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ZONE REFINING
Z ZONE REFINING H B Singh, University of Delhi, New Delhi, India & 2005, Elsevier Ltd. All Rights Reserved. This article is reproduced from the first edition, pp. 5668–5673, & 1995, Elsevier Ltd., with an updated Further Reading list supplied by the Editor.
Introduction Zone refining is a method for the ultimate purification of substances – element, inorganic compound, or organic compound. It denotes a family of methods for controlling the distribution of soluble impurities or solutes in crystalline materials. A short molten zone travels slowly through a relatively long solid charge and while traveling redistributes the solute in the charge. This article gives a brief account of the principles and application of zone refining. Impurities, even at trace levels, may exert a profound influence upon certain properties of metals and compounds. For instance, the presence of even a few foreign atoms in a metal may affect its deformation and fracture properties, electrical conductivity, or corrosion resistance. This necessitates a study of matter in a high state of purity, which may be achieved following a large number of fractionations or chemical separations using high-purity reagents and guarding against contamination from vessels, laboratory, and even personnel. Handling samples at various stages introduces impurities and this limits the production of high-purity materials. In zone refining, this problem is solved by melting only a small part of the material and slowly passing the molten zone down the solid sample to be purified (Figures 1A and 1B). Subsequent passes of the molten zone increase the purity without requiring any handling. The technique of purification by zone melting is commonly called zone refining. Any substance that can be melted and solidified and exhibits a difference in impurity concentration in the liquid and solid states at the point of solidification can be purified by this method. The impurities redistribute, being displaced toward one end with corresponding purification of the portion being traversed by the molten zone. Though zone melting is largely restricted to
systems exhibiting liquid–solid transformation, successful refining has also been carried out where vapor–solid and even solid–solid transformations take place. In cases where any of these equilibria are not favorable for an impurity, zone refining may be combined with other techniques to achieve the desired purity level. It is essential to distinguish between zone refining and other common purification techniques. The process in which the whole of the charge is melted and gradually solidified unidirectionally is referred to as normal freezing. This may result in a mass of small crystals that may have entrapped liquid. Therefore, slow growth rates are required to secure a continuous and smooth solid–liquid interface during freezing from solvent or melt. Although many difficult separations, such as isolation of radium and separation of lanthanoids, have been achieved by repeated, fractional crystallization, zone refining is superior as it decreases the effort of making such repetitions. The main difference between the two techniques is that in zone refining only a part of the charge is melted at any time. This change alone tremendously increases the efficiency of crystallization as a separation and purification method. An impurity travels with or opposite to the movement of the zone depending, respectively, on whether it lowers or raises the melting point of the substance to be purified. In this way the impurities are concentrated in one or
4 2 4 1
2
3
x
L
(A)
l
3
L
1
l
1
x
(B)
Figure 1 Schematic diagram of a horizontal (A) and vertical (B) configuration of the zone melting process. 1, Resolidified zone; 2, unmelted zone; 3, molten zone; 4, heater.
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Theory of Zone Refining Consider a sample consisting of bulk component B contaminated with impurity I, the concentration of which is C0. I is analogous to the solute and B to the solvent. The sample, also called ‘the charge’, is taken in the form of a bar or rod of length L (p0.5 m). The heating zone, in the form of a narrow ring of width l capable of being heated to temperatures above the melting point of the sample, is positioned around the sample rod (Figures 1A and 1B). Generally, the heating zone is mobile and the sample is kept stationary, but the refining process can also be carried out by holding the heating zone stationary and moving the sample. Means of heating include electrical resistance coils, electric arc, electron beam, induction coil, lasers, plasma, radiant energy, and solar energy. Movement of the beating zone (4 cm h 1) along the sample produces a molten zone at the front and a solid zone at the rear. If the solute lowers the melting point of the solvent, its concentration in the freezing solid is lower than in the liquid. If the solute raises the melting point, the resolidified mass is more impure than the liquid phase. Thus, the freezing interface rejects certain solutes and attracts others. The measure of concentration of the impurity in the liquid phase is given by the distribution coefficient k, defined as the ratio of concentration of impurity I in the just-forming solid phase to that in the liquid phase, i.e., k ¼ CS =CL
When the solid and liquid phases are in equilibrium, then k ¼ k0, where k0 is the equilibrium distribution
k<1 k>1 kC 0 Concentration of impurity, l
the other end of the charge, thereby purifying the remainder. The degree of purification increases with each increase in the number of passes defined as the number of times a single zone passes through the charge. Pfann, working on the purification of germanium for making transistors, was able to produce this element having less than 1 part of impurity in 1010 parts of germanium. Besides purification, zone melting is useful in distributing any impurity uniformly throughout a system and in producing single crystals. Thus, precisely controlled discontinuities in impurity concentration, such as p–n or n–p–n junctions in semiconductors can be made in order to produce basic building blocks for transistors and solid diodes. Since zone refining can remove as well as concentrate the desired impurity, it can be used as a powerful analytical tool for detecting and determining impurities unobservable by conventional means.
