Electrode surface modifications and material transport into a high voltage vacuum spark discharge

Znternational Journal of Mass Spectrometry and Zon Processes, 71 (1986) 85-102 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

85

ELECTRODE SURFACE MODIFICATIONS AND MATERIAL TRANSPORT INTO A HIGH VOLTAGE VACUUM SPARK DISCHARGE

K. SWENTERS,

J. VERLINDEN,

University of Antwerp (First received

P. BERNARD

(U.Z.A.), Department

6 January

and R. GIJBELS

of Chemistry, B-2610 Antwerpen- Wilrijk (Belgium)

1986; in revised form 18 March 1986)

ABSTRACT The material eroded in a high voltage spark discharge was studied by scanning electron microscopy. It was found that the material collected near the spark discharge consisted mainly of very small particles, although some larger droplets were observed. Studies of the sparked electrode surface and of the deposited material by secondary ion mass spectrometry showed differences in their composition as compared to that of the solid bulk. The differences may be responsible for the different relative sensitivities observed in spark source mass spectrometry.

INTRODUCTION

Microparticles have long been suspected of playing a major role in breakdown initiation processes. According to Cranberg [l], an electrically charged “clump” is detached from one electrode and accelerated across the gap. On impact with the opposite electrode the particle is either itself vaporized or causes local evaporation of the bombarded electrode. According to cathode initiation models [2-41 the electrode gap is filled with vapour resulting from the explosion of micropoints on the cathode surface which are heated by the autoelectron current. According to the initiation model of Davies and Biondi [5,6], a rupture of anode material, heated by a field emitted electron beam to a critical temperature, should occur. The material is ejected into the gap as a drop which evaporates on its way to the cathode. Davies and Biondi [5] made objections against the clump model for small gaps ( < 1 mm). From calculations, it followed that no macroparticle is capable of producing sufficient vapour in the gap by virtue of its kinetic energy alone. Similarly, it was shown that the dimensions of cathode protrusions are too small to fulfil the requirements of a critical density of vapour in the gap for breakdown to occur for 1 mm gaps [5] or smaller [7]. 0168-1176/86/$03.50

0 1986 Elsevier Science Publishers

B.V.

86

Studies on the material consumption of - 20 metallic electrodes in spark source mass spectrometry (SSMS) [7-111 showed that the volume of sample atomized per spark pulse, obtained by weighing the electrode before and after sparking, is dependent on the breakdown voltage and on the melting point of the material. Assuming that the material is transported into the gap as a liquid drop, then the drop radius can be calculated: it was found to be proportional to the reciprocal melting point of the metal and proportional to the gap, at constant breakdown voltages. In addition, it was shown that the vapour pressure of the elements (matrices) does not play an important role in the weight loss per pulse [9,10]. In various reports it was demonstrated by scanning electron microscopy that surface melting occurs during sparking [9,10,12], even for refractory metals such as MO and Re. Surface melting, drop formation and evaporation seem to be essential in SSMS. Experimental observations of microparticles formed in the gap have been mentioned in the literature. However, many of them are performed in industrial vacuum [13-161 or in an argon atmosphere [17] and only a few refer to high vacuum conditions [l&19]. We have studied drop formation by collecting erosion products on a glass plate, positioned close to the spark. The size distribution of the collected particles was investigated by scanning electron microscopy and the results are compared with data of material consumption studies. The experiments were carried out for different matrices and at different breakdown voltages. The composition of the sparked electrode surface [20] and of the deposited material was studied by secondary ion mass spectrometry (SIMS) and compared to that of the solid bulk. The results may lead to a better understanding of the physics of a vacuum discharge. EXPERIMENTAL

