January, 1927
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Metallic Thorium' By J. W. M a r d e n and H. C . Rentschler RESEARCH LABORATORY, WESTINGHOUSE LAMPCo.,BLOOMFIELD, N. J.
The history of the preparation of t h o r i u m powders is used by BurgeP with some H E recent development herein reviewed and discussed. M e t h o d s commercially of radio tube filaments success and later patented by suitable f o r the preparation of t h o r i u m powder diKuzel and Wedekind." The activated with thorium rectly f r o m the oxide h a v e been described. A t t e n t i o n method was investigated by has made a study of the prepis particularly directed to t h e calcium-calcium chloride a r a t i o n a n d properties of Wedekind. l8 m e t h o d f o r the r e d u c t i o n of thorium oxide. T h e paper The calcium method is the this metal of special scientific describes t w o satisfactory methods f o r h e a t - t r e a t i n g i n t e r e s t . Although a numonly one which shows promise t h o r i u m . The physical properties of d u c t i l e thorium ber of attempts to produce of development to produce are describes a n d the r e s u l t s c o m p a r e d with those of thorium have been made. no thorium of high purity. The o t h e r investigatorsexperimenter seems to have difficulties of this method are: succeeded in producing it in 1-Incomplete reduction, due t o the chemical equilibrium such quantity and of such purity as to make it available and established between the calcium, thorium, and the oxides of the useful for scientific and commercial purposes. two metals. It is the object of the present paper to describe methods 2-The presence of impurities (silicon and iron particularly) yielding high-grade thorium powder, fused metal, and ductile in commercial calcium. 3-The building and maintaining of tightly sealed bombs. thorium metal.
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Preparation of Thorium Powder Historical
REDUCTIONOF HALIDEs-The preparation of thorium powder was first attempted in 1829 by Berzelius.'>*by causing sodium or potassium to act on thorium chloride. Chydenius12 Nilson13 Moissan and Honig~chmidt,~Matignon and Delephir~e,~ Von Bolton16 Meyer,' Karstens,* Lely and Hamburger19 and Arsemlo used various modifications of this method. Many difficulties were met in the attempt to reduce the tetrachloride with sodium. This was especially true if the powder must be free from oxide, carbide, and iron. Several of these difficulties were: 1-The instability of the chloride makes i t difficult to handle. 2-The difficulties in the preparation of the chloride are many, since gaseous chlorine, sulfur chloride, or carbon tetrachloride must be used but the final product must not contain sulfur or carbon. 3-It is a problem t o select a tube suitable for the distillation of thorium chloride a t red heat without the introduction of silicon and other objectionable impurities from materials with which the salt is in contact. 4-The reduction requires a perfectly tight bomb or a suitable atmosphere, with the entire exclusion of oxygen, nitrogen, and moisture. The commonly used substance is iron, and experiments have shown that considerable amounts of this impurity are introduced in such reductions if the bombs are not lined. The problem of making a really tight connection or a properly tight, ground joint t o the bomb which will remain vacuum-tight through a wide range of temperature changes and which can readily be opened after use is not easy, especially since sodium boils and begins t o generate pressure a t about 800" C. .5-A short exposure of thorium chloride to the air or a leak in the stopper of the bomb will often yield a thorium powder which contains as much as 20 per cent of oxide, which cannot be reduced by sodium. 6-This method of reduction yields a very finely divided powder, and great difficulty is experienced in washing away the by-products from the exceedingly active thorium. 7-The yield is only about 50 per cent of the calculated amount of thorium metal, a large proportion remaining unreduced and dissolving readily on washing.
REDUCTION OF THORIUM OXIDE-Various attempts have been made to reduce thorium oxide. Winklerl' used magnesium, Honigschmidtl* used silicon, and Troostl3 and Moissan and Etard14 used carbon and aluminum for this purpose. HuppertzI6 suggested the use of cdcium vapor, a method 1
Received July 22, 1926.
* Numbers in text refer to bibliography a t end of article.
4-The introduction of carbon as calcium carbonate, formed by exposure of the calcium to air. 5-The difficulty of proper regulation of temperature and other conditions t o get a sufficiently coarse powder t o avoid oxidation in washing.
ELECTROLYSIS-VOn WartenbergIg and Moissan and Honigschmidt4 attempted to electrolyze thorium chloride in fused potassium and sodium chlorides. ACETYLACETONATE METHOD-It is claimed that when sodium and thorium acetyl acetonate Th(CH&OCHCOCH& are distilled together into a red-hot tube, metallic thorium is deposited.13 It was found on trial, however, that the deposit consisted largely of thorium carbide or thorium containing much free carbon. Experimental
The experiments described in this paper have been made during the past five years with the following objects in mind: 1-To 2-To
try out methods described in the literature. recognize the best points of each. 3-To devise relatively simple processes which could be carried out by men only fairly well skilled in the art of making such products.