C0
kC 0
0
L Distance, x
Figure 2 A schematic impurity distribution for single-pass zone refining.
coefficient. When k0o1 addition of solute lowers the melting point; conversely, if the melting point is raised by the addition of solute then k041. Assuming ko1, the concentration C of the impurity I in the resolidified mass at a distance x from the starting end is given by C ¼ C0 ½1 ðkÞekx=l
where C0 is the concentration at x ¼ 0 (Figure 2). In such a case, moving the heating zone from left to right (Figure 1A) concentrates impurities on the right-hand side. Consequently, a second pass of the heater will result in further purification of the sample by depleting impurities from the left-hand side and accumulating these on the extreme right of the sample rod. This enrichment continues at a decreasing rate until its concentration attains a value C0/k. From this point on, the concentrations entering and leaving the zone are equal and the concentration of freezing solid remains C0. In the last portion of the charge equal to the zone length l, the impurity concentration is more than in the original sample (Figure 2). In the same way as higher purity is achieved by repeated crystallizations or separation and recombination of fractions in fractional crystallization, purity can be increased in zone refining by accumulating impurities in one or other end by multipass zone refining. This can be achieved either by repeated zoning by a single heater or better, by using a multiple heater arrangement (Figure 3). This eliminates all kinds of handling procedures. Another advantage of multipass zoning is the small size of the molten region, which ensures minimal contamination by the container or atmosphere. After several passes, the impurity concentration approaches a steady state or ultimate distribution.
ZONE REFINING 465
the critical value; the former is related to zone width and the latter to zone speed.
Ring heaters (stationary)
Matter Transport in Zone Melting Moving charge
Molten zone Figure 3 Multipass zone refining.
Factors Affecting Zone Refining Width of Hosting Zone
The width of the heating zone, defined as the length of the solid melted to form the zone (and not the length of the molten zone), is one of the most important operational parameters. It is desirable to express the length of the sample rod l as a multiple n of the zone width, i.e., n ¼ L=l
Thus, the greater the value of n, the greater the number of zone passes required to approach the ultimate distribution. After n passes, each successive additional pass produces less additional purification. In practice, no5 is not very useful. Further, the interzone spacing i is generally of the order of the zone width l, though for high thermal conductivity materials like copper, it may be several zone widths. Zone Traverse Velocity
Diffusion coefficients of the solid and impurities, and impurity rejection and its transport away from the solidifying face, are governing factors that determine zone travel speed. The rate of solidification consequent upon the speed of zone travel must be much more than the rate of solid diffusion but not so fast as to prevent efficient diffusion of the impurity into the molten zone. Mechanical aids like stirring, to facilitate liquid diffusion, increases the efficiency of the process and permits an increase in zone speed by a factor of 10 or more. However, at high freezing rates, dendrites are formed which entrap liquid, and at somewhat lower rates, prismatic substructures are formed. In fact, for attaining the highest purity in the shortest time, for a given value of k it is necessary to set the ratio n/f to its smallest possible value, f being the heating zone traverse speed. Also, the ratio G/R (temperature gradient/growth rate) must be less than
The process of zone refining is primarily based on the transport of impurities in one or other direction. The driving force in matter transport is provided by the difference in densities between the solid and liquid phases. If the density of the liquid formed on melting the solid is higher, then the matter is transported in the direction of zone travel. Conversely, if expansion occurs on melting, the matter is transported in the opposite direction to the movement of the heating zone. In situations like this, matter transport is avoided by tilting the sample ingot. The critical angle of tilt is such that the liquid surface at the trailing end is at the same height as that of the original solid before it was molten.
Factors Influencing the Design of Zone Refining Equipment These factors include: 1. The nature of the process, whether batch, semicontinuous, or continuous. 2. The yield of purified material to be obtained in relation to time and expense. 3. The chemical and physical properties of the material to be purified and to a lesser extent the properties of the impurities. 4. The properties of the container material with reference to the melting point, thermal conductivity, porosity, and thermal expansion of the material to be purified. Conventional container materials used in zone refining include plastics (e.g., polytetrafluoroethylene), metals, glass, silica, mullite (Al2Si2O13), alumina, and graphite. In some cases compounds like zirconia (ZrO2), beryllia (BeO), and various fluorides, silicides, and nitrides are used. Silicon nitride is of special interest in view of its low coefficient of expansion, high temperature strength, resistance to thermal shocks, inertness, and nonwettability by metals. If the properties of the container material are incompatible with those of the sample, refining may be carried out by the unsupported floating zone method (FZM) (see below). 5. The atmosphere in the vessel surrounding the charge. 6. The traversing mechanism for transport of the molten zone(s). The choice lies between moving the charge and moving the heat source. The former has practical limitations where controlled atmosphere and temperature are involved. It is possible to move both the bar and the atmosphere
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containment vessel simultaneously. The stirring of the molten zone by mechanical means during zoning, in order to aid diffusion, is more easily achieved if the heating zone is static and the charge is made to move.
