March 9, 1943. H, wALTHER 2,313,410 PREPARATION OF BORON COMPOSITIONS Filéd March 31, 1959

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/N VE/V TOR H. WAL THER BV mgm ATTORNEY Patented Mar. 9, 1943 2,313,410

UNITED STATES PATENT OFFICE 2,313,410 PREPARATION 0F BORON COMPOSITIONS Henry Walther, Millburn, N. J., assignor to Beil Telephone Laboratories, Incorporated, New York, N. Y., a cgrporation of New York Application March 31, 1939, Serial No. 265,183 14 Claims. (Cl. 117-107) This invention relates to the preparation of tallic halide combines with the hydrogen in the resistor elements which have a negative tem proximity of the filament to deposit the metal perature coefllcient of resistance and more par thereon simultaneously with the formation of ticularly to the preparation of boron thermistors. the boron. The resulting product is homoge An object of this invention is to modify a mass neous and the ñlament upon which the boron of boron to change the characteristics thereof for and metal is deposited serves as a convenient rendering it suitable for use as a thermistor. guide for securing a required length of resist Another object of this invention is to incor ance unit. Alternatively, other metals or silicon porate throughout a mass of boron homogeneous or nitrogen may be incorporated with the boron ly another element. ‘ ~ ¿ 10 by subjecting boron trichloride, a reducing me A more particular object of this invention is dium and elemental nitrogen or a vaporized com to prepare simultaneously boron and another ele pound of the metal or silicon, such as a metal ment to produce a homogeneous substance adapt lic halide or silicon tetrachloride, to a heated sur ed for use as a thermistor. face such as an incandescent iìlament. The boron Boron has an exceptionally high negative tem 15 and metal or silicon or nitrogen is deposited on perature coeñ'icient of resistance. The conduc the filament simultaneously to form a homoge tivity of pure boron doubles for a 17° C. tem neous unit. perature rise in the range from zero to 100° C. A more comprehensive understanding of this This property is of special value in electrical sys invention is obtained by reference to the accom tems in which temperature control devices are 20 panying drawing, in which: required. However, the resistivity of pure boron Fig. 1 shows diagrammatically apparatus which is too high to be of practical use for this pur may be employed in the method of incorporat pose. The resistivity of a boron unit employed as ing a- metal with boron by the use of the ele a. thermistor may be materially lowered by in mental metal; corporating with it a small quantity of another 25 Fig. 2 represents portions of apparatus which element, such as aluminum. While boron with may be used in a modification of the method. ap certain amounts of other elements, such as mag paratus for which is shown in Fig. l; nesium, or aluminum, cr both, has been hereto Figs. 3 and 4 are views of a thermistor pro fore prepared, the resulting product is hetero duced by the method of this invention; and geneous. The grain size of such products ranges 30 Fig. 5 is a graphic representation of the tem from relatively large crystals to a fine powder and perature-resistance characteristics of the ther the crystals are extremely irregular. mistors shown in Figs. 3 and 4. In accordance with this invention, small quan In the apparatus of Fig. 1 a supply of a reduc tities of metals or silicon or nitrogen are intro ing medium such as hydrogen is indicated by the duced into boron to form a homogeneous mass 35 block labeled “Hydrogen” The hydrogen passes which is admirably adapted for use as thermistors. through a stop-cock l0 to a means for freeing The grain size and crystals of the product result the hydrogen from any contaminating materials, in'g from the practice of this invention are uni such as oxygen. This means is represented by form. The metal or silicon or nitrogen is in the block labeled “Purifier” and may comprise troduced when the boron is formed by the inter 40 heated capper gauze. From the puriñer the hy action oi‘ boron halide and a reducing agent when drogen passes to a drier comprising, for example, subjected to a suitable heated surface. Prefer phosphorus pentoxide or other efficient de ably, the metal if it is above hydrogen in the hydrating agent. The hydrogen then passes electrochemical series is disposed in the form oi through a flow meter to a mixing chamber. a shield around a heated surface comprising a 45 A