Two electrodes of the same matrix were sparked under vacuum (ca. 10e7 torr) in a JEOL JMS-Ol-BM2 double focusing spark source mass spectromevoltage, 3.2 kV; repetition ter. The spark parameters were: breakdown frequency, 3 kHz; pulse length, 20 psec. The breakdown voltage (U,,) is defined as the selected anode voltage multiplied by the chosen spark level on the automatic spark controller (100% = no sparking, 0% = short circuit). The effective voltage between the electrodes is - 15 times higher than this breakdown voltage. Sparked-off material was collected on a glass plate (2.0 x 0.5 cm2) placed 4 mm from the spark. The sparking time for each matrix was ca. 5-10 sec. The deposited material was studied with a scanning electron microscope (SEM) [48] using an electron energy of 20 keV and a beam current of 1 nA. Automated sizing of 500 particles was performed with a program [21] to

87

obtain particle size distributions of the deposited material on the glass plates. X-ray analysis of each particle allowed one to differentiate between particles originating from the electrode material and those present as impurities on the glass. In the data reduction two conditions were inserted. (a) The shape factor, SF = (perimeter)2/[411(area)] must be < 1.1: in this way aggregates and splashed drops are excluded. (b) The X-ray spectrum of the drop should contain a significant number of counts in the characteristic X-ray region of the matrix investigated. The remaining information was plotted in a histogram as the number of particles measured vs. the particle size (diameter of the sphere). The above measurements involved the metals: In, Sn, Zn, Al, Cu, Au, Ni, Ti, Co, MO, Ta, Re and W; the semiconductor Si and pressed graphite powder. For one standard reference material (steel NBS-SRM-661), a relatively thick layer was deposited on a glass plate after 10 h of sparking. The aim was to compare the composition of this layer with that of the solid bulk of the electrode. For this purpose secondary ion mass spectrometry (CAMECA IMS-300) was used. The standard and the sample were bombarded with a 5.5 keV 0: primary ion beam rastered over a square area measuring ca. 400 pm on one side and with a primary ion current of 0.1 PA, while oxygen gas was flooded over the sample surface through a bleed-in system in the immersion lens. Quantification of the results was based on the MISR method [22]. This method makes use of the different behaviour of 2 different matrix ions, to calibrate element sensitivities for differences in surface sampling conditions. For quantitative analysis, a series of sensitivity factors (SF) is generated as a function of the reactive gas adsorption on the sample surface by progressively admitting oxygen gas in the sample chamber. The actual surface sampling condition is then measured by the ratio of two matrix ions which react differently upon gas adsorption. We used 54Fe0 ‘/ 56Fe: as the environment sensitive ratio. A series of SF’s was generated for various elements, thus allowing quantitative analysis with an accuracy of 10-20s. RESULTS

AND DISCUSSION

SEM observation of sparked electrodes Various observations of sparked electrode surfaces by scanning electron microscopy in this [7,9,10] and other laboratories [12] revealed craters in molten and resolidified surfaces. This is shown in Fig. 1 for the matrices Pb, Ti and W sparked at the experimental conditions given above. These pictures indicate that electrode material is brought into the gap from a liquid surface layer. Observation at higher magnifications shows that the craters

Fig. 1. Scanning electron micrographs of sparked (b) Ti (TM = 1933 K); (c) W (TM = 3683 K).

electrode

surfaces.

(a) Pb (TM = 600.6 K);

are surrounded by microprotrusions which can, however, also be randomly distributed. Such microprotrusions, which cause a local field enhancement, are responsible for the field emission current during the discharge initiation

89

1231. The surface roughness is usually more pronounced for low-melting metals and at higher spark voltages and/or repetition frequencies. Also, the surface of low melting matrices such as In is much smoother when cooling the electrodes with liquid nitrogen [24]. According to Ramendik [25] the sample can be molten to a depth Xip in the course of one discharge of duration t,

JGp=

(Yd'2

where y is the thermal conductivity coefficient. At t, = 1 ,us, which is a realistic value [26], the liquid layer thickness may reach XiP 2 1 pm. The value may increase at high pulse repetition frequencies (e.g. 1-3 kHz, used in this work). Material

consumption

during sparking in relation to melting point

From our study on the material consumption in the spark [7-111, it was found that more material is transported into the gap for metals with lower

Vol/pulse

(pm7 pulse)

0

! 2.6

I

2.8

3.0

3.2

3.4

3.6

Fig. 2. Volume of material consumed per pulse (pm3 pulse-‘) vs. melting point (TM K) at frequency, 3 kHz; pulse width, 20 ps. fixed spark parameters: U,,,, 3.2 kV; repetition - - -, Factor 2 from the full straight line, calculated with the method of least squares.