The sodium reduction of the chloride, the calcium reduction of the oxide, the electrolysis experiments, and even the carbon reductions of the oxides were carefully repeated. It became evident that radical changes must be made in the methods pursued by so many able experimenters over nearly one hundred years, if coherent ductile metal was to be made. In spite of 'claims for high purity by such investigators as Von Bolton and Lely and Hamburger, they did not succeed in getting ductile wire after heat treatment. REDUCTION OF THORIUMCHLORIDEWITH SODIUMAND WITH CALCIUM-It is practically impossible to produce a very high grade of thorium powder by the distillation of the tetrachloride and subsequent reduction with sodium. The product is very finely divided, and after washing with water and alcohol, it has a very light gray color characteristic of thorium containing a considerable amount of oxide. This is caused by oxidation of the thorium during the disintegration. To avoid the trouble met in the distillation of thorium chloride a method was devised which was afterwards found to be somewhat similar to the process of Chauvenet.20 It consisted in preparing the chloride without the necessary distillation in chlorine or in carbon tetrachloride or sulfur vapors, thus avoiding silicon impurities. The salt was
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reduced with sodium but the resulting product was as unsatisfactory as before. Far better results were obtained by reduction of the tetrachloride with calcium. The calcium was especially prepared for the experiments by the electrolysis method from C. P. calcium chloride from which the iron, aluminum, and silica had been carefully removed. The metal was very much purer than that on the market, analyzing about 0.01 to 0.02 per cent of combined iron and aluminum and less than that amount of silica. It was cut up to about 10 or 20 mesh by means of a coarse rasp or a milling machine. While it was very easy to cut calcium in cold winter weather, it was difficult to get good calcium on hot humid days. The calcium has to be cut in small lots and these frequently placed in stoppered bottles to prevent formation of calcium carbonate. This impurity produced carbide which caused difficulty when the metal was heat-treated.
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and any which stuck to the charge was therefore readily removed. When fairly coarse, the thorium powder can be treated in cold dilute nitric acid with no fear of oxidizing the thorium or dissolving it, so that it is easy to wash and handle. Further studies were made on the washing of the thorium powder to eliminate the products of oxidation which took place in handling the powder in the air. Immediately before use, the powder was put into a pure absolute alcohol solution of dried hydrogen chloride. This solution dissolved off any surface formations of thorium hydroxide or thorium oxide, which are readily soluble in dilute acid if not previously ignited. REDUCTIONOF THORIUMOXIDE WITH CALCIUMAKD CALCIUMCHLORIDE-This method was found the most satisfactory for the production of thorium. On many trials the product analyzed from 98 to nearly 100 per cent This carbide reaction is rather interesting since thorium carbide free metal. It could be made directly into ductile wire. is said to be decomposed by cold water. When much carbon is This method is therefore given in detail. Preparation of Materials. Chemically pure thorium nitrate present in the mass after reduction, a strong smell of acetylene is noticeable on disintegration with water, and after thorough was ignited very gently in a wide evaporating dish in an washing with water, a second evolution of acetylene is noticed electric furance, not over the open gas flame where carbon upon acidification of the wash liquors. If much carbon is present, samples will retain carbon, in spite of rigid acid washing, some- might be introduced. Thorium nitrate containing a little sulfate or sulfuric acid gives a compact powder on ignition. times to the extent of several tenths of 1 per cent, even after heat treatment. Samples having much carbon do not heatOn heating first to about 400' or 500" C., the pure nitrate treat satisfactorily. The effect is much the same as when molybpuffs up and forms a voluminous mass which afterwards denum or tungsten containing certain amounts of carbon are heatsinks down and becomes more compact. If the heating treated. A puffing and splattering takes place, spoiling the contour of the sample and making it quite unsatisfactory for me- in the porcelain dish is continued up to a bright red heat, chanical working. the thoria absorbs a small amount of silica which is very Best results were always obtained when a considerable undesirable. For this reason, after the puffing was over excess of calcium was mixed with the thorium chloride, and the nitrates practically destroyed, the powder was the excess being necessary to remove the gases which were placed in portions in platinum crucibles and the ignition enclosed in the bomb and to insure as nearly complete a continued to about 800" to 860" C. for 1 or 2 hours. The reduction as possible. A fairly satisfactory reduction took oxide was then pulverized to pass a 200-mesh sieve. The calcium chloride was heated for a long time to 450" C. place at a red heat, but the resulting thorium was not ento drive out all traces of moisture and afterwards kept i n tirely satisfactory for commercial use. REDUCTIOK OF THORIUM OXIDEWITH CALCIUM CHLORIDE sealed bottles till needed for use. The calcium was prepared from this chloride in cylindrical AKD SODIUM-It was thought possible to improve upon this method by using sodium and calcium chloride for the sticks, by electrolysis. The first piece of calcium made from reducing agent and thorium oxide instead of the tetra- a fresh batch of chloride was discarded, since the iron and chloride. It is well known that when sodium and calcium other impurities were concentrated in it. The metal for chloride are heated together in a closed container there is the reduction was prepared by filing away the surface of produced a certain amount of calcium, which should reduce several sticks with a coarse rasp to remove all the oxides thorium oxide. Therefore, if four gram-atomic weights and calcium chloride, fastening in a shaping machine, and of sodium, two gram-molecular weights of calcium chloride, cutting very rapidly into chips a t such a rate that several. and one gram-molecular weight of thorium oxide are heated pounds of calcium could be cut in an hour. This material together in a closed container, the calcium chloride should was kept in a stoppered bottle to prevent oxidation and be reduced to calcium metal, the chlorine going to the sodium, formation of carbonate. Construction and Preparation of Bomb. The bomb conand the calcium in turn reducing the thorium oxide, forming struction was modified from the usual type which consists calcium oxide and thorium according to the equation: of an iron cylinder with a screwed-in stopper. To avoid the Tho2 2CaClz 4Na = 4NaC1 2Ca0 Th leakage which sometimes occurs, the inside of the bomb Other metals such as chromium and zirconium are easily was machined to a 20-degree taper a t the top, and a plug was reduced by the sodium-calcium chloride combination and ground into this tapered joint with all the care used in grindvery pure powders are obtained. It was found that thorium ing a glass stopcock. Over the top of this, a screw cap was oxide could be reduced by this method. The powder ob- used to wedge the plug into place with considerable force. tained was very finely divided unless high bomb temperatures The bomb has a tendency to warp on use and after each were maintained. Another method, however, involving heat the stopper must be reground or reseated. It is unthe use of calcium and calcium chloride proved easier and doubtedly best to make one run in a bomb made from a is described more in detail. The percentage yield of metal drilled-out cold-rolled steel rod before its use for the proby the sodium-calcium chloride process is greater than with duction of the best grade of powder, because the iron conthe sodium reduction of the chloride, while the percentage tains small amounts of carbon and other impurities which of iron as impurity is reduced as a result of the absence of can be removed only by heat treatment, and also because free chlorine or hydrogen chloride from ThC14. the bomb is liable to warp and leak on its first trial. Experiments were made to avoid the introduction of iron It was very necessary that the bomb be cleaned and freed from the bomb, including an attempt to carry on the re- from rust by means of 1:l hydrochloric acid and water which action in an alundum crucible and finally in a bomb lined contained a small amount of acetic, formic, or almost any with pure, highly ignited calcium oxide to eliminate all other organic acid. Care was taken not to injure the ground carbonates. The calcium oxide was soluble in dilute acid joint.
