Molar Heat Capacity and Heat of Fusion of Silicon Tetraiodide*

229 Molar Heat Capacity and Heat of Fusion of Silicon Tetraiodide* By Toshio Kurosawa**, Ryosuke Hasegawa** and Tetsuo Yagihashi** The molar heat capacity of silicon tetraiodide, SiI4, used as a source material in the iodide process of silicon prepara- tion was measured by means of an adiabatic calorimeter which can be used for continuous measurement. The molar heat capacities of solid and liquid silicon tetraiodide obtained were Cp=19.59+0.0209T eal/mol deg and Cp=35.25+0.00987Tcal/mol deg, respectively. The melting point was 120.5℃, and the heat of fusion and the entropy of fusion were 4.70kcal/mol and 11.9cal/mol deg, respectively. The heat contents, free energy functions and standard entropy of S298=43.7cal/mol deg were derived for silicon tetraiodide from the experimental values and thermochemical data. Moreover, the molar heat capacities of pure chromium, fused quartz and highly pure silicon were also measured, the values of which were in good agreement with those in the previous literature. (Received April 26, 1965) helices. The sampling capsule was also made of quartz I. Introduction and theshape is shown in Fig.1(A). Thiscapsule was hprevious reports(1)～(4), various processes relating COnlleCted With the diStilation column and filled with to the hydrogen reduction of silicon tetraiodide have been the main fraction of silicon tetraiodide in argon atmos- described as a part of the research program on the pre- phere. After separated from the column, it was sealed parationof pure silicon.In regardto thermochemicalhy a ball joint①, and then two capillaries ② were fused off under evacuation from the capsule. The distilled properties of silicontetraiodide, the measurement of vapor pressure by Andersen (5) and the decomposition equii- brium by Schafer were known previously, but much re- mains unknown.for the molar heat capacity and the heat offusion. By theuse ofhighly pure silicon tetraiodide preparedby a synthesisfollowed by distillation,the molarheat capacity and theheat of fusionof silicontetraiodide were measured in the present study. The heat contents and the free energy functions from room tem- peratureto the boilingpoint, were alsoderived. An investigation of the molar heat capacity and the heat of fusion.is considered to be very significant from the metallurgical point of view as well as for the treatment of this material. Ⅱ. Sample and Measuring Procedure 1.Silicon Tetraiodide Silicon tetraiodide was prepared by reacting the argon- carried iodine at 700℃ with crushed crude silicon of 98.4% purity and 30～60mesh size. The synthesized silicon tetraiodide was introduced into the distillation Fig.1(A) Silica vessel for the chemical compound. columnand purifiedin argon atmosphere, The number (B)Sample for solid block. of theoretical windings was thirteen and the column was ①Ball joint ②Capillary ③Side tube for argon made of transparent quartz and packed with single turn inlet or vacuum ④Hole for internal heater ⑤Hole for temperature measurement ⑥Hole for adiabatic controll * This paper was published in Japanese in the Journal of the Japan Institute of Metals, 29 (1965), 267. **_National Research Institute for Metals, Tokyo, Japan. silicontetraiodide was hydrated with excess deionized (1) T. Kurosawa and T. Yagihashi: J. Japan Inst. Metals, 26 water,and the precipitatewas calcinedto silica.Silica (1962), 122. thus obtainedwas analyzedby emissionspectrography. (2) T. Kurosawa, T. Ishikawa and T.Yagihashi: ibid, 26 (1962), 274. the impuritesin the originalcrude siliconwere not ob- (3) T. Kurosawa, T. Ishikawa and T. Yagihashi: ibid,26 (1962), servedexcept for a traceof aluminum. 314. Gravimetricanalysis of the sample showed 5.2%of (4) T. Kurosawa and T. Yagihashi: ibid, 27 (1963), 221. siliconand 93.6%of iodine(theoreticalvalues:5 .2%Si (5) H. C. Andersen and L. H. Belz: J. Amer. Chem. Soc., 75 (1953), 4828. and 94.8%I). Moreover,the presenceof SiI2in silicon Trans. JIM 1965 Vol.6 230 Molar Heal Capacityand Heat ofFusiou of SiliconTetraiodide tetraiodide prepared at 500～700℃ was assumed to be standard potential of the required temperatures was negligible from the thermodynamic consideration based present. Then the time requried for the sample tempera- on the equation by Schafer(6), ture to become equal to the ,present temperature, namely the time required for the temperature increment of 2℃, was recoreded by the automatic controller. Further, iodine due to the decomposition of the sample was not found. 