0

, 1.5

20

25

3.0

3.5

4.0

45

Ubr I kVi

Fig. 3. Volume of material consumed per pulse (pm3 pulse-‘) at various breakdown voltages U,, (kV) for a copper matrix. Fixed spark parameters: repetition frequency, 3 kHz; pulse width, 20 ps.

melting point, and this was also observed by Derzhiev et al. [27]. Our results are presented in Fig. 2 as the volume of material consumed per spark pulse vs. its melting point (constant spark parameters). At the given breakdown voltage, 10’2-10’4 atoms per pulse are transported into the gap depending on the element. As the breakdown voltage is increased, there is also an increase in material loss, as shown in Fig. 3 for a copper matrix. Measurements of the crater area and the gap width permitted the estimation of the critical particle density in the spark plasma (assumed to be cylindrical) and this was found to be typically in the range 10’8-10’9 atoms of the cme3, in agreement with literature data [28]. Typical dimensions diameter and height of the plasma filament are 40-300 pm and lo-120 pm

91

respectively, [7-111.

depending

SEA4 observation

on the material

of sparked-off

and

on the breakdown

voltage

material

If we assume that material is ejected into the gap as one liquid particle with a typical radius of l-5 pm [7-111, then it would be expected that such particles, after collection, can be observed by scanning electron microscopy. Figures 4(a) and (b) show some size histograms for the matrices Cu and MO and these are typical for all our observations. The distribution shows a high abundance of small particles and only a few particles of larger size in the expected range of l-5 pm. This observation is in agreement with experiments of Helmer and Walters [17] who performed similar studies in an argon atmosphere. There are several reasons for smaller particles to occur. The microparticle detached from the anode has to travel a certain distance through the electron beam (and plasma from the breakdown initiation [26]) before leaving the gap. When entering the interelectrode region, it is rapidly heated up because it has lost its thermal contact with the bulk and strong evaporation may occur. The larger its path length through the beam of field emitted electrons, the smaller its final dimensions. It should be mentioned that on the average the initial drop radius is - 25 times smaller than the crater (electron beam) radius [7]. Also, collision of the drops on the glass plate followed by splitting into smaller drops as well as explosion into the vacuum might be responsible for the occurrence of the smaller sizes. The smaller particles collected on the glass plate may also result from the condensation of atomized material during cooling, after plasma dispersal. Nevertheless, some particles are observed with rather large diameters; if these are the ones which have survived the effects mentioned, it seems logical to connect them with our results on weight loss measurements. In Figs. 5(a) and S(b) the radius of the largest drop r* measured by SEM is compared with the radius r obtained from material loss studies under the same experimental conditions. Figure 5(a) shows the results for various matrices sparked at fixed spark parameters, Fig 5(b) shows Y* and r for a copper matrix sparked at various values of U,,. A very satisfactory agreement is found: on the average r* - 0.8 r. (If the drops are able to escape from the interelectrode gap without being affected by electron bombardment, one would perhaps have found r* 2: r.) This observation suggests that the material is indeed brought into the gap as a drop of critical dimensions. Figure 6 shows secondary electron and X-ray images obtained for a molybdenum matrix: the large drops have nearly the same dimensions (r*) as those inferred from the weight loss measurements (r). If such a particle completely evaporates, it

92 number

-

70

-

(a)

60

30

20

10

1r* I 1.0

1.5

I

I

3.5

t

4.0

4.5

diameter

f pm)

number 50

(b)

40

30

20

IO

2r*

I

0

0.75

1.00

1.25

1.50 diameter

1.75 q.frnj

Fig. 4. Size distribution of particles collected near the spark, measured by SEM: (a) for a copper matrix; (b) for a molybdenum matrix. Fixed parameters: L’,,, 3.2 kV; repetition frequency, 3 kHz; pulse width, 20 ps.