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Charging and Heating of Bomb. A charge consisted of 1 jar for the disintegration of the sample and the elimination part of thorium oxide to 1 part of calcium chloride and of the excess calcium and the calcium oxychloride, which 0.75 part of calcium. The method was to weigh the thorium broke down in water into lime and calcium chloride. With oxide into a closed bottle, to weigh the calcium chloride out large samples good results were obtained with filtered water. separately, and pour that into the thorium oxide, and finally For a charge of about 1 pound of thorium and an equivalent to weigh out the calcium and pour the greater part of it into amount of calcium and calcium chloride about 20 liters the mixture. The bottle was then stoppered and shaken of water were necessary for the first washing and disintevigorously to produce a thorough mixture. The charge was gration, in order that the solution might not become heated then poured quickly into the bomb, and the small remaining and the thorium oxidized. The loosened charge was added portion of calcium dropped on top, and the wedge stopper to the water a little a t a time, with a stirrer throwing the put in place and turned gently to make tight connection. water vigorously so that the particles would not settle in The cap was then screwed down on the bomb, which was any one place, resulting in heating and oxidation of the placed in an electric furnace. A pyrometer was introduced. thorium powder. The importance of this vigorous agitation After the furnace temperature reached 950" C. it was main- cannot be emphasized too strongly. The solution was ,tained there to allow the bomb to reach the same temperature. stirred until no more gas developed, that is, until the calcium With equipment suitable for 1 pound of thorium, about 1.5 was all disintegrated. The stirrer was removed from the hours were required to raise the temperature to 950" C., solution, the insoluble material allowed to settle for 3 to where it was maintained for 30 minutes to 1 hour. The 5 minutes, and the supernatant liquid poured away. This bomb was lifted out of the furnace while bright red, allowed liquid was not clear but contained a considerable amount to stand in the air to cool below a dull red heat, and then of lime in suspension, which, of course, was discarded. A placed in an iron vessel containing running water. The second portion of 20 liters of water to a charge of this size water was kept below the edge of the screw cap so that the was added and vigorously stirred for 5 or 10 minutes. The threads were not loosened. The bomb was more easily residue was allowed to settle as before and the supernatant opened if cooled rapidly than if allowed to cool slowly in liquid again poured away. About four such washings the furnace. The bomb was not opened until it was cold were usually required to give a clear liquid. Above the to the hand, since calcium and thorium oxidize very readily insoluble material, the liquid was a little dark from a very when hot. Care was used in cleaning the outside of the small amount of extremely finely divided thorium. This bomb before removing the tapered stopper in order to avoid could safely be poured away since the amount of thorium contamination with iron scale which forms on the outside lost was very small and it was desirable to get rid of any during the heat treatment. exceedingly fine powder. The thorium powder thus preRemoval of Charge. The charge was chiseled out with long pared was heavy, and about 2 minutes settling was usually steel chisels similar to cold chisels. This charge consisted sufficient before each decantation. of a mixture of calcium chloride, calcium oxychloride, thorium, After the final washing with water, 2 liters of filtered and excess calcium. For best results, it has been found water were added to the residue, the stirrer started with desirable to use 25 to 50 per cent more than theoretical such vigor that the thorium powder and the water were quantity of calcium. thrown violently around on the inside of the container, and The calcium chloride in this mixture serves three purposes: 250 or 300 cc. of concentrated nitric acid were poured in very slowly. Evolution of acetylene occurred, indicating that I-It acts a s a flux which forms a more or less fluid medium in which there was in the starting products very little carbide, which t h e reaction takes place. was not decomposed by the first treatment with water. 2-It forms an oxychloride with the calcium oxide which is one of t h e products of reaction, thus removing i t and allowing the reaction to go to comHydrogen sulfide appeared here, resulting from a small pletion. amount of sulfates in the commercial thorium nitrate. 3-It allows the thorium, which is heavy, t o settle down, thus e5ectually No sulfur has ever been detected in the final product. sealing i t away from oxidizing influences a t the top of the bomb. After about 5 minutes of this vigorous washing, the solution The action of calcium chloride in bringing the reduction t o completion will be better understood by a consideration of the was diluted to 20 liters and allowed to settle after the resolvent action of CaCL on CaO. Arndt and Loewenstein21 moval of the stirrer, and the acid liquid was poured away. have found the solubility of CaO in CaClz t o be 16.28 grams in The acid washing was repeated three or four times, de100 grams of CaClz a t 802" t o 942" C. If a compound is formed pending on the gases which were evolved. If, for example, in this fusion it would correspond t o the formula Ca0.3CaClz. On the other hand GorgeanZzfound a compound of the formula there was any odor of hydrogen sulfide on a third washing, CaC12.CaO and MilikanZ3has found hydrated compounds from the acid treatment was repeated at least twice more. aqueous solution having the formula