photographs; and Barbara Agule for preparing the samples. LITERATURE CITED
(1) Borchardt, H. J., Daniels, F., J . Phys. Chem. 61, 917 (1957). (2) Holtzberg, F., Reisrnan, A., Berry,
M., Berkenblit, M., Ibid., 79, 2039 (1957). (3) Reisman, A., J. A m . Chem. SOC.81, 807 (1959). (4) Reisman, A , Holtzberg, F., Zbid., 80, 6503 (1958). (5) Reisman, A., Holtaberg, F., J . Phys. Chem. 64, 748 (1960). (6) Reisman, A., Holtzberg, F., Banks,
E., J . Am. Chem. Xoc. 80, 37 (1958). (7) Reisman, A., Karlak, J., Ibid., 80, 6500 (1958). RECEIVEDfor review May 23, 1960 Accepted August 10, 1960. Presented in part, Division of Analytical Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960.
Applications of Zone Melting to Analytical Chemistry W. G. PFANN and H. C. THEUERER Bell Telephone Laborafories, Inc., Murray Hill,
b Some aspects of zone melting, and controlled freezing in general, which may be of interest to analytical chemists are discussed. Topics include: concentration of trace impurities to detectable levels, provision of highpurity standards, microtechniques, direct analysis by zone melting, and phase diagram work. A laboratory zone refiner can be a useful analytical aid.
T
this paper is to answer questions about zone melting which may arise in the minds of analytical chemists. Is i t something we should know about? Can it bc useful to us? Does it pose any problems for us? Th(. answer to all three, illustrated by examples in this Faper, is “Yes.” First of all it is appropriate to define zone melting and describe its relation t o other techniques. Zone melting is the genwic name for a family of techniques of physical separation based on the scgrcgation of impurities during freezing. All these techniques have in common the establishnient of a short melted region, the molten zone, which slowly travels through a relatively long charge of solid. In zone refining, the particular technique of greatest intermt to analytical chemists, a sequence of molten zones is passed through the charge in one direction. Impurities that lower the freezing point of the parent material travel with the zones and become concentrated near the end of the charge. Impurities that raise the freezing point travel in a direction opposite to the zones and become coneentratcd a t the beginning of thc charge. The number of zone passes is commonly of the ordcr of ten but may range from two to sevrral hundred. Since the backward-moving impurities-those nliich raise the freezing point-travel backward only one zone lcngth per pass, it is common for such impurities to use a t icast as many passes as there are zone lengths in the charge. The distribution coefficient, k , drtermines how effectivcly a giwn iniHE hlAIN PURPOSE O f
1574 *
ANALYTICAL CHEMISTRY
N. J.
purity is concentrated. k is the ratio of the impurity Concentration in the just-freezing solid, to that in the main body of the zone. The impurity travels with the zone if k is less than unity, opposite the zone if k is greater than unity. The equilibrium value of k , designated ko, is the ratio of solidus to liquidus concentrations a t a given temperature. The effective value of k always lies between & and unity. If the rate of zone travel is small, and the zone is well mixed, k will approach ko. If the rate of travel is large, k will approach unity. A typical average rate is 1 cm. per hour, although in favorable circumstances i t may be as high as 20 cm. per hour. An impression of the effectiveness of zone refining can be obtained from the calculated curves of Figure 1, A , R, and C, which are for k’s of 0.2, 0.5, and 0.7, respectively (21). The maximum attainablr separation, for the ratio of ingot length to zone lcngth pertinent to thrse figures, is shonn by the dashed straight lines. What is the relation of zone refining to fractional crystallization” The latter technique has been used for many, many years, particularly by chemists. Crystallization from solvents has been more common than crystallization from the melt. A fairly well known example of the latter technique, although by no means the earliest, is the purification of benzoic acid for use as an analytical standard by Schwab and Wichers (27). Freezing from the melt avoids the possibility of contamination from a solvent; it also minimizes, when properly performed, the entrapment of mother liquor in the frozen material. The difficulty with either of these techniques of fractional crystallizatioiifrom melt or solvent-is the time and labor involved in separating and recombining fractions when repenttd crystallizations are necessary. The valut. of the zone rcfining ttchniyue lies lorircly 111 its elimination of t h e w tediou\ operations Nc controlls t i freezing of :in twtircb mclt.