Modifications of Zone Refining Continuous Zone Refining
Zone refining, as outlined above, is a multistage batch technique in which successive operations are performed on a single batch of sample. The main limitation of such a procedure is the decrease in efficiency of zone passes as their number increases. Thus, ultimate purity is obtainable only at a prohibitive cost of time and money. To circumvent the various limitations of the basic technique, several continuous zone refining methods have been developed in which feed enters at one point in the sample while the product and waste leave at other points (Figure 4). The effect of countercurrent movement of solid and liquid phases is achieved by the movement of molten zones. In addition to the horizontal continuous refiner, the zone void vertical refiner and zone transport refiner are other modifications of this class. Cross-flow zone refiners and rotating drum multistage crystallizers based on the above principle are mainly used in growing single crystals rather than in purification of materials. Zone Leveling
It is virtually impossible to obtain a uniform concentration of a soluble impurity from one end to the other of a crystal grown from the melt by a conventional zone melting method because the separating crystals differ in composition from that of the melt. Zone leveling denotes a group of zone melting methods that aim to produce a uniform, or level,
Heaters Enriching section
Stripping section
distribution of impurity in the charge. In this method, the initial concentration in the first zone length is arranged to be C0/k and in the remainder, C0. Once this condition is met, the impurity concentrations entering and leaving the heat zone remain constant until the zone reaches the end of the ingot and normal freezing commences. The method is highly valuable in producing homogeneous single crystals having a uniform concentration of impurity, for example, crystals of a germanium-rich germanium– silicon alloy. This alloy is highly prone to segregation; k for silicon in germanium is B3. Another freezing technique has been developed in which the distribution of solute (i.e., impurity) is perturbed during freezing. This technique, called the method of perturbing the concentration, has led to the production of solid electronic devices having p–n and n–p–n junctions. Floating Zone Method
The FZM is a melt growth technique that does not require the use of a crucible. Thus, sample contamination by the container is not a problem. A vertical sample is heated with a focused laser light source or another heat source that can produce a molten zone capable of being held in place by surface tension. The molten zone can be seeded to produce oriented or single crystals. A miniaturized version of FZM called a laser-heated pedestal growth method uses a CO2 laser operating at 10.6 mm; in certain cases several lasers have also been used. The source rod is produced by cold pressing or sintering. To start, a molten region is formed at the top of the source rod. The seed rod, which may be of an inert or refractory material like platinum or indium, or polycrystalline or a single crystal of interest, is introduced into the melt and slowly withdrawn to initiate the crystal growth. The method was first applied to grow crystals of superconducting material of composition Bi2Sr2CaCu2O8. Another version of the FZM applied to incongruently melting materials is called the traveling solvent float zone (TSFZ) method. In this method the composition of the feed rod is that of the desired final product. Single superconducting crystals of La2–xSrxCuO4 are reported to have been prepared by TSFZ.
Feed Waste Movement of heaters
Molten zone Figure 4 Continuous zone refining process.
Liquid Encapsulation
Materials with volatile components are taken in an enclosed system so as to maintain the vapor pressure of the volatile constituents equal to or more than the equilibrium dissociation pressure over the melt. The loss of the volatile constituent is prevented simply by maintaining an inert gas pressure greater than
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the equilibrium dissociation vapor pressure of the volatile component. It is desirable that a liquid encapsulant should be less dense than the melt, be optically transparent, have a low viscosity at the melting point of the sample, and be stable and nonreactive with melt, crucible, and environment. B2O3 is the encapsulant of choice above 10001C. Below 8001C, BaCl2, CaCl2, and CaCl2 þ KCl are used and at still lower temperatures LiI þ KI is used as an encapsulant for zone refining of CdSnAs3. Certain organic acids are used for growing bismuth crystals at B1501C. On the other hand, high-pressure float zone melting is employed for achieving high purity of dissociating samples like GaP. Where sample availability is in the range of a few micrograms to a few milligrams, microscale zone refining is very advantageous. Besides processing very small quantities, the operational time is drastically decreased because a large number of heating zones is used and the zone speed can be increased because of the more diffusive mixing achieved. Solution Zone Refining
Solution zone refining, also called traveling heater zone refining (THZR), is identical to zone refining. It consists of a liquid zone (B1 cm thick) of a concentrated solution containing the phase to be refined as solute; the solvent should be such as to dissolve the solute and the impurities but to be insoluble in the solid solute phase. This process is particularly promising for purification of materials that decompose on melting, have very high melting points, have a high vapor pressure at the melting point, and are highly reactive. By using THZR, purification can be achieved at a much lower temperature than is normally possible. Other Methods
Thin alloy zone crystallization (TAZC) and temperature gradient zone melting (TGZM), in which the driving force is, respectively, an electrochemical potential and a temperature gradient, have also been developed based on the principle of zone refining. An alloy of composition 80% indium and 20% antimony (m.p. 4251C) for refining thin layers of InSb has been prepared by TAZC whereas TGZM has been used in fabricating semiconductor junctions, and joining and growing single crystals.