boron halide such as boron trichloride is heated filament of tungsten. A boron halide, contained in a flask i2 which is cooled by solid such as boron trichloride, and a reducing mate carbon dioxide I3 in a Dewar flask I4. From the rial, such as hydrogen or carbon monoxide, are' ñask l2 the boron trichloride passes through a then passed over the heated filament. The re mercury valve i 5 containing two fritted glass ducing material interacts with the boron trichlo 50 filters I6 and il and thence through a iiow ride or other halide of boron to form boron and meter Yto the mixing chamber. The fritted glass hydrochloric or hydrohalogen acid. The boron filters I6 and i'l-are fused in the valve l5. The is deposited on the filament. The halogenated pores of these filters are so iine that mercury at product. such as hydrochloric acid, reacts with vacuum or at a pressurel even in excess of one the metal to form a halide thereof. The me 55 atmosphere cannot pass through, while gases 2 2,313,410 readily pass. A convenient fluid for the flow to a reservoir 38 containing mercury 39. A three meter in the hydrogen line is alpha monobromo way stop-cock 40 associated with the reservoir naphthalene. In the boron trichloride line no 38 is capable of applying pressure to the mer such organic liquid may be employed since the cury by means of air through a line 4I or re boron trichloride reacts with these materials. ducing the pressure thereof by connection to the Mercury is employed in the flow .meter vof the atmosphere through line 42. boron trichloride line and is also employed in A source of nitrogen gas may be employed for the valve I5. The valve I5 is supplied with mer flushing the apparatus. In the illustrated ap cury I8 contained in a reservoir I9 through a paratus the nitrogen passes through a stop-cock stop-cock 20 to control the fiow of boron trichlo 10 44 through the puriñendrier, flow meter to the ride to the mixing chamber. mixing chamber and thence through the deposit To eiiectuate this control, a three-way stop chamber 24, ‘ivalve 33 to the exhaust tube 39. cock 23 is connected to the reservoir I9 to con After the apparatus has been flushed, it is de trol the pressure on the mercury by means of sirable to remove the nitrogen in the system be air pressure supplied through a tube 2| from a fore heating the filament 21, since the nitrogen source not shown, or to reduce the pressure would combine with the boron deposited on the thereon to that of the atmosphere `through a filament. Accordingly the nitrogen is turned off tube 22. When it is desired to stop the flow of by means of a stop-cock 44 and hydrogen passed boron trichloride to the mixing chamber the through the system until the system is entirely stop-cock 20 is opened to permit the mercury freed of nitrogen. from the reservoir I9 to flow into the two arms If the shell 43 be of aluminum, for example, of the valve I5. The stop-cock 23 is opened to the hydrochloric acid resulting from the inter allow air pressure to be applied to the mercury action of boron trichloride and hydrogen reacts I8 in the reservoir I9. Under these conditions with the aluminum to form aluminum chloride. the mercury is forced up the two arms of the This aluminum chloride in turn is decomposed valve I5 to the height of that of the glass filters to form aluminum as a result of the interaction I6 and I1. When it is desired that the boron of aluminum chloride with the hydrogen present. trichloride be passed to the mixing chamber, the When boron trichloride and hydrogen pass over air pressure is removed and the pressure exerted a heated surface such as hot tungsten filament on the mercury I8 reduced to atmospheric by 30 the reaction is' turning the stop-cock 23 to connect the reser voir I9 to the tube 22. When this condition pre vails, the mercury in the arms of the valve I5 To deposit boron on the tungsten filament the flows back to a point below the junction of the temperature of the filament is preferably main two arms. Preferably, the boron trichloride in tained between 800° C. and 1500" C. Toward the flask I2 is free of chlorine. While chlorine either end of this temperature range the time does not constitute a harmful impurity from the rate of deposit decreases, at the lower end due to standpoint of >the boron to bedeposited, its pres slow chemical reaction and at the higher end due ence contaminates the mercury and may render to excessive evaporation. ’I'he most suitable tem the mercury seal valve l5 inoperative. 