CaCl2.3CaO.xH~0. FurDrging of Thorium Powder. The sample was next washed ther, it may be t h a t the calcium chloride has some solvent action twice with 20-liter portions of filtered water to remove the on the thorium oxide with the formation of thorium tetrachloride which is readily reduced in the bomb. The result is the same acid, and the metal powder was filtered on a Biichner funnel, washed with alcohol and ether, and sucked dry with an whether the equation is written: aspirator. The powder was placed upon a 60-mesh sieve ThOz 2CaClz = ThCld 2Ca0 and, as it dried, brushed through the sieve and placed in a 2 C a 0 = 2CaO.CaClz 2CaClz (1) ThCla 2Ca = T h 2CaCl2 tube which was then exhausted and the remaining alcohol and ether pumped away. It is to be noted that powder ThOz 2CaClz 2Ca = T h 2CaO.CaClz made in this way would scarcely pass the 60-mesh sieve, or while samples made by most of the other processes readily Tho2 2Ca = T h 2Ca0 (2) passed through a 200-mesh sieve and were too finely divided 2 C a 0 + 2CaClz = 2CaO.CaClz and too active chemically for the present work. It might Tho2 2CaClz 2Ca = T h 2CaO.CaClz also be stated that after an hour of evacuation, one of these The formula CaO.CaCl2 has been used in writing these equasamples still contained 0.5 to 1 per cent of alcohol and other tions since experiments show t h a t a quantity of CaClz equivalent volatile material. For this reason the analyses given in to the calculated amount of CaO seems enough t o complete the this paper have been made on heat-treated thorium. reduction. Under the microscope the powder consisted of bright Disintegration and Washing. Before the cutting of the shiny particles with apparently clean surfaces but, on standing charge, filtered or distilled water was prepared in a large in the air, the surfaces became dimmed over with oxide
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coatings. Under good conditions no trouble was experienced in getting 90 per cent of the theoretical yield. Some thorium material soluble in water or dilute nitric acid remained, and a little very fine thorium which did not settle quickly enough for rapid decantation was lost. A magnet was passed through the powder to remove free iron and iron scale which might have got into the powder in cutting out from the bomb.
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FIRSTMETHOD O F HEAT TREATMENT-In all of the earlier experiments during the present work the heat-treating was done in the high-frequency induction f~rnace.2~Figure 1 presents a diagram of the equipment, which consists of a glass bulb fitted with a ground stopper which is sealed to a press through which pass leads to vacuum mercury spark gaps (l), condenser (2), and transformer arrangements (3), for the production of high-frequency currents. The coil Sintering, Fusing, and Working of Thorium is represented by (4), within which is a support of pure thorium oxide, upon which the samples were heated. The Metal tube ( 5 ) is connected to mercury diffusion pumps and the Historical spectrum tube (6) is connected to a transformer ( 7 ) which The method of Lely and H a m b ~ r g e rconsisted ,~ in pressing gave 3400 volts for the purpose of detecting the degree of bars 3 X 3 X 150 mm., sintering in an evacuated porcelain vacuum. The high-frequency current can be controlled tube, and after cooling, placing in a water-cooled treating by a rheostat or choke coil (8) in the primary of transformer bottle and heating nearly to the fusion point by direct (3). In some of the later experiments the high-frequency curpassage of the electric current. Unfortunately, these authors were unable to maintain a high vacuum during the rents were generated by using two 10-kilowatt, 3-electrode 5 s i n t e r i n g process a t 1100". oscillator tubes in a special apparatus constructed by this With high-grade thorium it is company. This apparatus had the advantages of being quite unnecessary to resort to noiseless, and of giving more steady heating and extremely any such device for satisfac- high temperatures when desired. Thorium Buttons. In the first experiments thorium was tory heat-treating. The cathode ray method of pressed into small cylindrical buttons about 0.5 inch in heat treatment for t h o r i u m diameter and 0.37 to 0.5 inch long. Pure thorium powder suggested by Tiede and Birn- is very soft and presses to form a remarkably strong, white, brauerZ4possesses many draw- metallic-appearing button, which need not be handled backs. The time required for with special care to avoid breaking. A satisfactory pressure operation is great, the size of for a button of this size is-about 6 or 7 tons. Thorium Oxide Crucibles. At first difficulty was met sample which can be handled is small, a n d t r e a t i n g i s in finding a substance on which thorium could be supported localized, so that the method and in which it could be melted without contamination. is n o t s u i t a b l e for the heat Although thorium oxide is at least partly reduced by calcium treatment of samples for later to form calcium oxide and thorium in a closed bomb, where a certain pressure of calcium vapor may be attained, this working. Since the metal must have reaction reverses in high vacuum. In other words, if thorium maximum density and be free was supported on lime and heated strongly in a high-vacuum of voids or pit holes in order furnace, thorium oxide was formed on the surface of the samto be adaptable for s u c h a ple and calcium vapor appeared. When magnesia was purpose as an x-ray target, the used with thorium, magnesium metal was vaporized. Contreatment described by Arsemlo tamination was also encountered when alundum was tried Figure 1 has not been repeated. Heat- as a support. For this reason a special process was devised for the proing of thorium in contact with magnesia in high vacuum duction of thorium oxide crucibles to be used