very light pressure has been used For polishing. We experienced no difficulty in ~,.
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achieving the desired thicknesses of less than 0.2 mm, especially after switching off the air-conditioning unit which unfortunately is situated rather nearby. Finally we must mention a further function of the " M i r a c l e " stone which seems to play an important part, in addition to embodying and holding the diamond, which was less important when using vacuum chucking. If the diamond edge is exposed, skimming of the scaife can occur easily, the diamond powder throw-off rate increases considerably (clearly visible on the splash guard) and in extreme cases the diamond digs in, causing breakage and damage to the scaife. The only explanation that can he offered is that the artificial stone acts as a "guide-in'.
7. Cleaning of diamonds 7. l. C O N T A M I N A T I O N MeV helium ion backscattering is used to reveal the presence of foreign atoms on the diamond surface and near-surface and to enable their massnumbers to be determined. Application of this technique showed that all the diamond samples had dirt on the surface, despite a clean appearance and despite having been ultrasonically cleaned in various solvents (trichloroethylenealcohol and acetone). There were three obvious possible sources: 1) The kimberlitic matrix in which the diamond was found (such impurities are referred to as "earthy"). 2) Processing, including crushing, heavy medium separation and recovery, and possibly polishing. 3) The vacuum environment of samples already experimented on (silicone diffusion pump oil). 7.2. S Y S T E M A T I C C L E A N I N G A P P R O A C H Tests were carried out on 12 diamonds which had been selected as they showed both earthy mass continuum and prominent pump-oil (silicon and oxygen) peaks (fig. 6). Rutherford backscattering of He + ions was used to inspect the surfaces before and after cleaning. Three general types of cleaning were tried. 1) A hot hydrofluoric and sulphuric acid teflon bomb process. This had been used successfully in our laboratories on diamonds for neutron activation analysis.
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2) Ultrasonic cleaning for one hour in various organic solvents. This was chosen after testing a wide variety of solvents for miscibility with silicone oil. 3) Ultrasonic cleaning with the commercial detergent " C O N T R A D " 7). It is believed to contain a surfactant and a chelating agent. It is designed for radiochemical decontamination and other demanding cleaning jobs. The concentration used was 20%, in deionized water. Tests showed that ultrasonic agitation for thirty minutes was more than sufficient in all cases. A small oxygen peak always remained after cleaning, equivalent to about a monolayer. The Contrad method showed itself to be superior to all other methods.
7.3. R O U T I N E Ultrasonic minutes was dure applied and before
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cleaning in 20% Contrad for thirty adopted (fig. 7) as the standard proceto all diamonds before experimentation procedures such as annealing. Each
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specimen was cleaned in an individual PTFE beaker. The cleaning was finished with several ultrasonic rinses in deionised water and the diamond allowed to dry by evaporation. It was handled by the edges using only " f i u o r o w a r e " plastic tweezers or a plastic-tipped vacuum pencil. Diamonds were stored in clean Fluoroware boxes, but preferably mounted and used immediately. Contamination from the vacuum environment was minimised by always using a cryopump and keeping the liquid nitrogen traps cold. 8. Improvement of diamond surfaces It is of course valid to question whether after sawing, polishing and cleaning, it is possible to still further improve the quality of the diamond surface in terms of crystal integrity. This surface disorder is sensitively and quantitatively assessed by backscattering measurements. To this end a number of techniques have been investigated. Gas etching is readily carried out by passing a stream of dry oxygen over the diamond at elevated temperatures. At 750°C etching is very slow with much pitting. At 1000°C etching rates of about 2 , u m / m i n are achieved. The quality of the result varied from stone to stone. At Feast in one case the surface disorder was reduced. However pitting is generally severe indicating that the etching rate is enhanced at defect sites. It is also not evident whether the improvement of the surface in this one case is due to the removal of disordered surface material or to the effect of annealing at the elevated temperature. Another technique tried was ion milling. A beam of 1 keV argon ions is effective in the removal of
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surface layers by sputtering. Backscattering channeling measurements showed unambiguously that the effects of the associated radiation damage increased the degree of surface disorder. A further approach was to anneal the specimen in ultra high vacuum conditions ( - 1 0 10 tort) at 950°C. Again in some-cases no improvement was found, but in others there was evidence of improvement. We may note that studies of thermal conductivity of diamond have shown (for the relevant temperature regions) that annealing heals small surface microcracks. The dominant conclusion however is that assuming good cleaning practice always, the quality of the crystal surface of diamond is a function of the sawing and polishing procedures. In the case of diamond the situation is distinctly different from that of silicon and germanium where the damage depth is usually about twice that of the abrasive grit size diameter. It is likely that for diamond the grit breaks down rapidly in size so that effectively one is always using very fine grits. However, it is also evident that there is an additional mechanism which is more chemical in character and temperature dependant. The interplay between these two mechanisms is by no means fully understood yet. One experiment that was carefully carried out was to cleave a diamond and examine the freshly cleaved s u r f a c e - i t was by no means as good as a carefully polished surface. Indeed it showed considerable strain and had a step!Oed appearance. Finally it is clear that annealing is generally beneficial for the removal of strain and possibly even for the healing of microcracks.