40 perature was found to be approximately 1300" C. The boron trichloride may be purified by shak In the event that the hydrogen is not freed ing it with mercury for a relatively short period from all traces of oxygen and moisture, the oxy of time. l From the mixing chamber the hydrogen gen would tend to oxidize and eventually destroy and boron trichloride pass to a deposit chamber the tungsten filament. The moisture resulting 24. The chamber 24 is sealed by means of a from the reaction of the hydrogen and such cover 25 and mercury 26 contained in a suitable oxygen also gives rise to the reaction ` recess in the tube 24. A lead wire 46, preferably comprising a heavy tungsten rod, passes through the cover 25 connected to a receptacle 45 for The boric acid formed in this reaction condenses holding one end of a tungsten filament 21. The 50 on the walls of the deposit chamber 24 and pre other end of the tungsten filament 21 is con vents visual observation of the filament. nected to another lead 41, preferably a heavy The hydrochloric acid formed in the interac tungsten rod, by means of a chuck 48. The lead tion of boron chloride and hydrogen reacts with 41 is in contact with a pool of mercury 28 con the shell of the metal to form a chloride thereof. tained in the bottom of the tube 24. A source of If the shell be of aluminum the following re current 29 supplies electrical energy to the fila action occurs: ment 21 from one pole through a variable re sistance 30, lead 46, filament 21, lead 41, mercury pool 28, a lead 3l connected to the pool of mer The metallic halide, such as aluminum chloride, cury, a switch 32, an ammeter 'I to the other formed at the shell, evaporates and is decom pole of the source 29. 60 posed again at the filament, permitting the If it is desired to incorporate a metal in the metal, such as aluminum, to be formed with the boron which is higher in the electrochemical boron on the ñlament. They amount of alumi series than that of hydrogen, it is advantageous num incorporated with the boron deposited on to provide a 'semicircular shell 43 of the metal the filament may be varied by changing the dis enclosing the filament 21. The shell is advan tance between the filament and the exposed me tageously spaced from V8 inch to one inch from tallic surface. the filament by means of a `disc 49 attached to Alternatively, the filament 21 may be’enclosed the receptacle 45. From the chamber 24 any in a cylinder comprising wire cloth of the metal resulting gases pass through a mercury valve 33, 70 desired to be incorporated with the boron. For similar to the valve I5 and containing fritted example, aluminum wire cloth may be formed glass filters 34 and 35, to the atmosphere in a circle to surround the filament 21, or pow through a tube 36. The valve 33 operates in a dered aluminum may be sprinkled on a mesh of similar manner to the valve I5, heretofore de copper or other metal below hydrogen in the scribed, and is connected through a stop-cock 31 75 electrochemical series. However, the semicircu v2,313,410 3 lar metallic shield is preferred. The flow of gases react in the chamber 24 to deposit a material through the reaction chamber and the genera comprising boron and a compound of boron and tion of convection currents within the chamber . nitrogen on the filament 21. The hydrogen re are often troublesome in the use of the wire acts with the boron trichloride to form elemen cloth construction since they tend to prevent tal boron and hydrogen chloride, while the nitro uniform temperature along the vertically sit gen combines with the boron to form a com uated filament. Uneven temperature results in pound of these elements. a non-uniform deposit of boron along the fila Figs. 3 and 4 show a resistance unit prepared ment. The metallic shield remedlesthis con in the manner heretofore described. 'I'he unit dition and boron resistance units prepared from 10 comprises, preferably, a deposit 50 comprising different parts of the filament when the shield boron and another element on a core of tungsten is employed cannot be distinguished from one 5I. Platinum leads 52 and 53 are attached to another. the deposit 50 in a manner described in Patent The tungsten filament 21 may be ‘.004 inch in 2,184,847 of G. L. Pearson, issued December 26, diameter and approximately 6‘/2 inches long, sus 1939. The presence of a small amount of alu pended under a slight tension with the lower minum incorporated in the boron in accordance mounting of rod 41 dipping into the mercury pool with this