93

0

r*(pml 3 .

1

2

3

r fuml Fig. 5. Radius r* (pm) of the largest drops measured by SEM vs. radius r(pm) of the drops calculated from material consumption (see Figs. 2 and 3). (a) For various matrices at fixed spark parameters: U,,, 3.2 kV; repetition frequency, 3 kHz; pulse width, 20 ps. Straight line calculated with the method of least squares for all elements excluding Sn and Zn. (b) For a copper matrix at various breakdown voltages (kV). Fixed spark parameters: repetition frequency, 3 kHz; pulse width, 20 ps. Straight line calculated with the method of least squares.

Fig. 6. Scanning electron micrographs of deposited metal drops collected when sparking 2 molybdenum electrodes. Fixed spark parameters: U,,,, 3.2 kV; repetition frequency, 3 kHz; pulse width, 20 ps. (a) and (b) Secondary electron images; (c) X-ray image of MO corresponding to the image in (b).

would produce enough vapour in the gap to reach the critical density to cause breakdown in the plasma filament N, = 1018 cmH3 necessary (assumed to be cylindrical).

95

.Fig. 7. Scanning electron micrograph image of deposited graphite pieces collected when sparking 2 graphite electrodes. Fixed spark parameters: U,,, 3.2 kV; repetition frequency, 3 kHz; pulse width, 20 ps.

In the case of graphite, SEM analysis revealed that solid pieces are collected instead of liquid drops. This is shown in Fig. 7. From the phase diagram of carbon [29] it can be predicted that at the given experimental conditions (low pressure), it is impossible to melt the surface. For this matrix a different mechanism is thus expected, perhaps similar to that proposed by Cranberg [l]. The different sparking behaviour of graphite was reported previously [9,10]: it is possible to obtain a stable spark for this matrix at values of U,,, for which metals with similar melting point would be difficult to spark. We will therefore not discuss further the behaviour of graphite. Solid-liquid transition and its relation to relative sensitivity coefficients in SSMS In some reports [20,30] it was shown that the composition of a metallic electrode surface after sparking was different from that of the solid bulk. Possible reasons for this surface modification were reported to be oxide layer formation [30-321, segregation [30-331, selective vaporization [34], fractional condensation [35], zone refining [10,20] and others. Relative sensitivities, which have plagued quantitative SSMS analysis, may therefore

96

be expected to be due, at least to some extent, to differences in surface and bulk composition. Which of the above processes is dominating may be dependent on experimental conditions such as pressure in the source region, spark parameters, discharge duration, electrode shape, electrode temperature, and various others, in particular, the chemical composition of the electrodes. When determining an impurity X relative to Y (internal standard) in a matrix Z, the relative sensitivity coefficient can be defined by the equation

2 =(gF)RSC($i, where I&/I; is the measured ion current ratio corrected for discriminations in the mass spectrometer (especially line width and detector mass response) and Cx/C, the true concentration ratio in the sample. In the case of a pure iron matrix, for instance, where Fe is used as the internal standard, Eq. (1) becomes RSC

i

2

Ii/I& 1Fe =

where Cx/Cr, i.e.

(2)

CX/CFe

is meant to be the impurity

concentration

in the solid bulk,

(cx/cFe)s-

For steel electrodes, we found that a process similar to zone refining may be important in connection with the RSC [lo]. In the zone melting method [36] impurities are displaced in the metal due to differences in solubility of these impurities in the solid and liquid phases. A partition coefficient K, is often used as a measure of the difficulty of eliminating an impurity from the solid phase. K, is the ratio of the concentration of an impurity X in the solid phase (Cx) s to that in the liquid state (Cx) 1.