9. Diagnostic techniques A wide range of these is relevant in tandem beam diamond experiments. For classification of stones, ultraviolet absorption and excitation is used, followed by electron spin resonance and electrical resistivity measurement. Infrared absorption is also a useful gross distinction but shows additionally fine structure features that can be associated with different forms in which nitrogen occurs. X-ray diffraction is a very useful technique for assessing crystal quality and in particular for determining orientation. This saves considerable time in ion channeling experiments and hence minimizes radiation damage to the crystal. Microscopic examination of specimens is always essential and the use of polarized light is a good indicator of strain, particularly in the vicinity of V.
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small inclusions. The addition of cathodoluminescence to the microscope stage is very valuable indeed, showing up very clearly any growth bands that may be present in the crystal. Pleochroic halos are also revealed very clearly in this way. The best diagnostic techniques for channeling properties is indeed channeling itself! Electron microscopy shows up defects such as planar aggregates (in the {100} directions) and dislocation loops, but requires a very thin specimen. The combination of all these diagnostic probes provides us with a very powerful set of tools for characterizing diamond, and sets the stage for the use of nuclear probes for quantitative analysis. To date these embrace thermal, epithermal and fast neutron activation analysis; photon activation analysis; proton induced X-ray analysis; proton, helium-3 and helium-4 activation analysis followed by /3~, 7 and X-ray detection. Heavy ion beams such as fluorine have been used with prompt gamma analysis. Although a great deal has been revealed
about the physics and geochemistry of diamond as a result, the stage is now set for an exciting decade of further discovery. We acknowledge with appreciation the support of the De Beers Diamond Research Laboratories, the Council for Scientific and Industrial Research and the South African Atomic Energy Board. References 1) H.W. Fesq, D.M. Bibby, C. S. Erasmus, E. J. D. Kable and J. P. F. Sellschop, Phys. Chem. Earth 9 (1975) 817. 2) j. p. F. Sellschop, H. J. Annegarn, R. J. Keddy, C. C. P. Madiba and M.J. Renan, Nucl. Instr. and Meth. 149 (1978) 321. 3) H.W. Fesq, D. M. Bibby, J. P. F. Sellschop and J. 1. W. Watterson, J. Radioanal. Chem. 17 (1973) 195. 4) Cynabond 5000, Sumitomo Chemical Company Limited. $) Durok, ~'The Miracle Stone", Product of the Ransom & Randolph Company. o) Seccotine, Lapage's - A division of tlenry C. Stephens Limited. 7) Contrad, Decon Laboratories Limited, Brighton, England.
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Part 1". h#vest&,atiotl o1 target properties PREPARATION OF NATURAL DIAMOND TARGETS FOR T A N D E M EXPERIMENTS M. REBAK, J. P. F. SELLSCIIOP, T. E. DERRY and R. W. FEARICK
Nm'l¢,ar Phv~ic~ Re~c,arH7 L'nit. L"Hivcrsitv (!1 the H'itwater.sraml. JM~anneshur,~, South ..l/)'ica
Charged particle beams have recently proved lo be ~ery useful for the analysis of lighter element impurities in diamond. In this paper ~e describe the successful development of sav, ing, polishing and cleaning techniques, as well as the diagnostic methods used to establish the suitabilit.~ of the diamonds so prepared for tandem beam experiments.
1. hltroduction The dominant emphasis in a target-making conference is unquestionably on the production of very thin, highly uniform composition, self-supporting if possible, targets frequently of separated isotopic material. This paper is concerned with a highly specialized target which is usually infinitely thick to the beam and over the microscopic and sub-microscopic composition of which one has no control. Hence target preparation takes on a completely different set of approaches: these include selection of suitable specimens, classification of such specimens, orientation of specimens, cutting, polishing and cleaning, to name a few. The specimen in question is diamond, and the tandem accelerator has a very special role to play in diamond studies. Diamond is perhaps the most remarkable mate-. rial known to man. The impressive and long list of physical properties, for which diamond represents an extreme form, has long presented the physicist with a challenge. But it presents to the geo-scientist equally an outstanding source of information on the genesis of the crystal itself as well as on the conditions in and composition of the upper mantle itself~). For over a century research in diamond physics has relied on optical probes, but more recently many other diagnostic techniques arising from physics have delved into the electrical and atomic properties. Most recently nuclear probes have been added to this armamentarium and have already made a meaningful and extensive contribution~). The first of these to be used was neutron activation analysis which with its sensitivity going down to the parts per billion level could be exploited only by the development of effective cleaning methods to remove surface contaminants at the trace level3).