-
termed normal freezing (ZO), has virtues of its own, and, in this paper, uses of both techniques are considered. Calculated normal freezing curves for various values of k are shown in Figure 2 . PROVISION
OF
HIGH-PURITY STANDARDS
There has aluays been a need for materials of the highest possible purity, for use as analytical and physical standards, for studies of basic material properties. and for use in the synthesis of more complex materials. Zone refining is playing, and certainly will continue to play, an important role as a source of such standards. For example, the semiconducting elements, germanium and silicon, metals such as tin, lead, aluminum, zinc, bismuth, and antimony all have been brought to their highest purity by zone refining. The floating zone technique, using either induction heating or electron bombardment, gives excellent promise of similar results for high-melting metals _such as molybdenum, tungsten, platinum, palladium, rhenium, and niobium. Zone refined tin and lead are the most accurate of various high purity samples as melting-point temperature standards (16). Recently, zone refined aluminum has been used in the precise determination of the density of aluminum (30). Sometimes the advent of these extremely pure metals has been accompanied by unusual properties. For example : Zone refined aluminum cannot be worked hardened a t room temperature. It rrcrystallizes almost immediately (2). Castings of zone refined lead exhibit a preferred orientation texture entirely different from that of lead slightly less pure (35). The rate of grain growth of zone refined lead after plastic deformntion is extremely rapid, but is reduced a thousand-fold b:. the aresence of l o p 3 yGof silver (3). Zone refined iron exhibits a vanish-
I
'0.5 1
9
,
I
95
'.a
1.5
2.0
2.5
3.0
3.5
~
4.0
, 4.5
A
' B 50
0
D STANCE lhi ZONE L E h G T H S , X / l
I
2
Figure 1 . Relative solute concentration, (C/C,),vs. distance in zone lengths, x / l , from beginning of charge, for various zone numbers of passes, n. 1 denotes charge length A. 8.
k = 0.2
k = 0.5 C. k = 0.7
3
5 6 7 DISTANCE IN ZCNE LENGTHS, X / L 4
8
9
IO
carbazole, and 3-nicthyl pyridine; and they note that these could not readily be treated by distillation. Schildknecht and coworkers, in Gtrniany, have published extensively on the zone refining of organic compounds (11, 25). Pauly and cow orkers in France have described the zone mc.lting of inorganic salts, including cryohydrates (18, IO). Higher melting inorganic compounds pose the problem of finding a suitable container. Neverthelcss, Grundig ( 9 ) , in Germany, has reduced the contcnt of divalent cations in KBr and KC1 to less than 10-670. Eeynon and Saunders (e), in England, dcscrihc several zone refiners iuitahle for organic chrinicals and list n-any rhernicds that have been succc,ssfully trc,atetl. CONCENTRATION OF TRACE IMPURITIES
Both nornial freezing and zone refining can be very effective in building up the concentrations of trace impurities to a detectable level. as a glance at Figures 1 and 2 \\ill show. Recent examples of the use of normal freezing are the work of Matthews and Coggeshall ( 1 7 ) on benzene and the nork of Schildknecht, Rauch, and Schlegelmilch (24). With the aid of zone refining certain critical impurities in Chile copper used for copper-oxide rectifiers have been identified by mass spectrosCOPS ( 1 ) . In principle, one can assume a value of k and, using either of these techniques, calculate, from curves like tliose above, the degree of enrichment of an impurity in a certain end region of the chargp. In practice, this procidure is risky, however, because tile distrihution coefficient depends on the VOL. 32, NO. 12, NOVEMBER 1960
1575
Y I
0.02.