for melting has led to serious contamination. Von Boltone" patented his well-known arc process for thorium and other metals. Thorium oxide seems to be the the heat treatment of thorium in which it was claimed most stable refractory and has proved entirely satisfactory. that metals could be fused in pure condition in a partially No contamination was observed, since even after melting, evacuated container. The difficulties of the method lie in thorium does not stick to thorium oxide as prepared for this the need of extremely pure gas, in order to maintain the arc work, but draws away from it. The method consists, and not contaminate the thorium, and in the fact that briefly, in suspending previously calcined thorium oxide in little control can be exercised over the rate of such a method water containing a certain percentage of cryolite. This of heating. Later, however, Von Bolton suggested another makes a fairly satisfactory suspension of thoria which can method0 which has been repeated and is briefly described be cast in plaster-of-Paris molds just as porcelain crucibles are cast. After casting, the crucibles are slowly and carebelow. fully dried and finally slowly fired up to nearly 2000" C. Experimental The crucibles are practically non-porous, are strong enough Thorium powder was poured into a copper tube and to be dropped sometimes from a height of several feet to the rolled down to small diameter. The copper-coated thorium floor without breaking and can be made of almost any size thread thus formed was brazed on to tungsten leads and suitable for the present kind of work. The pressed thorium buttons were usually placed upon sealed into a flask, tubulated a t both ends. Dilute nitric acid was drawn up into the flask t o dissolve the copper, but an inverted thoria crucible supported within the tungsten not high enough in the flask to attack the brazed joints. coil shown inside of the glass bulb a t (9) of Figure 1. For Since nitric acid (1:l)does not act appreciably upon thorium high temperatures it was essential that the coupling of the a ribbon of pressed powder could be obtained. The flask tungsten coil be as close as practicable to the sample. The was sealed to the vacuum pump, evacuated, and torched coil was made from tungsten wire, varying in size from 60 out, and finally a current was passed through the ribbon. to 100 mil. With a sample 0.5 inch in diameter a 60-mil The latter was not uniform in cross section and some diffi- coil of 9 to 12 turns wound on a 0.75-inch mandrel was quite culty was experienced in heat-treating it satisfactorily. satisfactory. The larger the diameter of the coil, the larger
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the size of wire needed, in order to avoid sagging of the tungsten when it became very hot. The number of turns of the coil depended also somewhat on the shape and size of the sample. h thin flat disk required a rather long coil in order to get good heating a t the center. When the sample was in place, the ground-glass stopper was inserted in the flask as shown in Figure 1, and the system connected to the mercury diffusion pumps. The stopper was held in place by atmospheric pressure. For the average experiment the container was a 12-in. Pyrex bulb, having at the lower end a 2- or 2.5-inch ground-glass stopper, lubricated with heavy stopcock grease. This joint was usually cooled by means of a blast of air and by wrapping with wet cloths. Degasijication. The heat-treating of samples of thorium a t low temperatures to remove the gases must be done with care. As has been stated, thorium, like many other metal powders, always contains adsorbed or absorbed gases, such as moisture, hydrogen] oxygen, nitrogen, carbon monoxide, etc., and these gases must be removed a t a low temperature so that the thorium will not interact with them chemically. The degasification of thorium practically all takes place below a red heat. Two methods of degasification were employed. The first was to heat the tungsten coil by passing the current from the 220-volt line directly through the wire and to allow the sample to warm up very gradually by radiation, while the mercury diffusion pumps were in full operation. If the temperature of the coil was gradually raised, the degasification could be accomplished without any spectrum discharge. A liquid-air trap was found desirable for the purpose of maintaining a very high vacuum. It is interesting to point out here t h a t thorium, especially if it is not compacted and if the surface of the metal particles are entirely clean, cannot be exposed to the atmosphere after degasification and before sintering without very serious contamination. Loose thorium powder was placed in a small flask which was sealed to the vacuum pumps, evacuated, and baked out a t 400" C. When the flask was opened after cooling t o room temperature, the thorium took fire. This was true even of very coarse thorium powder, but only true when the powder was fresh and clean on the surface. Apparently, when the surface is clean, sufficient oxidation OCCIUS on exposure t o the air, after degasification and cooling t o room temperature, to produce heat enough t o raise the powder to the kindling temperature. If the thorium powder is placed in a glass bulb thoroughly exhausted and slowly heated to about 400" C. so as to drive off the occluded gases and moisture, i t will be active toward certain gases beside oxygen. If hydrogen was admitted rapidly to the bulb, the hydride formed so violently t h a t the whole mass became red hot. The subject was investigated to a limited extent. It was found t h a t a sample weighing a few grams would clean up a liter or more of hydrogen at room temperature t o a few p pressure. It was calculated that if the hydride ThHd was formed, 6 grams of thorium should combine with about 1 liter of hydrogen (standard temperature and pressure).