This work revealed a vast body of uniquely valuable data but could not cover the lighter elements and particularly the light volatiles which we believe to play an important role in relationship to the physical properties of diamond. Naturally, neutron techniques can say nothing on questions such as lattice location of defects. Charged particle beams lend themselves admirably to specific rather than multi-elemental analysis of the lighter elements and present in addition the possibility of three-dimensional spatial determination. Well-collimated beams can be used in channeling and de-channeling studies which provide unique information. These prospects can be realized however only if the surface of the diamond can be prepared not only to be meticulously clean but also to have the absolute m i n i m u m of crystal disorder. 2. Properties of diamond Diamond is usually appreciated by the technologist for its supreme hardness and by the female of the species for its high refractive index. However, ~50 o
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TARGET PROPERFIES
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these are but two of its remarkable physical properties. If we bear in mind that carbon crystallizes in two main allotropic forms, diamond and graphite, then the uniqueness of the properties of diamond as compared against those of graphite become even more striking. Only some of these can be considered here. Diamond hardness is best appreciated not by just quoting its index on the Mohs scale (viz. 10) but by considering that in relation to the bond energy/ unit volume (fig. 1) from which it is seen that diamond is an order of magnitude harder than the nearest next materials (Carborundum and A1203) on the Mohs scale. The optical properties of diamond place it in a special position both in terms of its gem qualities and in terms of its value in science and technology. An elegant example of the latter point is that the space probe now on its way to Venus, is equipped with diamond windows with their wide-band transmission characteristics, resistance to chemical attack, hardness and high temperature capabilities. Diamond can be transparent from 2 2 0 0 A to 2.5/xm, and then again from 6/~m u p w a r d s - a vast range. It has a refractive index up to 2.72. Virtually all diamonds have very high electrical resistance, greater than 1016 ohm cm, but by illuminating them with light of the correct wavelength they become excellent (photo)-conductors. Another outstanding property is the thermal conductivity: at 20°C the value for diamond can be 26 W/°C cm, as compared with copper at the same temperature which is only 4 W / ° C c m . At -190°C, diamond is at least 20 times better than copper as a thermal conductor (fig. 2) There are many more physical properties for which diamond is distinguished which we cannot 400
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hope to cover. However we may note that it has the highest known bulk modulus (5.4 × 10 ~2 d y n / cm2), the highest known critical tensile strength for cleavage. In specific directions however, it is very easy to cleave. Diamond has an extremely low coefficient of friction, and very high surface energy. It is evident that the set of physical properties that are embraced in diamond represent many extreme values and these warrant explanation. However the diamond researcher very soon discovers that natural diamonds are extremely variable and one must be very careful to ensure that what we investigate is specific to diamond and not to its impurities. There are three types of defect in diamond. The first of these is that of lattice defects (e.g. twinning, dislocations, stacking faults). The second is the inclusion in the bulk diamond of a well-defined, but not necessarily crystallographic, foreign body. Such inclusions vary from cavitites with fluid in them, to chunks of graphite, to perfectly shaped crystals of cogenetic minerals such as garnet, ilmenite, olivine and even diamond. The third type of defect can again be considered as an inclusion but not in the form of a specific entity: this therefore includes all the dispersed impurity elements, substitutional or interstitial in the lattice, which may also form aggregates of sub-microscopic size. The evidence from nuclear analysis is that no less than 58 impurity elements are commonly found in even the purest gem-quality diamonds. A dominant component of this impurity picture is always the set of right volatiles (H, N, O). Frequently the remainder occur on trace level only, emphasizing therefore the importance of nuclear analysis. Generally it is thought that nuclear analysis gives elemental information only. However if quantitative analysis is carried out, then inter-element correlations can be established and thereby chemical and mineralogical facts unravelled. In the case of diamond, the trace element composition has been unambiguously shown to have a well-defined chemistry. Since this is c o m m o n to all stones studied, it is now accepted that the trace element chemistry is representative of the magma of the upper mantel of the earth where diamonds had their genesis, and that some of this magma is always incorporated into the stone, probably in the form of minute droplets which are fairly uniformly dispersed. In this way the earth scientist has been able to use, arising from nuclear analysis, the
NATURAL
DIAMOND
diamond as a window into the otherwise inaccessible upper mantle of the earth. But it is in the area of the physical properties of diamond where most open questions still remain, and the relationship to the impurity picture remains to be established. It is particularly here that tandem research is essential, since the range of beams in terms of species and energy, and the ease of changing in both cases, lends itself directly to studying particularly the lighter impurities and also the distribution of these both macroscopically and microscopically. 3. C l a s s i f i c a t i o n of d i a m o n d s
Since the properties of diamond are known to cover a range of values, considerable effort has been put into establishing whether groupings of these exist. This has resulted in a classification scheme based on optical properties. The ultraviolet absorption edge distinguishes diamonds into Type I and Type II (fig. 3). A small proportion of Type I stones has an electron spin resonance s i g n a l - these are termed Type Ib and the larger remainder Type Ia. Among Type II stones a small proportion is semi-conducting (resistivity 10-10 3 o h m cm) and are termed lib, with the remainder termed Ila.