Applications of Zone Refining Zone refining, an extremely simple technique in principle, provides an excellent means of preparing ultrapure crystalline materials. Handling of the
material and its consequent contamination is kept to an absolute minimum. Further, the use of a controlled atmosphere or vacuum in the system removes the danger of atmospheric contamination or reaction with components of the air. Besides purification, the importance of zone melting methods lies in the uniform distribution of the desired impurity throughout a single crystal, a problem which remained unsolved for a long time. Thus, precisely controlled discontinuities in impurity concentration can be made to produce p–n or n–p–n junctions in semiconductors. Further, joining single crystals and measuring diffusivities in liquids can also be achieved. Some examples of the applications of zone refining are given below. Refining of Metals
A large number of metals have been obtained in a state of ultrapurity by zone refining. Quantification of impurities is done by measuring the ratio of resistivities at two different concentrations of the impurity. Uranium is purified by zone refining in a uranium oxide boat and an atmosphere of purified argon; a travel rate of 6 mm h 1 was found to be the most satisfactory. By starting from a relatively pure material, a 90% decrease in iron and carbon content is achieved; oxygen and nitrogen are also removed by vaporization. A final purity 499.99% is obtained. Refining of Semiconductors
Elemental semiconductors Germanium has been widely used for semiconductor diodes and transistors. Six zone passes through an ingot of the purest available germanium sufficed to decrease the concentration of impurities to 1 atom in 1010 atoms of germanium. Ultrapure silicon is not only important for transistors, it is also required for solar batteries. Zone refining of silicon using a quartz boat is unable to remove the last traces of boron except when a chemical purification is simultaneously applied. The reactivity of molten silicon creates problems of contamination with oxygen. For higher purity, the floating zone technique has been used to quantify the concentration of boron, an electron acceptor, and phosphorus, an electron donor. The electrical conductivity measured as a function of distance along the rod is found to be proportional to the difference in concentration of boron and phosphorus; their concentration after refining is of the order of 103 atoms cm–3. Copper impurity in selenium is related to a trapping center in CdSe. After more than eight passes, copper accumulates at one end. Its concentration at
468 ZONE REFINING
the other end was below the detectable limit; k for copper is o1. Compound semiconductors A (III–V) compound, InSb, is of particular interest in view of its highest electron mobility. In its preparation, zone refining of the compound, of the components, or both, may be utilized. During zone refining, the optical energy gap of InSb decreases from 0.39 eV for relatively impure material to 0.18 eV for an extensively refined sample. The electrical conductivity and Hall coefficient of pure and tellurium-doped InSb were measured as functions of temperature from 90 to 470 K. From the analytical standpoint, it was possible to identify the critical impurities in InSb. Both tellurium and zinc are present. The former, a donor, lowers the melting point and is slower to segregate but is removed by zone refining, while zinc, an acceptor, raises the melting point and is removed only on electroplating. Thallium bromide semiconductor for X-ray and g-ray detectors is purified by multipass zone refining. A vacuum-deposited thin film of TlBr is used as a single photon detector and in X-ray imaging including xeroradiography. Preparation of Films and Single Crystals of Superconductors
Several complex compounds have been investigated for their superconducting properties. These usually comprise oxides of a lanthanoid, an alkaline earth metal, and a transition metal. Superconducting properties of such complex oxide systems are very sensitive to the presence of impurities. Films of the Y–Ba–Cu–O system with a critical temperature for superconducting transition of 80 K have been obtained by zone crystallization. Single crystals of Bi2Sr2CaCuOg are obtained by TSFZ. Measurement of the superconducting properties of various parts of the crystal shows that the later grown part exhibits semiconducting behavior. After 1 day of annealing, homogeneous crystals are obtained. Large single crystals of Bi-2212 superconducting along the c-axis have been prepared by FZM. Refining of Organic and Inorganic Compounds
Many organic and inorganic compounds have been zone refined. Since the physical properties of compounds differ significantly from metals, these require different apparatus. The low thermal diffusivity requires artificial cooling between zones. Low chemical diffusivity makes agitation of the liquid desirable; agitation also ensures better heat transfer to the liquid.
Ultrapure crystals of nitrobenzene for nonlinear optical devices are prepared by zone refining of the fractionally distilled, purified sample. Single crystals of m-nitroaniline for use as waveguides in optical second-harmonic generation are produced by zone melting crystallization using a ridge heater. Sodium nitrate has been purified by infrared laser-assisted zone refining. Besides a large number of hydrocarbons, fatty acids, alcohols, nitrogen- and sulfur-containing organic compounds, halides, and nitrates have also been purified by various zone refining methods. Analytical Applications
Zone refining is a powerful tool for separation and concentration. Following preconcentration, conventional analytical techniques are used to identify and quantify the impurity. This has enabled certain observed abnormalities in properties to be correlated with the presence of impurities in ultratrace amounts that had earlier remained undetected. In another version, the distribution coefficient, k, of a metallic solute is determined by adding its radioisotope under the conditions of the zoning experiment. If the concentration of the nonradioactive form of the solute in the starting material is measurable by an available analytical technique but subsequently falls below the detection limit after refining, its final concentration can still be calculated from the value of k and the zone refining curves. See also: Radiochemical Methods: Radiotracers.
Further Reading Brice JC (1973) The Growth of Crystals from Liquids. Amsterdam: North-Holland. Gatos HC (1979) Methods for Crystal Growth. Amsterdam: North-Holland. Grubel O (1961) Metallurgy of Elemental and Compound Semiconductors. New York: Wiley-Interscience. Ito Y and Ma Y (1996) pH zone refining counter-current chromatography. Journal of Chromatography A 753(1): 1–36. Pamplin BR (ed.) (1980) International Series in the Science of the Solid State, vol. 16, Crystal Growth, 2nd edn. Oxford: Pergamon. Parr NL (1960) Zone Refining and Allied Techniques. London: George Newnes. Pennanec R, Viron C, Blanchard S, and Lafosse M (2001) Original uses of the pH zone refining principle: adaptation to synthesis imperatives and to ionic compounds. Journal of Liquid Chromatography and Related Technologies 24(11–12): 1575–1591. Pfann WG (1958) Zone Melting. New York: Wiley.