It is also possible to effect the degasification of samples of - thorium in the high-frequency induction furnace by means of very slow regulation of the high-frequency induction currents passing through the coil. The furnttce gives a discharge between the turns of the coil if gas is present. This glow discharge is a very delicate means of detecting gas. When heat-treating the sample the operator immediately discontinues the induction heating when any considerable amount of glow discharge or arcing between the turns of the coils occurs. Very definite details with regard to the heat-treating of dzerent samples cannot be given, as the amount of gas varies in different samples. For a very coarse, very clean thorium powder, the process of degasiiication requires a relatively short time, perhaps 0.5 hour. A much longer time is required for very fine samples of thorium powder made by some of the older processes. The induction heating was continued as rapidly as the gas
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pressure would permit until the sample began to show red heat, after which very little gas developed. By means of the choke coil or rheostat, the high-frequency induction currents were increased until the sample had either melted or reached the desired temperature. There was no difficulty in melting thorium in this type of furnace. When a small sample was melted on the flat bottom of an inverted crucible, the metal did not run down but drew up into a more or less round bead and it could be maintained in the molten condition in the form of a bead on the flat thoria support. The thorium had to be cooled before removal from the furnace, lest it take fire or a t least be oxidized over the surface. When the support was removed from the furnace the metal had not made any contact with it. The thorium and the crucible did not stick together, and the portion of the metal which had been in contact with the support was bright and clean. It is not necessary to melt thorium in order to make it workable. If a button or slug of thorium is held just below t h e fusion p o i n t for a few minutes, it can be cold-rolled or hammered to almost any e x t e n t . By the process of heat treatment just described, s a m p l e s of t h o r i u m h a v e been made ranging in size from small b u t t o n s about 0.26 or 0.37 inch in diameter to disks over 2 inches i n d i a m e t e r . The mocess seems meciallv suitabie for the production i f disks, plates, cylindrical butFigure 2 tons, etc. OTHERMETHODOF HEATTREATMENT-Another method of heat treatment was devised for the specific purpose of making wire and slugs for inserts in x-ray targets. The furnace is shown in Figure 2. I n the earlier experiments bars 8 inches long, and ranging from smaller sizes up to 0.26 inch diameter (1) were placed in a 10-inch glass bulb (2) fitted with two hollow iron stoppers (3), which were tapered and ground into glass joints a t the ends of the bulb (4). They were made hollow for the purpose of water-cooling. The construction of the furnace is evident from the diagram, (5) representing mercury and (6) an iron clamp fastened to the thorium bar for the purpose of distributing the heat in the mercury. In operation, the upper stopper was removed, a thorium bar, which had been pressed from the powder, was clamped to it by simple screw clamps and the small iron clamp (6) fastened to the lower end of the bar. The upper stopper was then placed in the flask with the mercury level so adjusted that the iron clamp was immersed in it. Since thorium does not form an amalgam under these conditions, there is no objection to its contact with mercury. The flask was evacuated with the mercury diffusion pumps in the same way as before and the bars were subjected to slow and careful preliminary degasification. When the thorium is clean, there is no difficulty, as suggested by Lely and Hamburger, in getting the bar to conduct the current. The primary of the transformer (7) used in this circuit was connected with the 220-a. c. volt line and the secondary gave 11 volts. The pressed bars were excellent conductors of the current without preliminary heat treatment. The heating was continued until the bar melted off. In making rods for wire or insert targets,
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
the bars were treated for about 10 minutes after degasification a t 90 per cent of the fusion current. Lely and Hamburger state that thorium appears to vaporize much more readily than tungsten. I n a clean glass bulb several 0.25inch bars have been melted off without any apparent blackening from vaporization of the thorium. In making larger bars 0.62 and 0.76 inch in diameter, a water-cooled, metal treating bottle was used in which the bar could be placed between a special type of butt clamp for heat-treating, thus getting away from the loss of metal from the ends of the bar by use of the other types of clamps, COLD-WORKING-Thorium can be directly cold-worked like copper or soft iron. It hardens to some extent and, a t times, after long and rapid cold-working has a tendency to split along the length of the bar, perhaps the result of the separation of large grains which have been elongated during the working process. For the production of rods which may be sawed into pieces and machined into insert targets for x-ray tubes, it has been the practice to press a 0.62-inch hexagonal bar 8 inches long. After heat treatment this bar is swaged cold to the desired diameter to produce thorough compacting. For making small wire the best method has been to enclose heat-treated thorium slugs in soft iron and to roll this thorium under the iron coatings in grooved rolls to about 30 mil. This iron-sheathed thorium has then been drawn through dies down to very small diameter. Wire has been made as small as 0.3 mil in diameter and it is undoubtedly possible to produce even smaller sizes. The iron is dissolved away from the thorium with dilute nitric acid. Thorium is very pliable and may be bent. The wire may be ma'de into coils of almost any desired form. Thorium has been drawn without the protective sheath, but since it does not have a high tensile strength like tungsten, it is preferable to draw the thorium under some metal coating. Thorium which has become hard on cold-working can be made soft and ductile by a short heating to about 1000° C. in vacuum. There is no difficulty in drawing thorium coatings if vacuum annealing is frequently resorted to. Disk targets for x-ray tubes have been made more than 2 inches in diameter by heat-treating disks in the highfrequency induction furnace and cross-rolling them between 0at rollers until the desired thickness was obtained. This metal may be sawed and machined with great ease. Analysis and Examination of Thorium