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require substantially more than this. For example, nuclear analysis is always accompanied by radiation damage which blackens increasingly the stone making optical measurements of the intrinsic properties less feasible. Hence it is our practice to remove from each stone a piece for optical experiments which in their turn preferably require two parallel fiat faces. Another situation is the need to mount the diamond target, and hence the preparation of suitable surfaces. The most demanding case is that where the best crystal properties are required with the absolute m i n i m u m of disorder at the surf a c e - t h i s is the case where ion channeling and dechanneling are studied. An extension of this is the case of transmission channeling where high quality thin crystal sections (less than 10/~m thick) are needed with faces closely parallel. Yet another situation is where the surface physics and surface chemistry are under study and hence faces of specific crystal orientation for presentation to the tandem beam must be prepared. To satisfy all these demands we are confronted with the need to cut, polish and clean, optimizing crystal integrity through to the outermost monolayer. 5. S a w i n g of d i a m o n d s
4. Criteria for d i a m o n d s as targets in t a n d e m e x p e r i m e n t s
From its birth in the upper mantle, through its rapid passage upwards and finally through the recovery plants, diamond has had a tortuous history. lts transit to the surface is a race against time as it is rapidly dissolving since the pressure and temperature conditions are no longer in the diamond stable region of the phase diagram. The surface of the diamond has therefore been in intimate contact with many materials that bear no relevance to the diamond per se. Cleaning is thus always a prime requirement, whatever the experiment. However many if not most experiments
5.1.
DESCRIPTION OF THE SAW
The saw consists of a rigid cast iron base, pivoted swinging arm and mandrel, The diamond is mounted in the yoke assembly at the end of the swinging arm. The arm may be loaded by an adjustable counterweight, pivotted by an antiback-
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lash threaded shaft (1 m m pitch) through precision sealed bearings. The mandrel rotates horizontally at 12 000 rpm, driven by a flat belt and pulley and a 1/4 H.P. electric motor. The swinging arm can be tilted and shifted, and the yoke assembly can be rotated in order to engage the diamond always on the top of the saw blade in the centre-line of the shaft. Finally the swinging arm is provided with a stopping screw (fig. 4). 5.2. MANDREL ASSEMBLY
Diamond sawing requires several mandrels with various flange sizes. A mandrel is made out of hardened and ground steel alloy, dynamically balanced. For bearing material we found after a long search, carbon blocks to be the most suitable, lubricated by ordinary candle wax. Load on the bearings is considerable. The play on the shaft (longitudinally) is taken up by two adjusting screws which in turn press against two oilsoaked hide strips against the end of the shaft. The cutting blades have to be dressed in position after mounting and quite frequently during sawing especially if sawing is not performed in an " e a s y " direction. Dressing the blade can be performed by gently pressing a block of alumina (A1203), carbon-tungsten, or a highspeed lathe tool against the rotating blade rested on the bracket. The choice of dressing tool depends on the diamond concentration of the blade. Always the final dressing was carried out by a high speed tool ground for parting-off purposes, in order to achieve a fiat top on the blade. Burs have to be removed by emery stick.
5.3. CONDITIONING OF SAW BLADE Experience has taught us that diamond powder of 4 - 8 / x m size gives the best sawing speed and finish. Coarse grit sizes of 2 0 - 4 0 ,tm cut well but one loses a lot from the blade by centrifugal force and at the exit side o f the cut, chipping o f the diamond occurs. Fine diamond sizes such as 3 1!2 p m cut smoothly but very slowly. We found the most effective way of conditioning the blade is to mix the diamond powder with olive oil into a thick pastelike consistency and to smear the paste evenly on an extremely hard steel roller (diam. 24 m m × × 3 0 ram) which rotates freely in a fork provided with a handle. With extreme care the rotating blade is approached with the stationary roller until contact is made and then with an oscillating m o v e m e n t the powder is pressed into the top of the blade. The -