The impurity is thus harder to remove from the solid phase if its value approaches unity and of course also if K, > 1. This coefficient is usually derived from binary phase diagrams in the case of rather pure metals (or semiconductors). Relevant literature data can be found in the compilations of Hansen [37], Elliot [38] and Shunk [39]. The relationship between K, and RSC (X/Fe),, can be derived in the following way (Cx/C,,

I&/Ii& Fe =

(cX/cFe_h

’

11

(4

(C~/CFe)s

If no other discriminations occur between element X and Fe after sampling the liquid phase (i.e. during evaporation of the liquid drop, and

97

during ionization of the vapor), then (Ii/r&)/( C,/C,,) since (C,,), 2: (C,,) s = 1 for pure iron, Eq. (4) reduces

Fe

_ (WI =K-’ CC,>, ’

1= 1. Furthermore,

to

(5)

Figure 8 shows experimentally determined RSC values from the literature [44] for an iron [40-431 plotted vs. K,’ values, tabulated by Chaudron value of 10 in zone melting actually indicates a strong matrix. A K,’ enrichment of the element of interest in the liquid phase, without real numerical meaning. It should also be mentioned that K, values derived from binary phase diagrams [37-391 sometimes deviate from Chaudron’s data by 50% and more, even if they were checked at very low impurity concentration. The RSC values of Van Hoye et al. [40] were obtained on a JEOL JMS-Ol-BM2 spark source mass spectrometer with electrical detection

RX 40

/ / / /

Fig. 8. Values of RSC (0, Van Hoye [40]; +, Ito [41]; TY,Oda [42]; 0, MC Crea [43]) vs. the reciprocal of the partition coefficient K, [44] for various elements in a steel matrix. - - -, Factor 2 from the full straight line, calculated with the method of least squares.

98

in the peak-switching mode and are corrected for discrimination effects occurring after ion production. The values of Ito et al. were also determined on a JEOL spark source mass spectrometer with electrical detection [41]. Mass spectrometric measurements were started after at least several minutes of presparking. If the assumption is made that thermodynamic equilibrium between liquid and solid phases is reached, one should still realize that the effective equilibrium constant Kerr actually depends on temperature and impurity concentration, and may also be affected by interactions between impurities (either in th e 1iquid or in the solid). Even if there is equilibrium at the solid-liquid interface (Ki = K,), Kerr varies with the melting rate [45]. In general, however, no equilibrium is reached at the solid/liquid interface, e.g. because of adsorption phenomena or for kinetic reasons, so that K, # K,. For the fast processes taking place when sparking the electrodes, one does not expect to obtain equilibrium conditions. Also, the literature data on K, are often in error (especialy the solidus data) because equilibrium between solid and liquid is difficult to achieve. In spite of these considerations, a remarkable agreement between RSC and K,’ is observed in Fig. 8, indicating that processes such as zone refining must be important here. A typical diffusion coefficient in the liquid phase is lo-’ cm2 s-l [46] which means that for times of 1O-6 s the diffusion length of an element in the liquid phase may be of the order of 50 nm. These values seem realistic; however, there are no numerical data available to reliably estimate the thickness of this molten layer, nor the time it stays molten. On the other hand, it is realized that convection processes can be caused by temperature, concentration and surface tension gradients, by electric forces pulling drops out of the liquified anode surface and also by electromagnetic stirring caused by a passage of current through the molten phase, implying that mass transfer may be more efficient than by diffusion processes alone. A similar log-log plot [RSC (X/Cu),-U vs. K,‘] can be made for the matrix Cu (Fig. 9). Again, a reasonable correlation is found, although, for high values of K,-r the RSC values tend to be numerically lower than K,‘. The RSC values suggest that enrichment of the element in the liquid phase sampled by the spark, does occur but to a lesser extent than expected from equilibrium conditions. RSC values < 1 are not observed, even if K,’ < 1. In the case of aluminium, RSC (X/Al),, is independent of K,’ [lo] suggesting that phenomena other than solid-liquid transition are more important for this matrix. Nevertheless, we would like to stress that, for all three matrices investigated, differences in RSC values are reflected by differences in surface composition of sparked electrodes relative to bulk composition [10,20].