There exists some confusion regarding the testing of supposedly pure thorium. The customary method of analysis is to burn the powder and determine the increases in weight due to oxidation, which amounts to only about 14 per cent for pure thorium. It is difficult to get the original weight on most samples of thorium powder owing to the fact that they often contain several per cent of volatile gases, such as hydrogen and moisture. When attempts are made to remove the gases from the fairly coarse powders produced in the present work by heating i n vacuo, they spontaneously take fire on reexposure to the air after cooling to room temperature. Most of the samples produced by previous experimenters have been prepared in iron bombs, where it is impossible to obtain iron-free powders. Iron increases in weight 43 per cent on ignition. I n the present work samples of very finely divided thorium powder containing about 3 per cent of adsorbed gas and moisture have been ignited to give an increase in weight corresponding to about 80 per cent free thorium, but after fusion these samples show at least 97 per cent metal, while one sample containing 5 per cent of oxide and 2 per cent of iron has given almost the theoretical increase in weight on oxidation and the analysis indicated
nearly 100 per cent purity. Iron is, however, removed to a large extent by proper heat treatment. As a result of this confusion, descriptions of metal powders given in the literature are often incorrect. For example, it is claimed that thorium powder has a very light gray color. Pure thorium powder, produced as described in the experimental part of this paper, has a dark, steel gray color. Many samples of heat-treated thorium have been examined during the course of this work and all samples prepared by the calcium-calcium chloride method and heat-treated in either of the two methods suggested above, will show better than 98 per cent metallic thorium. Samples have been obtained which analyze after heat treatment as high as 99.7 to 99.8 per cent thorium metal; that is, not total thorium but thorium as free metal uncombined with oxygen, assuming that the oxygen still remaining is combined as Thoz. This is a very high degree of purity when one considers that commercial aluminum rarely analyzes higher than 98.5 per cent. ANALYTICAL METHODS-In one method of analysis, dry, oxygen-free chlorine was passed over the sample, care being taken to have no compounds of carbon present. The thorium forms a chloride which may be distilled away a t a red heat, while the oxide remains unattacked. This method of analysis is difficult, because the chlorine must be freed from all traces of oxygen and other impurities except halogens. The method of purifying the chlorine consisted in condensing the gas, after removing the oxides of carbon, with liquid air, pumping the gases away, closing the chlorine container, melting the chlorine, freezing it out, again pumping it, and finally distilling it into another flask cooled with liquid air. This gave chlorine 01 sui%cient purity for the analysis, but the difficulties of handling this chlorine and keeping it in the laboratory were too great to be satisfactory for ordinary purposes. Sample Analysis by Chlorine Method Weight of oxide Per cent
Weight of sample Gram 0.6059 0.4288
Gram 0.0053 0.0048
oxide 0.88 1.12
The older methods, where a considerable amount of material is needed for the analysis, were also used. Samples were dissolved in acid and careful determinations made for iron, silica, heavy metals, etc. Separate samples of the same material were carefully cleaned and ignited to oxide. I n the later samples of good working thorium, there was no determinable quantity of silica and the iron never ran over 0.1 per cent. The oxide content of thorium was calculated from the increase in weight on oxidation, due allowance being made for iron or any other impurities found in the metal. The metal was also free of carbon as determined by the customary methods of analysis. Sample Analysis by Ignition to Oxide Increase in weight on burning Gram 0.2762 2.001~ 0.2751 Silica not determinable Iron 0.10 per cent \%'eight of sample Grams 2.0046
Calcd. increase Per cent Gram Th 0.2766 99.8 99.6 0.2762 Average 99.7
By far the easiest and most satisfactory method to determine the oxide content of thorium is by planimetric count. The microscopic examination has the further advantage of showing the presence or absence of voids or pit holes which are particularly undesirable in samples to be used for x-ray targets. If oxide particles are present they are to be seen as small irregular dots or specks of a gray color. Samples of thorium which contain iron or silicon will show
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January, 1927
a very hard white material, usually between the grains. When samples of thorium containing 2 per cent or less of oxide show a tendency toward brittleness, this white material is usually to be seen under the microscope. The methods of oxide count and the calculation of the percentage of oxide by this method are too well known to require discussion here.
Properties of Ductile Thorium The various physical properties of thoriuni are given below. The tables include the work done in this paper and previous measurements for comparison. It must be borne in mind that the physical properties of a metal vary with the amount of working to which it has been subjected. For example, specific gravity is increased by rolling and drawing. Snnealing lowers the specific resistdance. The values given are those which are correct for this metal a t some stage during its working. SPECIFICGRavITy--The specific gravity of thorium in the powdered and in the sintered or fused condition a t room temperature as determined by various investigators is as follows : Chydenius.2 1861 Nilson,a 1882, 1883 \'on Bolton,e 1908 \'on Wartenberg" Lely and Hamburger,# 1914 Authors, a t 22' C.
Sintered or fused
Powder 7,65 11.01 10.99 10.78 11,32
. . .. ..