roller must not be allowed to speed up to the same speed as the blade. It has been proved that diamond powder transfer occurs only when the angular velocities are unequal. Two sizes of blade are used. A thick blade is appropriate for " n o t c h i n g " to start a cut. It is soon found that to penetrate the surface of a diamond is usually the most difficult operation in the whole sawing process. This fact underlines the need of choosing the " e a s y " crystallographic direction to saw. The "trial and error" method is best avoided if possible. After notching, which can vary from 0.2 mm to 2 mm in depth, depending on the size and shape of the stone, the sawing blade has to be changed in order to saw the diamond off. The notching blade is 0.09 m m thick, 90 mm in diameter while for sawing we use a blade 0,06 m m thick, 76 m m in diameter. These sizes are starting sizes, because during the process of impregnating and sawing, blades have to be dressed frequently, especially if one requires a cut in a " h a r d " direction. Blades are made out of hardened b e r y l l i u m - copper which, in our opinion, is much superior to phosphor-bronze. 5.4. LOAD The right amount of loading is definitely an important factor for a successful cut. With the adjustable loading weight the load on the saw can be varied between 0 - 4 0 0 g. In the case of notching (0.09 mm) our best results were obtained with a load of 5 0 - 6 0 g. To achieve optimum cutting speed with a thin blade (0.06 mm) we found 5 - 2 0 g load to be the best. An important feature is to damp arm movements by placing a piece of neoprene sheet, 3 mm thick, between the stopping (limiting) screw and the base. Furthermore forming a 1.5 mm radius sphere on the end of the stopping screw is necessary. The result is no bouncing of the arm, no smearing of the blade and the diamond charge lasts. Our average rate of cut has been measured in the " s o f t " direction and found in a width of a cut of 4 m m wide, to be 1 m m per hour. 5.5. DIAMONDMOUNTING The diamond is mounted in a " d o p " or holder for sawing purposes. A great variety of " d o p s " have to be used, basically every diamond requires its own " d o p " . Only the shank size is the same. Depths and diameters vary according to the size and shape of each diamond. The function of the
NATURAL
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DIAMOND TARGETS
" d o p " is to hold a diamond rigidly in position during the sawing process. It may sound simple but one should appreciate that the diamonds used are usually small in size and rather awkward in shape for mounting. Furthermore the fact is that whilst sawing the temperature rises up to 2 0 0 - 3 0 0 ° C , so that with the effect of vibration as well, the " d o p " has to support the diamond very well. The best results have been achieved with three types of mounting: 1) Cementing 2) Staking 3) Vacuum chucking. 1) After searching widely we found " S y n a b o n d 5000" 4) (alkyl :z-cyanoacrylate) to be the most suitable cement for diamond. The use of a counter support is necessary, in order to hold the diamond securely and at the final stage of sawing it will hold the off-cut in position. One must emphasize the fact that the diamond must be supported firmly in position, because during the sawing operation the smallest m o v e m e n t can result in the specimen shattering, and the destruction of the blade and mandrel. 2) The diamond is ~ s t a k e d " into a metal " d o p ' " tailored specifically for each stone. We usually machine in this " d o p " a taper or hole for the purpose of " n e s t i n g " the stone. The diamond is pressed in and finally located by a cap shrunk or pressfitted on. 3) Vacuum chucking proved to be the most effective way to hold a diamond. This technique has only one limitation, namely, the diamond has to have one facet or window polished on already. In most of our work this method was used, because we already had one or two parallel windows polished on the diamond before we sawed a thin slice off (usually 0.4-0.5 mm thick). After measuring the minimum diameter needed to sink the diamond into the vacuum holder the recess is turned on a lathe. The depth has proved not to be very important as long as it is not less than 0.5 m m in order to give adequate physical strength. A number of pumping holes were drilled according to the size of the window and the bottom of the recess is carefully deburred and lapped. After a thin layer of " S y n a b o n d " cement has been applied on the window, the stone is pressed into position. The setting is about 45 s . The role of the Synabond is to establish the stone square in position but it does not have the strength to hold the stone in the dop during the sawing. The dop is placed in the yoke
assembly of the saw and the vacuum connection coupled on the shank. A rotary backing pump is used, and within ten minutes we registered 10 2 torr vacuum or better.
6. Polishing of diamond 6.1. DESCRIPTION OF TIlE POLISHING MA(ItlNE This method uses a rotating cast iron wheel commonly known as a "scaife". The wheel is 300 mm in diameter and rotates about a vertical axis. The running speed is designed at a nominal 2750 rpm from a 3-phase 380 V supply. The spindle is fitted with the rotor, instead of a pulley. As the polishing wheel is driven by electrical induction the usual belt drive and motor bearings are completely eliminated. The bottom point of the spindle runs in a bronze bearing with oil bath and the top point runs in a self-adjusting lubricated bearing. The system is smooth, silent, rigid and trouble-free due to its simplicity. Since the wheel has a spindle, conventional scouring and balancing methods may be applied (fig. 5).