Introduction Zone refining is a method for the ultimate purification of substances – element, inorganic compound, or organic compound. It denotes a family of methods for controlling the distribution of soluble impurities or solutes in crystalline materials. A short molten zone travels slowly through a relatively long solid charge and while traveling redistributes the solute in the charge. This article gives a brief account of the principles and application of zone refining. Impurities, even at trace levels, may exert a profound influence upon certain properties of metals and compounds. For instance, the presence of even a few foreign atoms in a metal may affect its deformation and fracture properties, electrical conductivity, or corrosion resistance. This necessitates a study of matter in a high state of purity, which may be achieved following a large number of fractionations or chemical separations using high-purity reagents and guarding against contamination from vessels, laboratory, and even personnel. Handling samples at various stages introduces impurities and this limits the production of high-purity materials. In zone refining, this problem is solved by melting only a small part of the material and slowly passing the molten zone down the solid sample to be purified (Figures 1A and 1B). Subsequent passes of the molten zone increase the purity without requiring any handling. The technique of purification by zone melting is commonly called zone refining. Any substance that can be melted and solidified and exhibits a difference in impurity concentration in the liquid and solid states at the point of solidification can be purified by this method. The impurities redistribute, being displaced toward one end with corresponding purification of the portion being traversed by the molten zone. Though zone melting is largely restricted to
systems exhibiting liquid–solid transformation, successful refining has also been carried out where vapor–solid and even solid–solid transformations take place. In cases where any of these equilibria are not favorable for an impurity, zone refining may be combined with other techniques to achieve the desired purity level. It is essential to distinguish between zone refining and other common purification techniques. The process in which the whole of the charge is melted and gradually solidified unidirectionally is referred to as normal freezing. This may result in a mass of small crystals that may have entrapped liquid. Therefore, slow growth rates are required to secure a continuous and smooth solid–liquid interface during freezing from solvent or melt. Although many difficult separations, such as isolation of radium and separation of lanthanoids, have been achieved by repeated, fractional crystallization, zone refining is superior as it decreases the effort of making such repetitions. The main difference between the two techniques is that in zone refining only a part of the charge is melted at any time. This change alone tremendously increases the efficiency of crystallization as a separation and purification method. An impurity travels with or opposite to the movement of the zone depending, respectively, on whether it lowers or raises the melting point of the substance to be purified. In this way the impurities are concentrated in one or
4 2 4 1
2
3
x
L
(A)
l
3
L
1
l
1
x
(B)
Figure 1 Schematic diagram of a horizontal (A) and vertical (B) configuration of the zone melting process. 1, Resolidified zone; 2, unmelted zone; 3, molten zone; 4, heater.
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Theory of Zone Refining Consider a sample consisting of bulk component B contaminated with impurity I, the concentration of which is C0. I is analogous to the solute and B to the solvent. The sample, also called ‘the charge’, is taken in the form of a bar or rod of length L (p0.5 m). The heating zone, in the form of a narrow ring of width l capable of being heated to temperatures above the melting point of the sample, is positioned around the sample rod (Figures 1A and 1B). Generally, the heating zone is mobile and the sample is kept stationary, but the refining process can also be carried out by holding the heating zone stationary and moving the sample. Means of heating include electrical resistance coils, electric arc, electron beam, induction coil, lasers, plasma, radiant energy, and solar energy. Movement of the beating zone (4 cm h 1) along the sample produces a molten zone at the front and a solid zone at the rear. If the solute lowers the melting point of the solvent, its concentration in the freezing solid is lower than in the liquid. If the solute raises the melting point, the resolidified mass is more impure than the liquid phase. Thus, the freezing interface rejects certain solutes and attracts others. The measure of concentration of the impurity in the liquid phase is given by the distribution coefficient k, defined as the ratio of concentration of impurity I in the just-forming solid phase to that in the liquid phase, i.e., k ¼ CS =CL
When the solid and liquid phases are in equilibrium, then k ¼ k0, where k0 is the equilibrium distribution
k<1 k>1 kC 0 Concentration of impurity, l
the other end of the charge, thereby purifying the remainder. The degree of purification increases with each increase in the number of passes defined as the number of times a single zone passes through the charge. Pfann, working on the purification of germanium for making transistors, was able to produce this element having less than 1 part of impurity in 1010 parts of germanium. Besides purification, zone melting is useful in distributing any impurity uniformly throughout a system and in producing single crystals. Thus, precisely controlled discontinuities in impurity concentration, such as p–n or n–p–n junctions in semiconductors can be made in order to produce basic building blocks for transistors and solid diodes. Since zone refining can remove as well as concentrate the desired impurity, it can be used as a powerful analytical tool for detecting and determining impurities unobservable by conventional means.