invention reduces the speciñc resist 28. The tension for a given length of filament is ance to about 1000 ohms-centimeters as compared adjusted to prevent excessive transverse vibration with about 10s ohms-centimeters for pure boron. of the filament which occurs when the funda 20 The resistance of pure boron at room tempera mental frequency of this vibrating string coin ture is reduced by a factor of two for a temper cides more or less with existing building vibra ature increase of 17° C., while the deposited tions or harmonics of them. The stiffness of the boron containing aluminum requires 29° C. in filament changes as the size of the deposit there-­ crease for the same change in resistance. on increases, thereby changing its resonant fre The specific resistance agains temperature quency. curve for a pure boron and that containing alu For the preparation of boron resistors contain minum is shown in Fig. 5. Curve D represents lng a small quantity of aluminum the semi-cir the temperature-resistance characteristic of pure cular shell 43 may be 2,1; inch thick and rolled boron, while curve E shows that of a resistor unit into a semicircle of 1/2 inch inside diameter. 30 comprising boron and a small amount of alumi The filament is situated at the center of the num prepared in accordance with this invention. semicircle. The deposit chamber 24 may be a It is observed that the specific resistance of the glass vessel measuring 11A inches inside diam deposit containing aluminum is considerably be eter and 12 inches long. The flow meter in the low that of pure boron. hydrogen and boron trichloride lines may be of In Fig. 2 portions of apparatus employed in an well-known construction, measuring the pressure alternate method of preparing boron containing difference across the ends of a capillary tube. small quantities or silicon is shown.' Any metal The flow of hydrogen employed in apparatus of whether or not above hydrogen in the electro the type illustrated may be about 5 cubic centi chemical series may be incorporated by the appa meters per minute and that of boron trichloride 40 ratus shown in Fig. 2. The alternate form of ap about 116 of that amount. The rate of gas flow _ paratus illustrated in Fig. 2 is a portion of the of either the boron trichloride or the hydrogen system shown between the lines A--A and B-B is not especially critical since an excess of each in Fig. 1. From the mixing chamber the hydro is needed. The required time of deposit ‘with gen and boron trichloride pass to a deposit cham the temperature of the filament at about 1300” C. 45 ber A64. A cover 65 through which a lead 16 passes is about 1% hours. The resulting yield on the - encloses the deposit chamber 64 and the cover 65 tungsten filament is a rod of about .030 inch is sealed to the deposit chamber 64 by means of a diameter of boron including aluminum or other pool of mercury 66 _contained in a recession of metal. During the deposit, constant tempera ­ the chamber 64. A filament 61 is connected at ture is advantageously maintained' by gradually 50 one end to the lead 16 and at the other end to a increasing the applied voltage as the filament lead 11. The lead 11 is immersed in a pool of increases in diameter. The finished rod of boron mercury 68 in electrical contact with a lead 1I. and the other element is removed from the de The filament 61 is heated in the same manner as posit chamber 24, reheated in air between 600° C. the filament 21 of Fig. 1. The hydrogen and to 1000° C. and then quenched in water. Pref boron trichloride are intermixed with a volatile erably the rod is reheated to approximately compound of either the metal or silicon to be in 800° C. This treatment materially decreases the corporated with the boron such as the metallic brittleness of the material. For use as thermis or silicon halide. The metallic or silicon halide tors the rod is preferably broken into small cyl 13 is contained in a flask 12 which is heated if inders of about .040 inch long which, for this 60 necessary by a helical coil 14 supplied with cur material, is the approximate length necessary to rent from a source 15 through a switch 16 and produce resistance units for most circuit require variable resistance 19. The metallic or silicon ments. ' halide reacts with the hydrogen to form the meta: If it is desired to combine nitrogen with or silicon and hydrohalogen acid in accordance boron, elemental nitrogen is passed over the with the following typical equation: heated filament with the boron halide and the reducing medium. To accomplish this combi nation in the apparatus shown in Fig. l, no shell The metal or silicon is deposited on the filament 43 is used and nitrogen is passed through a stop 61 in the same manner as that described for the cock 8, a flow meter, a stop-cock 9 to the purifier. 