h=o.ol 0.01
4
\
/’
10-5
I 2
I 1
1
0
1 3
I
4 DISTANCE I N INCHES
1 5
1 6
I 7
Figure 3. Electrical conductivity vs. distance from beginning of rod for a silicon single crystal after various numbers of floating zone passes; n and p denote conductivity type
freezing conditions. A more reliable procedure is to establish the degree of enrichment by experiment, with known samples, and then to use the same experimental conditions on the unknown. Even then one has to consider possible effects of volatilization, absorption in the oxide coating and the possible effect of other impurities on the segregation of the impurity of interest. Despite these limitations, controlled freezing can be a valuable and reliable technique for concentrating impurities, especially when a series of more or less similar samples is to be analyzed. HANDLING SMALL AMOUNTS
OF
MATERIAL
A feature of zone refining which should appeal to analytical chemists is its applicability to very small samples. It has not yet been shown just how far one can go in this direction. However, a micro-zone refiner has been described by Handley and Herington (IO). An even tinier apparatus, described by Schildknecht and Vetter (16)) consists of a n axial array of tiny copperwire coils which forms a tube through which the sample in its container is drawn. Alternate coils are connected to a common heat source, the remainder to a common heat sink. Using this apparatus they obtained, from a 28-mg. sample containing 80% C&K,OH and 20% CmHuOH, a 10-mg. fraction of They used a extremely pure C&ssOH. combination of zone refining and fractionation (although, in the authors’ opinion, the same result could have been obtained with less trouble by zone refining alone).
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ANALYTICAL CHEMISTRY
It remains a challenge to find just how small a sample can be zone refined. As the zone becomes smaller, convection ceases to be a mechanism for the needed mixing of the liquid. However, at very small zone lengths diffusion becomes increasingly effective. The diffusion-limited rate, R, of zone travel is N DIL, where D is dsusivity and L is zone length. For D i Z3 X sq. cm. per second, R is 1 cm. per hour for a zone 1 mm. long, and 10 cm. per hour for a zone 0.1 mm. long, which are certainly practical rates. It has even been suggested that one might profitably apply micro-zone melting to frozen biological substances. N
-
DETERMINATION OF BORON AND PHOSPHORUS IN SILICON
Years of effort, by many investigators, have been expended in the search for ultra-pure silicon. I n so far as the needs of the semiconductor industry are concerned, the problem has been solved. Silicon containing less than 0.1 p.p.b. of all impurities known to affect the behavior of transistors or diodes has been produced. Most silicon devices now on the market can tolerate a much higher level of impurity. This rather remarkable achievement was beset with many difficulties. The impurity levels were far below the limits of sensitivity of conventional techniques. Therefore, certain basic semiconducting properties-namely, electrical conductivity and excess carrier lifetime-were used as analytical tools. Single crystals were required for these measurements, but single crystals grown from the best available crucible material, fused silica, became contaminated with boron, oxygen, and other impuri-
ties. Hence, the effects of changes in the sequence of purification steps designed to improve the purity of the silicon often were masked by contamination during crystal growth. This led to intensive development of the floating zone technique for the preparation of single crystals of silicon (8, 13, 33). Its advantages were several. First, as a zone refining technique, it could be used to remove by segregation almost all of the impurities in high-grade silicon. Boron, the only major exception, was not readily concentrated because its distribution coefficient was near unity. However, it was demonstrated that boron could be converted to a volatile oxide during the floating zone process by introducing water vapor (33) into the ambient gas. Second, as a method of preparing a contamination-free single crystal, the floating zone technique became a valuable research tool for the study of silicon purification methods and for the study of the effects of minute amounts of impurity on the bulk properties of silicon (34). Third, the floating zone technique was used to determine directly boron and phosphorus in silicon in the following way. Boron and phosphorus were known by experience to be the two major remaining impurities in transistor-grade silicon. Phosphorus is a donor; each atom contributes one conduction electron to the lattice. Boron is an acceptor; each atdm contributes one hole (or positive charge) to the lattice. The electrical conductivity is proportional to the concentration of either, when present alone. But when both are present, the conductivity is proportional to the difference in their concentrations. This