C0
kC 0
0
L Distance, x
Figure 2 A schematic impurity distribution for single-pass zone refining.
coefficient. When k0o1 addition of solute lowers the melting point; conversely, if the melting point is raised by the addition of solute then k041. Assuming ko1, the concentration C of the impurity I in the resolidified mass at a distance x from the starting end is given by C ¼ C0 ½1 ðkÞekx=l
where C0 is the concentration at x ¼ 0 (Figure 2). In such a case, moving the heating zone from left to right (Figure 1A) concentrates impurities on the right-hand side. Consequently, a second pass of the heater will result in further purification of the sample by depleting impurities from the left-hand side and accumulating these on the extreme right of the sample rod. This enrichment continues at a decreasing rate until its concentration attains a value C0/k. From this point on, the concentrations entering and leaving the zone are equal and the concentration of freezing solid remains C0. In the last portion of the charge equal to the zone length l, the impurity concentration is more than in the original sample (Figure 2). In the same way as higher purity is achieved by repeated crystallizations or separation and recombination of fractions in fractional crystallization, purity can be increased in zone refining by accumulating impurities in one or other end by multipass zone refining. This can be achieved either by repeated zoning by a single heater or better, by using a multiple heater arrangement (Figure 3). This eliminates all kinds of handling procedures. Another advantage of multipass zoning is the small size of the molten region, which ensures minimal contamination by the container or atmosphere. After several passes, the impurity concentration approaches a steady state or ultimate distribution.
ZONE REFINING 465
the critical value; the former is related to zone width and the latter to zone speed.
Ring heaters (stationary)
Matter Transport in Zone Melting Moving charge
Molten zone Figure 3 Multipass zone refining.
Factors Affecting Zone Refining Width of Hosting Zone
The width of the heating zone, defined as the length of the solid melted to form the zone (and not the length of the molten zone), is one of the most important operational parameters. It is desirable to express the length of the sample rod l as a multiple n of the zone width, i.e., n ¼ L=l
Thus, the greater the value of n, the greater the number of zone passes required to approach the ultimate distribution. After n passes, each successive additional pass produces less additional purification. In practice, no5 is not very useful. Further, the interzone spacing i is generally of the order of the zone width l, though for high thermal conductivity materials like copper, it may be several zone widths. Zone Traverse Velocity
Diffusion coefficients of the solid and impurities, and impurity rejection and its transport away from the solidifying face, are governing factors that determine zone travel speed. The rate of solidification consequent upon the speed of zone travel must be much more than the rate of solid diffusion but not so fast as to prevent efficient diffusion of the impurity into the molten zone. Mechanical aids like stirring, to facilitate liquid diffusion, increases the efficiency of the process and permits an increase in zone speed by a factor of 10 or more. However, at high freezing rates, dendrites are formed which entrap liquid, and at somewhat lower rates, prismatic substructures are formed. In fact, for attaining the highest purity in the shortest time, for a given value of k it is necessary to set the ratio n/f to its smallest possible value, f being the heating zone traverse speed. Also, the ratio G/R (temperature gradient/growth rate) must be less than
The process of zone refining is primarily based on the transport of impurities in one or other direction. The driving force in matter transport is provided by the difference in densities between the solid and liquid phases. If the density of the liquid formed on melting the solid is higher, then the matter is transported in the direction of zone travel. Conversely, if expansion occurs on melting, the matter is transported in the opposite direction to the movement of the heating zone. In situations like this, matter transport is avoided by tilting the sample ingot. The critical angle of tilt is such that the liquid surface at the trailing end is at the same height as that of the original solid before it was molten.
Factors Influencing the Design of Zone Refining Equipment These factors include: 1. The nature of the process, whether batch, semicontinuous, or continuous. 2. The yield of purified material to be obtained in relation to time and expense. 3. The chemical and physical properties of the material to be purified and to a lesser extent the properties of the impurities. 4. The properties of the container material with reference to the melting point, thermal conductivity, porosity, and thermal expansion of the material to be purified. Conventional container materials used in zone refining include plastics (e.g., polytetrafluoroethylene), metals, glass, silica, mullite (Al2Si2O13), alumina, and graphite. In some cases compounds like zirconia (ZrO2), beryllia (BeO), and various fluorides, silicides, and nitrides are used. Silicon nitride is of special interest in view of its low coefficient of expansion, high temperature strength, resistance to thermal shocks, inertness, and nonwettability by metals. If the properties of the container material are incompatible with those of the sample, refining may be carried out by the unsupported floating zone method (FZM) (see below). 5. The atmosphere in the vessel surrounding the charge. 6. The traversing mechanism for transport of the molten zone(s). The choice lies between moving the charge and moving the heat source. The former has practical limitations where controlled atmosphere and temperature are involved. It is possible to move both the bar and the atmosphere
466 ZONE REFINING
containment vessel simultaneously. The stirring of the molten zone by mechanical means during zoning, in order to aid diffusion, is more easily achieved if the heating zone is static and the charge is made to move.