Fig. 5. The diamond polishing machine. V. T A R G E T P R O P E R T I E S
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Wheels are changed easily and quickly by merely releasing the top bearing and slipping the wheel out. The spirit level setting is always maintained and a simple height adjustment allows each wheel to be set exactly to the correct height in seconds, thus compensating for varying thicknesses of wheel or varying lengths of spindle. The work top is designed to accommodate six diamond holders known as " t a n g s " , with only one in use at a time, as well as a " b l o c k i n g " attachment. 6.2. CONDITIONING THE SCAIFE The usual method of conditioning the scaife is to charge the wheel with a mixture of diampnd powder and olive oil. It works, but we found this method has its shortcomings. The most successful method developed is as follows: The cast iron scaife is dressed and scoured, electronically balanced, placed in position and cleaned, and finally the surface is degreased scrupulously. The motor is switched on for a few seconds, and the active area of the scaife marked with a felt pen approximately in three concentric zones. The diamond powders are mixed with sodium silicate and the mixture is spread thinly and evenly onto the surface with a perspex strip within the marks. Three different grit sizes of diamond powder are used: on the outside area 0 - 1 / z m , in the middle area 0 - 2 / l m and finally 4 - 8 / z m for the innermost zone. The scaife is heated with a hair dryer for about 15-20 rain. In order to work the diamond powder into the scouring grooves and into the pores of the scaife, a fairly large piece of boart diamond, held in the tang, is used. The scaife is run at half speed, and using firstly light pressure combined with an oscillating m o v e m e n t the diamond powder is worked in. Soon the surface of the scaife runs smoothly, and one can switch on to full speed and increase the load. In about three minutes the scaife is fully impregnated. Using this method the powder loss is negligible. The scaife now has three different grit sizes impregnated. The coarse ( 4 - 8 / x m ) inner ring is used for stock-removing, and only in the same area we keep on adding diamond powder mixed with olive oil. This is necessary for lubrication purposes in order to avoid burn marks. We have shown that diamonds polished on the 4 - 8 / ~ m area give ½-1 # m finish, which we establish by using comparative measurements. One theory that has been put forward in explanation of this observation is that
the diamond grit breaks up into smaller pieces during polishing. As the stone polishes down to near size, we oscillate the diamond on an everincreasing radius of the whole scaife in order to eliminate the scaife marks and to get a fine finish. All our specimens polished by this technique show 1/4 ~ m or better surface finish. 6.3.
POLISHING THIN PARALLEL DIAMONDS FOR SIZE
We have produced many varieties of parallel polished thin diamond slices. For example we prepared a suite of eight pieces of 0.35 mm thickness for ion dechanneling experiments, and a batch of twenty-four pieces 0.2 m m thick for the purpose of nitrogen analysis and associated infrared measurements. The requirements are demanding. The surface finish must be as fine as possible, on both sides, while parallelity and the size have to be accurate. Difficulties in polishing thin diamonds for size have arisen from four important factors, i.e. to hold a diamond securely in position during polishing, to find the best load per cutting speed ratio, vibrations, and sizing. We have resolved these in the following way. Firstly we saw a piece off from the bulk of the diamond, about (0.5_+0.1)mm thick (2 cuts), and then mount it in a dop with the second cut facing outwards and then polished to finish. The dop is mounted in the tang which is hand held. For mounting, we use "'Miracle" s) stone in an open dop. Miracle stone comes in fine powder form and is mixed with water prior to application to a thick paste-like consistency. We fill up the dop with the paste to form a half sphere and then place the diamond slice in the middle of the pool. With a small size vibrator we press gently and vibrate at the same time until it sinks in. It is dried in front of an infrared heat source or in a drying box, for about thirty minutes. The artificial stone has by now set but has no adhesive strength. We then scrape off from the top of the diamond the excess artificial stone, and with a sharp pair of harispring tweezers ease out the diamond, taking great care not to crumble the edge of the imprint. The diamond slice is cleaned in acetone ultrasonically "Secotine ''6) adhesive is then applied into the diamond imprint and the cleaned and dried diamond pressed home gently, taking care to maintain the same orientation. The assembly is dried in the same manner as above for 1-2 h. Diamonds pre-
NAIURAL
DIAMOND
pared in this manner usually hold in position, if the e n g a g i n g - d i s e n g a g i n g for polishing is done extremely carefully. R a m m i n g the diamond on the surface of the scaiFe will usually cause breaking (shattering) the specimen into pieces, and deep dugin marks in the scailE surface. The polishing direction is established before lowering the diamond, but often slight adjustment is necessary to find the optimal polishing direction, by rotating the dop. To adjust the specimen to the right plane, the stem of the tang (3/16" dia copper rod/ has to be bent accordingly. This operation has to be carried out carefully with slow rotating scaiFe speed lapprox. 3 0 0 - 5 0 0 rpm). In our experience loading the diamond during polishing needs no more than 2 0 0 - 5 0 0 g. Set rules cannot be applied here: the skilled hand and sensitive ears of the polisher are essential. Polishing of diamond is probably a combination of mechanical abrasion and thermo-chemical processes. After several experiments we became convinced that the load is not the important factor, but rather the speed to achieve high cutting ratio. It also has been proved that during polishing a diamond heats up to between 250~C and 300°C if it is polished in an " ' e a s y " direction, otherwise the diamond can heat up ir~ a matter of minutes to become red hot. We have no figures of our own to substantiate this claim but the temperature in the contact area must be ver~ high. The biggest problem has been to find an effective adhesive. Most of the commercially available adhesives are thermo-plastic at polishing temperature. We have tried to cool the diamond in order to make the adhesive hold, but the cutting speed dropped by almost an order of magnitude. In an extreme test case we cooled the diamond down to LN2 temperature, and using the same parameters found the cutting ratio bad reduced approximately eightl~ld. After the diamond face is flat, all saw marks removed and the surface finish satisfactory, the polishing is completed. To take the diamond out of the artificial stone holder the best method is to submerge the dop in water and with a sharp object, like an old-fashioned g r a m o p h o n e needle held in a pinvice, scratch the stone away all around the diamond. The stone is quite brittle and one can easily dislodge the diamond. In order to polish the second side, the diamond has to be placed in a v a c u u m chuck (described in sect. 5). Preparation of the v a c u u m chuck is the