70 apparatus shown in Fig. 1. From the purifier, both the reducing agent such Various metals may be employed for modify as hydrogen and the nitrogen pass through the ing the characteristics of boron to reduce mate drier and flow meter to the mixing chamber. rially the resistance thereof. For example, lithi Here the hydrogen and nitrogen are mixed with um, sodium, potassium, copper, silver, gold, cal the boron halide. such as boron trichloride, to 75 cium, strontium, barium, zinc, cadmium, alumi 4 2,313,410 num, titanium, zirconium, tin, vanadium, arsenic, a surface in such proximity to said heated surface antimony, bismuth, chromium, molybdenum, that metal from said body of metal is deposited magnesium, iron, cobalt, nickel, platinum, may with boron on said heated surface. be used for this purpose. The halides or other 6. The method _of preparing a composition in non-oxidizing compounds, suchas carbonyls of Cl cluding boron and a metal above hydrogen in the these metals, or silicon, may be employed asv de electrochemical series which comprises exposing scribed in the method illustrated by the apparatus boron halide and hydrogen, in an atmosphere free shown in Fig. 2; or the metals themselves if above of oxygen, to a body having a heated surface, hydrogen in the electrochemical series, may be which body has spaced therefrom a body of said used in the form of a semi-,circular shell as here metal which is unheated except for-heat supplied tofore described to effect their incorporation uni from said heated surface and which has a sur formly throughout a mass of boron. After de face in such proximity to said heated surface that posit the mass may be heated between 600° to metal from said body of metal is deposited with 1000,o C. and subsequently quenched in water. boron on said heated surface. While boron trichloride has been used in the 7. The method of preparing a composition in specific embodiments of this invention, other cluding boron and a metal above hydrogen in the boron halides such as boron tribromide or boron electrochemical series which comprises exposing tri-iodide may be used to produce the thermistor. boron halide and a reducing agent, in an atmos While preferred embodiments of this invention phere free of oxygen, to a heated tungsten fila have been illustrated and described, various modi 20 ment having spaced therefrom a body of said fications may be made therein without departing metal which is unheated except for heat sup from the scope of the appended claims. plied from said filament and which has a surface What is claimed is: disposed in such proximity to said filament that 1. The method of preparing a composition in metal from said body is deposited with boron on cluding boron and a metal above hydrogen in the 25 said filament. ` electrochemical series, which comprises subject 8. The method of preparing a composition in ing boron halide and a reducing medium to a cluding boron and aluminum which comprises ex body having a heated surface maintained between posing boron halide and hydrogen, in an atmos 800° C. to 1500° C. in the presence of a body of phere free of oxygen. to a body having a heated said metal, which body is spaced from said body surface, which body has spaced therefrom a body having a heated surface and is unheated except of aluminum which is unheated except for heat for heat supplied by said heated surface, but supplied from said heated surface and which has which has a surface disposed in such proximity a surface in such proximity to said heated sur to said heated surface that metal deposits from face that aluminum from said body is deposited said body of metal on said heated surface with 35 with boron on said heated surface. boron, heating the resulting product to approxi 9. The method of preparing a composition in mately 600° C. Ato 1000° C., and quenching it in a. cluding boron and a metal above hydrogen inthe liquid. electrochemical series which comprises exposing 2. The method of preparing a composition in boron halide and a reducing agent, in an atmos cluding boron and aluminum which comprises 40 phere free of oxygen, to a heated filament which passing a boron halide and hydrogen over a tung is at least partly