fact is of key importance; i t has been a stumbling block to early investigators, because one must know the absolute concentrations of each, not only the difference. Because boron and phosphorus have different values of k , their concentrations, a t a given point in the charge, Kill be reduced by different percentages, for any given number of zone passes. This can be seen from the curves in Figure 1. Therefore, if the electrical conductivity at some point is measured before and after some number of zone passes, and if the k-values are known for boron and phosphorus (which they are), the absolute concentrations of boron and phosphorus can be calculated from these two difference measurements. This technique has been used a t Bell Telephone Laboratories alniost on a routine baais as a n analytical tool in the study of silicon purification for a number of years. A typical set of data are plotted in Figure 3. After two floating zone passes, the conductivity was fairly high, being due primarily to phosphorus. -4fter 10 passes, enough phosphorus ( k = 0.4) traveled to the end of the rod to leave an excess of boron in the first 1*/2 inches. After 40 passes, the phosphorus was reduced to a negligible level, and the remaining p-type conductivity is attributable esstmtially to boron ( k = 0.8). I n this example, calculations show the original concentrations to be about 5 X 10-9 atom yo boron and about 9 X 10-8 atom 70phosphorus. PHASE DIAGRAM STUDIES
Space does not permit a proper account of the effort in recent years on tlie use of carefully controlled freezing operations in determining portions of phase diagrams. It is paradoxical that a process n-liich produces a decidedly nonequilibriuin condition, the segregation of solutes, can be used to determine phase equilibria. However, while equilibrium is not achieved between the solid and the liquid as a wholebccause of the extremely low rate of diffusion i n the solid-equilibrium does appear to be closely approached between the liquid and solid within a few atomic distances of the freezing interface. Therefore, if the liquidus curve is known, the solidus can be determined by a point by point analysis of a normally frozm crystal. This technique has been used to detcwnine the usually very small solid solubilities of donors and acceptors in germanium and silicon. A review by Trurribore ( S T ) summarizes much of this work. Recently, Tiller (36) has advocated the extension of this niethod to the study of multicomponent systems, and has shown that the liquidus curve, as wrll, can be determined during the
normal freezing operation by resting a thermocouple on the advancing interface. Zone melting has been applied profitably to the determination of binary and higher-order eutectic compositions by Yue and Clark (39). The basis of the method is that continued zone refining of a system, which a t equilibrium consists of primary plus a eutectic, causes an accumulation of pure eutectic at the end of the charge. The same might in principle be done by normal freezing. However, lack of ideality in freezing conditions (dendrite formation, entrapment of liquid) can easily lead to erroneous results. The repeated passes of a zone refining operation, however, ensure that, even for rather nonideal freezing conditions, the end of the charge will be pure eutectic, which can then be analyzed. Another use of controlled freezing is as an aid in determining disputed points on a phase diagram. For example, by conventional thermal and analytical methods, it is sonictimes difficult to determine whether there exists a eutectic or a peritcctic point, if the point is very close in temperature to the melting point of one of the major components. By subjecting alloys near the point in question to normal freezing or zone refining, onc can detcrmine from the direction of segregation 1%hether the slope of tlie liquidus is negative or positivc, MrHugh and Tiller (15) used this technique to demonstrate the existence of a eutectic a t 49.85% T e in the Ge-Te system. A recent Russian paper (SR) claims to have shown a eutectic a t the Pb end of the Pb-Ca system, using zone refining as a segregation method, whereas the existing phase diagram (26) shows a peritectic. -4 repetition of Russian experiments (6), hoblwer, failcd to confirm their conclusion and did confirm the existence of a peritectic. Presumably their analytical niethod or tht, lack of purity of their materials led to an erroneous result. DISCUSSION
The examplm givcn above show that zone refining, and controlled freezing in general, can be helpful to analytical chemists in several ways. A zone refiner suitable for most chemicals can easily be built from materials available in the laboratory or can be purchased at a moderate price. Hence, it can easily become a useful analytical aid. A comprehensive book on zone melting is available (21). Substances that are liquid at room temperature can also be zone refined. Refrigeration of the solid is required, but otherwise there is no difference in the principle of operation. Rock ( 2 2 ) and Schildknecht and Vetter (26) have described such apparatus.
Of course, the advent of purer and purer materials, made by other techniques as well as by zone refining, aggravates the problem of analysis. Electrical resistivity measurements (14) have been widely used for metals; also activation analysis (32). For semiconductors and halides (9), fundamental electrical properties of the material have proved to be the most sensitive. I n silicon, the infrared absorption peak of oxygen has been turned from a scientific discovery to a working tool in a few years (12). The trend is clearly toward physical methods of measurement. LITERATURE CITED
(1) Ahearn, A. J., private communication. (2) Albert, P., Lehericy, J., Compt. rend. 242, 1612 flQ.56). (3) Aust, Met. Soc. A I M E 218, 50 (1960). (4) Ball, J. S., Helm, R. V., Ferrin, C. R.. Petrol. Enar. 30, No. 13, C36 (1958)’. (5) Benyon, J. H., Saunders, R. A., Brit. J . A p p l . Phys. 1 1 , 128 (1960). (6) Bouton, G. hI., private communica-
tion.