99

20

-

RSC / / / /

lo-

Fig. 9. Values of RSC (0, Van Hoye [47]; *, Ito [41]). vs. the reciprocal of the partition coefficient K, [44] for various elements in a copper matrix. - - -, Factor 2 from the full straight line, calculated with the method of least squares.

Chemical

composition

of sparked-off

material

Further evidence for the deviation of the plasma composition from that of the solid bulk of the electrodes was found by analysing the eroded material. This was done for a steel standard reference material NBS-SRM-661, a thick (pm) layer of whi c h was deposited on a glass plate placed near the spark. For quantitative analysis by SIMS we used the MISR method [22]. It was found that the deposited material is of quite different composition than the as the ratio of the layer to the bulk signal solid bulk. We define (F,),,, intensity measured for impurity X and (F,),,, as the ratio of the electrode surface to bulk signal intensity (obtained from [10,20]), all normalised to the respective signals of Fe+, thus

(6)

100 TABLE

1

Values of ( Fx 1,_,a and ( Fx 1s/a obtained kHz, 20 ps

by SIMS, and of RSC for steel sparked

Element

tFX)L,B

(C&/B

V Cr Mn co Ni cu As Zr Nb Fe

2.0 0.44 1.3 15.0 0.57 7.1 2.8 3.3 4.6 1

1.5 1.5 1.1 0.95 1.7 2.9 2.1 2.4 1

at 3.2 kV, 3

1.6 1.7 2.7 0.86 0.84 1.6 3.0 1.9 1.6 1

and (see ref. 10, 20)

(7) The results are given in Table 1 for the elements V, Cr, Mn, Co, Ni, Cu, As, Zr and Nb. For comparison, RSC (X/Fe),, values, also corrected for discriminations occurring after ion production in the spark, are given [40]. As was argued in detail elsewhere [10,20] the RSC values (column 4) reflect quite well the modified surface composition of sparked electrodes (column 3). The composition of the sparked-off material certainly differs from the original composition (column 2), indicating that elemental discriminations occur (see below). Some (F,) L,B values agree with those of ( Fx)s,B, and but for other elements, e.g. Co, Cu, Cr, there is a thus of RSC (X/Fe),,, great discrepancy (more than a factor of 2, up to a factor of 15) When comparing the data in columns 2 and 3, one should realize what information data are influenced by the solid-liquid transition they contain: the (F,),,, and other phenomena specified in the last section taking place at the electrode surface. The ( F,) L,B data, however, include other effects superimposed on the previous ones: the deposited layer of sparked-off material may indeed contain liquid drops directly originating from the sampling of the liquid electrode surface, but also much finer particles [Fig. 4(a) and (b)] from condensation of atomized material and of atomized, ionized, recombined and finally neutralized material. Because of the condensation contributions to the (F,) L,B value, there is also a large deviation from the RSC value which does not contain the effect of condensation. In order to get a better agreement between columns 2 and 3, and thus also with column 4, it

101

would probably be necessary instead of analysing by SIMS a large area of the deposited layer, to study separate drops and select only the larger ones, with radius r* [Fig. 5(a) and (b)], which are thought to be derived directly necessitate from the liquified electrode surface. This would, however, quantitative analysis with an ion microprobe, which is not a trivial task. Quantitative analysis with the less sensitive electron microprobe would require one to study alloys instead of pure matrices. Studies on changes with time have not been made so far, neither of the electrode surface, nor of material collected near the spark. It would certainly be of interest to check whether their compositions reach equilibrium (steady state) or change steadily with time. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

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