surrounded by and has extend sten fllament heated to approximately 1300" C. in ing lengthwise thereof a body of metal having a proximity to an aluminum shell, semicircular in substantially cylindrical interior surface 'which form, heating the resulting product to approxi is spaced from but in such proximity to said fila mately 800° C., and quenching it in water. ment that metal from said body is deposited with 3. The method of preparing a composition in boron on said filament. cluding boron and a metal which comprisesex 10. The method of preparing a composition in posing boron halide and a reducing agent, in an cluding boron and aluminum which comprises atmosphere free of oxygen, to :a body having a­ passing boronhalide and hydrogen over a tung heated surface, whichfbody has spaced therefrom 50 sten filament, heated between 800° C. and 1500° C., a body of said metal which is unheated except for which is at least partly surrounded by and has heat supplied from said heated surface and which extending lengthwise thereof a body of aluminum has a surface in such proximity to said heated sur having a substantially cylindrical interior sur face that metal from said body of metal is de face which is spaced from but in such proximity posited with boron on said heated surface. to said filament that aluminum from said body is 4. The method of preparing a composition in deposited with the boron on said filament. cluding boron and a metal which forms a volatile l1. The method of preparing a composition in halide upon exposure to a hydrohalogen acid in cluding boron and aluminum which comprises the presence of mild heat which comprises ex passing boron trichloride and hydrogen over a posing boron halide and hydrogen, in an atmos tungsten filament, heated between 800° C. and phere free of oxygen, to a ‘body having a heated 60 1500° C., and which is, disposed at the focus of surface, which body has spaced therefrom a body a semicircular cylinder of sheet aluminum which of said metal which is unheated except for heat extends lengthwise of the filament and is mounted supplied from said heated surface and whìchlhas in such proximity to said filament that aluminum a surface in such proximity to said heated surface 65 from said cylinder is deposited on said filament that metal from said body of metal is deposited with boron. with boron on said heated surface. 12. The method of preparing a composition in 5. The method of preparing a compositionvin cluding boron and a metal which comprises ex cluding boron and a metal above hydrogen in the posing a boron halide and a gaseous reducing electrochemical series which comprises exposing 70 agent to a body having a surface at an elevated4 boron halide and a reducing agent, in an atmos temperature sufiiclent to reduce the boron halide phere free of oxygen, to a body having a heated .' and to a Ibody of said‘metal having adjacent said surface, which body has spaced therefrom a body heated surface a surface which is at a tempera of said metal which is unheated except for heat ture below that which will reduce said boron supplied from said heated surface yand which has 75 halide and which surface is in such proximity to 2,318,410 said heated surface that metal from said body of metal is deposited IWith boron on said heated sur 14. The method of preparing a composition in» face. cluding a boron and a metal above hydrogen in 13. 'I‘he method of preparing a composition in the electrochemical series which comprises ex cluding boron and a metal which forms a volatile posing a boron halide and a gaseous reducing halide upon exposure to a hydrohalogen acid in 5 agent to a body having a surface at an elevated the presence of mild heating whichcomprises ex temperature su?îcient to reduce the boron halide posing a boron halide and a gaseous reducing and to a body of said metal having adjacent said agent to a body having a surface at an elevated heated surface a surface- which is at a tempera temperature suñicient to reduce the boron halide ture below that which will reduce said boron and- to a body of said metal having adjacent said 10 halide and which surface is in such proximity to heated surface a surface which is at a tempera said heated surface that metal from said body of metal is deposited with boron on said heated sur tur below that which will reduce said boron face. halide and which surface is in such proximity to said heated surface that metal from said body >of HENRY WALTHER. metal is deposited with boron on said heated surface. .