Modifications of Zone Refining Continuous Zone Refining
Zone refining, as outlined above, is a multistage batch technique in which successive operations are performed on a single batch of sample. The main limitation of such a procedure is the decrease in efficiency of zone passes as their number increases. Thus, ultimate purity is obtainable only at a prohibitive cost of time and money. To circumvent the various limitations of the basic technique, several continuous zone refining methods have been developed in which feed enters at one point in the sample while the product and waste leave at other points (Figure 4). The effect of countercurrent movement of solid and liquid phases is achieved by the movement of molten zones. In addition to the horizontal continuous refiner, the zone void vertical refiner and zone transport refiner are other modifications of this class. Cross-flow zone refiners and rotating drum multistage crystallizers based on the above principle are mainly used in growing single crystals rather than in purification of materials. Zone Leveling
It is virtually impossible to obtain a uniform concentration of a soluble impurity from one end to the other of a crystal grown from the melt by a conventional zone melting method because the separating crystals differ in composition from that of the melt. Zone leveling denotes a group of zone melting methods that aim to produce a uniform, or level,
Heaters Enriching section
Stripping section
distribution of impurity in the charge. In this method, the initial concentration in the first zone length is arranged to be C0/k and in the remainder, C0. Once this condition is met, the impurity concentrations entering and leaving the heat zone remain constant until the zone reaches the end of the ingot and normal freezing commences. The method is highly valuable in producing homogeneous single crystals having a uniform concentration of impurity, for example, crystals of a germanium-rich germanium– silicon alloy. This alloy is highly prone to segregation; k for silicon in germanium is B3. Another freezing technique has been developed in which the distribution of solute (i.e., impurity) is perturbed during freezing. This technique, called the method of perturbing the concentration, has led to the production of solid electronic devices having p–n and n–p–n junctions. Floating Zone Method
The FZM is a melt growth technique that does not require the use of a crucible. Thus, sample contamination by the container is not a problem. A vertical sample is heated with a focused laser light source or another heat source that can produce a molten zone capable of being held in place by surface tension. The molten zone can be seeded to produce oriented or single crystals. A miniaturized version of FZM called a laser-heated pedestal growth method uses a CO2 laser operating at 10.6 mm; in certain cases several lasers have also been used. The source rod is produced by cold pressing or sintering. To start, a molten region is formed at the top of the source rod. The seed rod, which may be of an inert or refractory material like platinum or indium, or polycrystalline or a single crystal of interest, is introduced into the melt and slowly withdrawn to initiate the crystal growth. The method was first applied to grow crystals of superconducting material of composition Bi2Sr2CaCu2O8. Another version of the FZM applied to incongruently melting materials is called the traveling solvent float zone (TSFZ) method. In this method the composition of the feed rod is that of the desired final product. Single superconducting crystals of La2–xSrxCuO4 are reported to have been prepared by TSFZ.
Feed Waste Movement of heaters
Molten zone Figure 4 Continuous zone refining process.
Liquid Encapsulation
Materials with volatile components are taken in an enclosed system so as to maintain the vapor pressure of the volatile constituents equal to or more than the equilibrium dissociation pressure over the melt. The loss of the volatile constituent is prevented simply by maintaining an inert gas pressure greater than
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the equilibrium dissociation vapor pressure of the volatile component. It is desirable that a liquid encapsulant should be less dense than the melt, be optically transparent, have a low viscosity at the melting point of the sample, and be stable and nonreactive with melt, crucible, and environment. B2O3 is the encapsulant of choice above 10001C. Below 8001C, BaCl2, CaCl2, and CaCl2 þ KCl are used and at still lower temperatures LiI þ KI is used as an encapsulant for zone refining of CdSnAs3. Certain organic acids are used for growing bismuth crystals at B1501C. On the other hand, high-pressure float zone melting is employed for achieving high purity of dissociating samples like GaP. Where sample availability is in the range of a few micrograms to a few milligrams, microscale zone refining is very advantageous. Besides processing very small quantities, the operational time is drastically decreased because a large number of heating zones is used and the zone speed can be increased because of the more diffusive mixing achieved. Solution Zone Refining
Solution zone refining, also called traveling heater zone refining (THZR), is identical to zone refining. It consists of a liquid zone (B1 cm thick) of a concentrated solution containing the phase to be refined as solute; the solvent should be such as to dissolve the solute and the impurities but to be insoluble in the solid solute phase. This process is particularly promising for purification of materials that decompose on melting, have very high melting points, have a high vapor pressure at the melting point, and are highly reactive. By using THZR, purification can be achieved at a much lower temperature than is normally possible. Other Methods
Thin alloy zone crystallization (TAZC) and temperature gradient zone melting (TGZM), in which the driving force is, respectively, an electrochemical potential and a temperature gradient, have also been developed based on the principle of zone refining. An alloy of composition 80% indium and 20% antimony (m.p. 4251C) for refining thin layers of InSb has been prepared by TAZC whereas TGZM has been used in fabricating semiconductor junctions, and joining and growing single crystals.