(7) Burstein, E., Phys. Rev. 93, G32 (1954). (8) Erneis, R., Z. A’aturforsch. 9a, 67 (1954). (9) Grundig, H., 2. Physik 158, 596 (1960). (10) Handley, R., Herington, E. F. G., Chem. & Ind. (London), 1956, NO. 16, p. 304; 1957, p; 1184. (11) Hesse, G., Schildknecht, H., Angew. Chem. 68, 641 (1956). (12) Kaiser, W.,Keck, P. H., J . Appl. Phys. 28, 882 (1957). (13) Keck, P. H., Golay, >I. J. E., Phys. Rev. 89, 1997 (1953). (14) Kunzler, J. E., Wernick, J. H., 212,881 (1958). (15) SlcHueh. J. P.. Tiller, IT. A., Ibid.. 218, 187 fi960). ’ (16) McLaren, E. H., Murdock, E. G., Can. J . Phys. 38, 100, 577 (1960). (17) Matthew, J. S., Coggeshall, N. D., ANAL.CHEM.31,1124 (1959). (18) Pauly, J., Sue, P., Compt. rend. 244, 1505 (1957). (19) Ibid., p. 2722. (20) Pfann, W. G., Trans. A m . Inst. Mining Met. Engrs. 194, 747 (1952). “Zone Melting,” (21) Pfann, W. G Appendix, W h y , fiew York, 1958. (22) Rock, H., fiaturwissenschaften 43, 81 (1956). (23) Schildknecht, H., 2. Naturforsch. 12b, 23 (1957). (24) Schildknecht, H., Rauch, G., Schlegelmilch, F., Chem. 2.-Chem. Apparatus 83, 549 (1959). (25) Schildknecht, H., Vetter, H., Angew. Chem. 71, 723 (1959). (26) Schumacher, E. E., Bouton, G. M., Metals & Alloys 1 , 405 (1930); Metals Handbook, p. 1186, Am. SOC.Metals, 1948. (27) Schwab, F. W., Richers, E., J. Research h’atl. Bur. Standards 32, 253 (1944). (28) Sifferlen, R., Compt. rend. 244, (9), 1192 (1957). (29j Smith, R. L., Rutherford, J. L., l r a n s . A m . Inst. Mining Met. Engrs. 209,478 (195.7). (30) Straumanm, M. E., Ejima, T., J. Chem. Phys. 32, 629 (1960). (31) Tanenbaum, hl., Briggs, H. B., \
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VOL. 32, NO. 12, NOVEMBER 1960
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Phys. Rev. 90, 153 (1953). (32) Tailor, I’. I., Havens, W. \V,-Jr., “Physical hlethods in Analytical Chemistry,” Vol. 3, pp. 539-601, Academic Press, Yew York, 1956. (33) Theuerer, H. C., Trans. A m . Inst. iMznzng Met. Enyrs. 206,1316 (1956). (34) Theuerer, H. C , Whelm, J. AT., Bridgers, H. E., Buchler, E., J . Electro-
chem. SOC.104, 721 (1957). (35) Tiller, W. A., Trans. A m . Inst. Mining Met. Enyrs. 209, 847 11957). (36) Tiller, W. A,, Trans. Met. SOC.A I J f E 215,555 (1959). (37) Trumbore, F. A,, Bell System Tech. J . 39, 205 (1960). (38) Vigdorovich, V. G., Nashel’skii, A.
Ya., Russ. J . Inory. Chem. 4 (9), 922 (1969). (39) Yue, A. S., Clark, J. B., Trans. M e t . SOC.A I M E , to be published. RECEIVEDfor review July 21, 1960. Accepted September 14, 1960. Division of Analytical Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960.
Zone Melting and DifferentiaI Therma I An a lysis of Some Organic Compounds MICHAEL J. JONCICH’ and DELLA RUTH BAILEY2 Department o f Chemistry, University o f Tennessee, Knoxville, Tenn.
b Techniques of zone melting were applied to systems of organic compounds using a simple apparatus which allowed the passage of 18 molten zones through the sample during a single run. Spectrophotometric analysis indicated that the methyl violet concentration in naphthalene could be lowered to 1 p.p.m. by a single passage through the 18-zone column. Similarly, combustion of samples followed by counting of the CO? in an ionization chamber showed that the concentration of carbon-1 4labeled naphthalene in benzoic acid could b e lowered to less than one part per 10 million using this zone refiner. A differential thermal analysis apparatus, using thermistors as the temperature sensing element, was constructed for use with organic compounds. Zone melting and differential thermal analysis were used on phenanthreneanthracene mixtures to determine the phase diagram of this two-component system. A tentative phose diagram is proposed based on these two types of measurements.