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same as described before except the depth of the recess has to be accurately machined in. The depth in the dop will be the thickness of the final diamond. Now the v a c u u m chuck should be placed in the carefully aligned blocking attachment (parallel phme to scaife), which consists of a rigid cast carrier and slide to sweep in and out over the surface of the scaiFe, and the vertical m o v e m e n t is quided by two rows of precision ballbearings. The vacuum p u m p is coupled up and when pressure reaches 10 m torr, polishing can be carried out. An electric multimeter is hooked up on the 10 k£2 scale between the scaiFe and the blocking attachment for thickness monitoring purposes. The blocking attachment is electrically insulated from the scaiFe. As polishing the diamond advances, the meter starts indicating until at Full scale deflection the specimen is the same size as the depth of the recess in the dop, because the dop now itself makes contact electrically with the scaife. With this method of gauging our average variation of thickness is +0.001 nm or better, Parallelity one can clearly see depends on the alignment of the attachment versus scaife, plus accuracy of the slides. With the blocking attachment to achieve 0.5 m m parallel pieces represents no difficulty in our present set-up, but to produce 0.35 and 0.2 m m thick pieces our failure rate increased rapidly. On investigating the cause of this failure we have come to the conclusion that vibrations cause the diamond to shatter and break. It has also been observed that breaking appears to occur at the m o m e n t of rebounce. Several types of damping device have been considered. Designs have been drawn up which proved to be very complicated and costly. We are facing a challenge here of competing with the skilled hand which has all the sensors and feedback systems built in. We turned back to the hand operated tang. In our considered opinion, the scaiFe should be standing on a platform separately from the building on a deep foundation, far away even from traffic vibrations. The scaiFe should be dressed and balanced in position after diamond charging and conditioni n g - this is not a trivial requirement. Using a precision tang with a sensitive spirit level built in, we adjusted it carefully with an empty dop in position for level without the diamond mounted in. Handling it with extreme care so as not to upset the adjustments, the diamond is mounted in. Vacuum chucking and the electric metering technique are employed. Very light pressure has been used For polishing. We experienced no difficulty in ~,.
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PROPERTIES
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achieving the desired thicknesses of less than 0.2 mm, especially after switching off the air-conditioning unit which unfortunately is situated rather nearby. Finally we must mention a further function of the " M i r a c l e " stone which seems to play an important part, in addition to embodying and holding the diamond, which was less important when using vacuum chucking. If the diamond edge is exposed, skimming of the scaife can occur easily, the diamond powder throw-off rate increases considerably (clearly visible on the splash guard) and in extreme cases the diamond digs in, causing breakage and damage to the scaife. The only explanation that can he offered is that the artificial stone acts as a "guide-in'.
7. Cleaning of diamonds 7. l. C O N T A M I N A T I O N MeV helium ion backscattering is used to reveal the presence of foreign atoms on the diamond surface and near-surface and to enable their massnumbers to be determined. Application of this technique showed that all the diamond samples had dirt on the surface, despite a clean appearance and despite having been ultrasonically cleaned in various solvents (trichloroethylenealcohol and acetone). There were three obvious possible sources: 1) The kimberlitic matrix in which the diamond was found (such impurities are referred to as "earthy"). 2) Processing, including crushing, heavy medium separation and recovery, and possibly polishing. 3) The vacuum environment of samples already experimented on (silicone diffusion pump oil). 7.2. S Y S T E M A T I C C L E A N I N G A P P R O A C H Tests were carried out on 12 diamonds which had been selected as they showed both earthy mass continuum and prominent pump-oil (silicon and oxygen) peaks (fig. 6). Rutherford backscattering of He + ions was used to inspect the surfaces before and after cleaning. Three general types of cleaning were tried. 1) A hot hydrofluoric and sulphuric acid teflon bomb process. This had been used successfully in our laboratories on diamonds for neutron activation analysis.
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2) Ultrasonic cleaning for one hour in various organic solvents. This was chosen after testing a wide variety of solvents for miscibility with silicone oil. 3) Ultrasonic cleaning with the commercial detergent " C O N T R A D " 7). It is believed to contain a surfactant and a chelating agent. It is designed for radiochemical decontamination and other demanding cleaning jobs. The concentration used was 20%, in deionized water. Tests showed that ultrasonic agitation for thirty minutes was more than sufficient in all cases. A small oxygen peak always remained after cleaning, equivalent to about a monolayer. The Contrad method showed itself to be superior to all other methods.