Applications of Zone Refining Zone refining, an extremely simple technique in principle, provides an excellent means of preparing ultrapure crystalline materials. Handling of the
material and its consequent contamination is kept to an absolute minimum. Further, the use of a controlled atmosphere or vacuum in the system removes the danger of atmospheric contamination or reaction with components of the air. Besides purification, the importance of zone melting methods lies in the uniform distribution of the desired impurity throughout a single crystal, a problem which remained unsolved for a long time. Thus, precisely controlled discontinuities in impurity concentration can be made to produce p–n or n–p–n junctions in semiconductors. Further, joining single crystals and measuring diffusivities in liquids can also be achieved. Some examples of the applications of zone refining are given below. Refining of Metals
A large number of metals have been obtained in a state of ultrapurity by zone refining. Quantification of impurities is done by measuring the ratio of resistivities at two different concentrations of the impurity. Uranium is purified by zone refining in a uranium oxide boat and an atmosphere of purified argon; a travel rate of 6 mm h 1 was found to be the most satisfactory. By starting from a relatively pure material, a 90% decrease in iron and carbon content is achieved; oxygen and nitrogen are also removed by vaporization. A final purity 499.99% is obtained. Refining of Semiconductors
Elemental semiconductors Germanium has been widely used for semiconductor diodes and transistors. Six zone passes through an ingot of the purest available germanium sufficed to decrease the concentration of impurities to 1 atom in 1010 atoms of germanium. Ultrapure silicon is not only important for transistors, it is also required for solar batteries. Zone refining of silicon using a quartz boat is unable to remove the last traces of boron except when a chemical purification is simultaneously applied. The reactivity of molten silicon creates problems of contamination with oxygen. For higher purity, the floating zone technique has been used to quantify the concentration of boron, an electron acceptor, and phosphorus, an electron donor. The electrical conductivity measured as a function of distance along the rod is found to be proportional to the difference in concentration of boron and phosphorus; their concentration after refining is of the order of 103 atoms cm–3. Copper impurity in selenium is related to a trapping center in CdSe. After more than eight passes, copper accumulates at one end. Its concentration at
468 ZONE REFINING
the other end was below the detectable limit; k for copper is o1. Compound semiconductors A (III–V) compound, InSb, is of particular interest in view of its highest electron mobility. In its preparation, zone refining of the compound, of the components, or both, may be utilized. During zone refining, the optical energy gap of InSb decreases from 0.39 eV for relatively impure material to 0.18 eV for an extensively refined sample. The electrical conductivity and Hall coefficient of pure and tellurium-doped InSb were measured as functions of temperature from 90 to 470 K. From the analytical standpoint, it was possible to identify the critical impurities in InSb. Both tellurium and zinc are present. The former, a donor, lowers the melting point and is slower to segregate but is removed by zone refining, while zinc, an acceptor, raises the melting point and is removed only on electroplating. Thallium bromide semiconductor for X-ray and g-ray detectors is purified by multipass zone refining. A vacuum-deposited thin film of TlBr is used as a single photon detector and in X-ray imaging including xeroradiography. Preparation of Films and Single Crystals of Superconductors
Several complex compounds have been investigated for their superconducting properties. These usually comprise oxides of a lanthanoid, an alkaline earth metal, and a transition metal. Superconducting properties of such complex oxide systems are very sensitive to the presence of impurities. Films of the Y–Ba–Cu–O system with a critical temperature for superconducting transition of 80 K have been obtained by zone crystallization. Single crystals of Bi2Sr2CaCuOg are obtained by TSFZ. Measurement of the superconducting properties of various parts of the crystal shows that the later grown part exhibits semiconducting behavior. After 1 day of annealing, homogeneous crystals are obtained. Large single crystals of Bi-2212 superconducting along the c-axis have been prepared by FZM. Refining of Organic and Inorganic Compounds
Many organic and inorganic compounds have been zone refined. Since the physical properties of compounds differ significantly from metals, these require different apparatus. The low thermal diffusivity requires artificial cooling between zones. Low chemical diffusivity makes agitation of the liquid desirable; agitation also ensures better heat transfer to the liquid.
Ultrapure crystals of nitrobenzene for nonlinear optical devices are prepared by zone refining of the fractionally distilled, purified sample. Single crystals of m-nitroaniline for use as waveguides in optical second-harmonic generation are produced by zone melting crystallization using a ridge heater. Sodium nitrate has been purified by infrared laser-assisted zone refining. Besides a large number of hydrocarbons, fatty acids, alcohols, nitrogen- and sulfur-containing organic compounds, halides, and nitrates have also been purified by various zone refining methods. Analytical Applications
Zone refining is a powerful tool for separation and concentration. Following preconcentration, conventional analytical techniques are used to identify and quantify the impurity. This has enabled certain observed abnormalities in properties to be correlated with the presence of impurities in ultratrace amounts that had earlier remained undetected. In another version, the distribution coefficient, k, of a metallic solute is determined by adding its radioisotope under the conditions of the zoning experiment. If the concentration of the nonradioactive form of the solute in the starting material is measurable by an available analytical technique but subsequently falls below the detection limit after refining, its final concentration can still be calculated from the value of k and the zone refining curves. See also: Radiochemical Methods: Radiotracers.
Further Reading Brice JC (1973) The Growth of Crystals from Liquids. Amsterdam: North-Holland. Gatos HC (1979) Methods for Crystal Growth. Amsterdam: North-Holland. Grubel O (1961) Metallurgy of Elemental and Compound Semiconductors. New York: Wiley-Interscience. Ito Y and Ma Y (1996) pH zone refining counter-current chromatography. Journal of Chromatography A 753(1): 1–36. Pamplin BR (ed.) (1980) International Series in the Science of the Solid State, vol. 16, Crystal Growth, 2nd edn. Oxford: Pergamon. Parr NL (1960) Zone Refining and Allied Techniques. London: George Newnes. Pennanec R, Viron C, Blanchard S, and Lafosse M (2001) Original uses of the pH zone refining principle: adaptation to synthesis imperatives and to ionic compounds. Journal of Liquid Chromatography and Related Technologies 24(11–12): 1575–1591. Pfann WG (1958) Zone Melting. New York: Wiley.