A
the majority of the work in zone melting has been in the fields of metallurgy and solid-state physics (fd), the technique may be considered an extension of the work of Schwab and Wichers (19) on the purification of benzoic acid and acetanilide. One of the techniques they used involved the lowering of a fused impure sample through a heating coil, allowing the freezing process to start a t the bottom of the tube and progress upwards as the sample emerged from the heater. The final liquid, containing a larger concentration of impurities, was then siphoned off. 1 Present address, National Science Foundation, Washington, D. C. 4 Present address, E. I. du Pont de Nemours &a Co., Aiken, S. C . LTHOUGH
1578
ANALYTICAL CHEMISTRY
Since the development of the zone melting technique by Pfann (23) in 1952, a number of organic compounds have been purified by the simple process of allowing one or more molten zones to pass through an impure sample (5-7, 25-18, 24). -4 large part of this work has involved the design and construction of zone refiners. I n tne testing of zone melting apparatus, usually only qualitative results were obtained. The authors were interested in carrying out quantitative experiments on organic systems using several methods of analysis, particularly in the possibility of zone refining equimolar mixtures of compounds and in using this technique for the determination of phase diagrams. This technique, therefore, coupled with results obtained from differential thermal analysis (DTA), was used to obtain a phase diagram for the phenanthrene-anthracene system. Z O N E MELTING APPARATUS
Although a number of rather elegant systems were constructed (fd),a simple apparatus gave best results. An 18-zone system was constructed by providing 18 heated regions and 19 cold regions (alternating hot and cold) surrounding a 20-mm. outer diameter borosilicate glass tube clamped in a vertical position. The heated regions were 14 mm. in length; the cold regions \vere 35 mm. long. T o provide the hot zones, aluminum disks (40-mm. outer diameter) with 20-mm. holes bored in the center to fit the column were used. Insulated Xichrome wire was wound on each flat side of each disk in thr form of a spiral and the Lvire attached to the disks If using asbestos and Sauercisen. the Xichrome wiring of all 18 aluminum disks was connected in series, then to a Variac power supply, the length of the molten zones did not remain constant, but fluctuated $18 the laboratory voltage fluctuated. In the extreme case, the molten zones became so narrow
(or so wide) that the liquid-solid interfaces moved in the wrong direction. To avoid this difficulty the two sides of the aluminum disks were wired separately and two separate circuits were used. The Nichrome wires a t the bottom end of each disk were connected in series, and finally to a Variac which was used to control the voltage. This formed the constant heater circuit. The wires a t the top of each disk were connected in series and were used as part of an intermittent circuit, controlled by a mercury thermoregulator placed a t the bottom of the column. I n this way the heating zones were essentially a t constant temperature and the widths of the molten zones were maintained constant during a run. Instead of using aluminum disk heaters around the thermoregulator, faster response of the thermoregulator was possible by wrapping Nichrome wire directly around the glass tube in this part of the system. The cold zones between the aluminum heaters were formed by coils of natercooled copper tubing which surrounded the glass tube. A continuous and rapid flow of cold water t.hrough the copper tubing provided sufficient cooling for all systems studied, although use of a refrigeration system may be desirable in some cases. A11 cold zones were in series and the copper tubing was looped to avoid close contact with the aluminum heating disks. During zone melting, the sample, placed in a tube of approximately 7 mm. outer diameter and usually 15 em. in length, was drawn up through the 20-mm. glass tube, causing the sample to pass through 18 heated regions. Glass twine attached to the top of the sample tube was connected to the shaft of a 1 r.p.m. electric clock motor, mounted above the large glass tube. The winding of the glass twine on the shaft of the motor caused the sample to be raised a t the rate of 2.5 rm. per hour. I n all work described here the column was mounted in a vertical position. If clamped in a horizontal position, huhblea occasionally formed which