7.3. R O U T I N E Ultrasonic minutes was dure applied and before
CLEANING
cleaning in 20% Contrad for thirty adopted (fig. 7) as the standard proceto all diamonds before experimentation procedures such as annealing. Each
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specimen was cleaned in an individual PTFE beaker. The cleaning was finished with several ultrasonic rinses in deionised water and the diamond allowed to dry by evaporation. It was handled by the edges using only " f i u o r o w a r e " plastic tweezers or a plastic-tipped vacuum pencil. Diamonds were stored in clean Fluoroware boxes, but preferably mounted and used immediately. Contamination from the vacuum environment was minimised by always using a cryopump and keeping the liquid nitrogen traps cold. 8. Improvement of diamond surfaces It is of course valid to question whether after sawing, polishing and cleaning, it is possible to still further improve the quality of the diamond surface in terms of crystal integrity. This surface disorder is sensitively and quantitatively assessed by backscattering measurements. To this end a number of techniques have been investigated. Gas etching is readily carried out by passing a stream of dry oxygen over the diamond at elevated temperatures. At 750°C etching is very slow with much pitting. At 1000°C etching rates of about 2 , u m / m i n are achieved. The quality of the result varied from stone to stone. At Feast in one case the surface disorder was reduced. However pitting is generally severe indicating that the etching rate is enhanced at defect sites. It is also not evident whether the improvement of the surface in this one case is due to the removal of disordered surface material or to the effect of annealing at the elevated temperature. Another technique tried was ion milling. A beam of 1 keV argon ions is effective in the removal of
123
TARGETS
surface layers by sputtering. Backscattering channeling measurements showed unambiguously that the effects of the associated radiation damage increased the degree of surface disorder. A further approach was to anneal the specimen in ultra high vacuum conditions ( - 1 0 10 tort) at 950°C. Again in some-cases no improvement was found, but in others there was evidence of improvement. We may note that studies of thermal conductivity of diamond have shown (for the relevant temperature regions) that annealing heals small surface microcracks. The dominant conclusion however is that assuming good cleaning practice always, the quality of the crystal surface of diamond is a function of the sawing and polishing procedures. In the case of diamond the situation is distinctly different from that of silicon and germanium where the damage depth is usually about twice that of the abrasive grit size diameter. It is likely that for diamond the grit breaks down rapidly in size so that effectively one is always using very fine grits. However, it is also evident that there is an additional mechanism which is more chemical in character and temperature dependant. The interplay between these two mechanisms is by no means fully understood yet. One experiment that was carefully carried out was to cleave a diamond and examine the freshly cleaved s u r f a c e - i t was by no means as good as a carefully polished surface. Indeed it showed considerable strain and had a step!Oed appearance. Finally it is clear that annealing is generally beneficial for the removal of strain and possibly even for the healing of microcracks.
9. Diagnostic techniques A wide range of these is relevant in tandem beam diamond experiments. For classification of stones, ultraviolet absorption and excitation is used, followed by electron spin resonance and electrical resistivity measurement. Infrared absorption is also a useful gross distinction but shows additionally fine structure features that can be associated with different forms in which nitrogen occurs. X-ray diffraction is a very useful technique for assessing crystal quality and in particular for determining orientation. This saves considerable time in ion channeling experiments and hence minimizes radiation damage to the crystal. Microscopic examination of specimens is always essential and the use of polarized light is a good indicator of strain, particularly in the vicinity of V.
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PROPERTIES
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M. REBAK et al.
small inclusions. The addition of cathodoluminescence to the microscope stage is very valuable indeed, showing up very clearly any growth bands that may be present in the crystal. Pleochroic halos are also revealed very clearly in this way. The best diagnostic techniques for channeling properties is indeed channeling itself! Electron microscopy shows up defects such as planar aggregates (in the {100} directions) and dislocation loops, but requires a very thin specimen. The combination of all these diagnostic probes provides us with a very powerful set of tools for characterizing diamond, and sets the stage for the use of nuclear probes for quantitative analysis. To date these embrace thermal, epithermal and fast neutron activation analysis; photon activation analysis; proton induced X-ray analysis; proton, helium-3 and helium-4 activation analysis followed by /3~, 7 and X-ray detection. Heavy ion beams such as fluorine have been used with prompt gamma analysis. Although a great deal has been revealed
about the physics and geochemistry of diamond as a result, the stage is now set for an exciting decade of further discovery. We acknowledge with appreciation the support of the De Beers Diamond Research Laboratories, the Council for Scientific and Industrial Research and the South African Atomic Energy Board. References 1) H.W. Fesq, D.M. Bibby, C. S. Erasmus, E. J. D. Kable and J. P. F. Sellschop, Phys. Chem. Earth 9 (1975) 817. 2) j. p. F. Sellschop, H. J. Annegarn, R. J. Keddy, C. C. P. Madiba and M.J. Renan, Nucl. Instr. and Meth. 149 (1978) 321. 3) H.W. Fesq, D. M. Bibby, J. P. F. Sellschop and J. 1. W. Watterson, J. Radioanal. Chem. 17 (1973) 195. 4) Cynabond 5000, Sumitomo Chemical Company Limited. $) Durok, ~'The Miracle Stone", Product of the Ransom & Randolph Company. o) Seccotine, Lapage's - A division of tlenry C. Stephens Limited. 7) Contrad, Decon Laboratories Limited, Brighton, England.