12
is
... 11 6 11.2 . . Worked wire orrod 11.3-11.7 Max. calcd. density from x-ray measurements of Hull,?e 11.9. Since the density of T h o 2 is 10.2, even 10 or 20 per cent of oxide in thorium would not lower the density determinations appreciably. It is difficult to understand, however, how Von Bolton got the high value of 12.16. MELTINGPoIsT-The
melting points are given as follows:
c. Von Boltons Von Wartenberg19 Authors
1450 (in vacuo) 1745 1842 * 30 ( i n vacuo)
(?A
The melting point has been determined in the present work in the high-frequency induction furnace by a method which allows the determination of black body temperatures. A cylindrical button of thorium powder (diameter about 0.5 inch, height of 0.37 to 0.5 inch) was heat-treated nearly to the melting point of thorium as described under "Sintering and Fusing." After thorough cooling, the button was removed from the furnace and a small hole drilled in its center from one of the flat ends, the hole having a diameter of about 0.062 inch and a depth of about 0.25 inch. This button was replaced in the furnace and an optical pyrometer sighted so that readings could be made from the bottom of the hole. This gives a much higher value than readings made on the outside edges of the button. The pyrometer was carefully and repeatedly checked against standard lamps calibrated by the Bureau of Standards, in terms of true temperature. The average value of several determinations yielded 1842" C. * 30". SPECIFICRESISTAxcE-The specific resistance given by Von Bolton was 40 X 10+ ohms per cubic Centimeter, while the present samples of thorium at room temperature (20" C.) varied from 13 to 18 X 10+ ohms per cubic centimeter, depending upon the diameter of the wire and the amount of working. Samples containing oxide showed higher specsc resistance than pure thorium. The temperature coefficient of electrical resistance be-
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tween 0" and 1730" C. was 0.0021, and between 22" and 135" C. was 0.0034. COEFFICIENTOF ExPANsIox-The linear thermal coefficient of expansion of the sample between 0" and 100" C. was 0.000117. HARDNESS-The hardness of cold-worked thorium was about 73 on the Rockwell tester, which is about the same as brass or severely cold-worked copper. TEKSILE STRENGTH-The tensile strength of thorium after a large amount of cold working without annealing was about 80,000 pounds per square inch. ~roLATILITY-The volatility of thorium metal a t its melting point is very low. A thorium wire does not blacken a lamp bulb even on fusion. ELECTRON EMIssrolr-The measurement of the electron emission of pure thorium is attended with much difficulty, on account of the rapidity with which the metal becomes oxidized before it can be introduced into evacuated bulbs in a condition suitable for this determination. Since there is some question about the removal of oxide films on the thorium, there is doubt as to whether the full value for the electron emission has been as yet observed. The values found are slightly lower than those given by LangmuirZ7 for thoriated tungsten. Temperature K. 1385 1396 1445 1550
Electron emission M. A. per s q . cm. 0.405 0.844 2.21 4.52
The temperatures are given in terms of true tungsten temperature since the light emissivity of pure thorium is not at present accurately known. The electron emission is given as milliamperes per square centimeter of wire surface. X-RADIATIONS-on account of its high atomic weight thorium has been used as an x-ray target. It gives more total x-radiations than tungsten and on account of its characteristic K-radiations gives deeper penetration in x-ray therapy. Other metals have been investigated in much the same way as t'horium and it is hoped in the near future to publish similar papers on vanadium, zirconium, titanium, etc. Bibliography 1-Berzelius, Pogg. Ann., 16, 385 (1829). 2-Chydenius, Ibid., 119, 43 (1861). Ber., 16, 153 (1883); 16, 2519 (1882); Compt. rend., 96, 3-iYilson, 346 (1883). 4-Moissan and Honigschmidt, A n n . chim. p h y s . , [SI 8, 182 (1905). 5-Matignon and Delephine, Ibid., [SI 10, 130 (1907). 6-Von Bolton, 2. Elektrochem., 14, 768 (1908); ( a ) D. R. P. 169,928. 7--Meyer, 2. Eleklrochem., 14, 809 (1908); 16, 105 (1909). 8-Karstens, Ibid., 16, 33 (1909). h L e l y and Hamburger, Z . anorg. chem., 87, 209 (1914). 10-Arsem, U. S. Patent 1,085,098 (1914). 11-Winkler, Ber., 24, 873 (1891). 12-Honigschmidt, Compt. rend., 142, 157, 280 (1905). 13-Troost, Ibid., 116, 1227 (1893). 14-Moissan and Etard, Ibid., 122, 573 (1896); A n n chim. phys., [7! 1 2 , 427 (1807). 15-Huppertz, Chem. Zentr., 1904, I, 1383. 16-Burger, Dissertation, Basel (1907); Gmelin Kraut, "Handbuch der Anorganische Chemie," 111, 1, 1207. 17-Kuzel and Wedekind, V. S. Patent 1,088,909 (1914). lS-Wedekind, Edel-Erden-Erse, [3] Nos. 19-20, 109 (1922). 19-Von Wartenberg, Z . Elektrochem., 15, 866, (1909). PO-Chauvenet, Compt. rend., 148, 1267 (1909). 21-Arndt and Loewenstein, Z . Elektrochem., 16, 784 (1909). 22-Gorgean, Comgl. rend., 99, 256 (1884). 23-Milikan, Z . physik. Chem., 92, 59 (1916). 24-Tiede and Birnbrauer, Z . anorg. Chem.. 87, 129 (1914). 25-U. S. Patent 1,480,301 (1924). 26-Hull, Phys. Rev., 18, 99 (1921). 27--Langmuir, Phys. Rev., 22, 3.57 (1923).