2.Apparatus and procedure The adiabatic calorimeter used for specific heat and differential thermal analysis was the apparatus made by Rigaku Denki Co., the calorimetry assemaly of which is shown in Fig.2. The sample ⑥ was set in the center Fig.3 Diagramaticalrepresentation formeasurement. The Alumel-Chromel thermocouplesused in this ex- perimentwere made by HoskinsGo, which were calibrat- ed by the meltingpoints of iodine,indium and tin. In order to standardizethe apparatus,the molar heat capacitiesof electrolyticchromium of 99.9%purity prepared by thisinstitute and high-puritysilicon of P type were measured as examplesof high thermal conductivity. Fused quartzwas used as an example of low thermal Fig.2 Adiabatic calorimeter. conductivity,These samples were made in the shape ①Outer shell ② Shield plates ③ Electric furnace shown in Fig.1(B). ④Adiabatic container ⑤Quartz supporter ⑥Sample ⑦Internal heater ⑧Thermocouple ⑨Differential In the.caseof chromium and silicon,a silicacap was thermceouple. put on the internalheater for insulation,and a piece of mica was plugged between the sample and the tipof the of the nickel adiabatic container ④ which was mounted thermocouple. In addition, measurements were carried on the quartz supporter ⑤. The internal heater out at one atmospheric pressure of argon in order to located in the center of the sample was a nichrome spiral, avoid oxidation of the sample. and two nickel wires were welded in parrallel to measure If the sample is heated by the constant wattage W, and the voltage, In order to heat the sample, a constant if it takes Δt seconds for the sample at θ℃ to become wattage was supplied to the internal heater using a watt (θ+Δ θ)℃, the following equation(1)is obtained on the stabilizer. The difference in temperature between the assumption that the heat supplied is completely absorbed sample and the nickel container was checked during the in the sample. experiment with an Alumel-Chromel differential thermocouple⑨, and tbe difference was maintained almost (1) nil by the furnace ③ which is adjusted by the controller Here 0.239 is the conversion factor from wattage to system. The fluctuation in wattage supplied to the in- calorie and consequently the right side of equation(1)is ternal heater was within 1%, and the difference between the thermal amount supplied to the sample for dt seconds, θ1 and θ2 was within ±0.05℃, Athermocouple for Mand Cp are the molar quantity and the molar heat measuring the temperature of the sample was also insert- capacity of the sample, respectively. Therefore the first ed into another hole. The nickel wires, differential ther- term of the left side is the heat absorbed in the sample. mocouple and temperature measuring thermocouple were The heat absorbed in the quartz capsule,cap and in- all insulated by thin alumina tubes, ternal heater and others is expressed by the second term, In this experiment the punched tape which was pro- and mi and Cpi are the molar quantities and molar heat grammed for the standard thermal potential of tempera- capacities of these materials, The amounts of heat tures at intervals of 2℃ was used, and a continuous consumed by the internal heaters, thermocouples and measurement was made by the system shown in Fig.3. small high-aluminatubes locatedin the sample capsule The punched tape was set on the tape reader and the were estimatedto be 0.05cal/deg,being very smallwhen compared with the other values. (6)H.Sehafer und B.Morcher:Z.anorg. allgen.Chem.,290 (1957),279. The heat of fusionof silicontetraiodide, Hf was ob- Toshio Kurosawa, Ryosuke Hasegawa and Tetsuo Yagihashi 231 tained from the peak recorded in the temperature-time lines are the values ohtained by Perry(7)and Goldsmith diagram. If the time required for melting is t, Hf can (8) be expressed by the following equation(2): The equations of the materials obtained in this experiment were determined as follows: (2 Chromium:Cp=4.39+0.00354T(room temp.～500℃) Ⅲ.Experimental Results and Discussion 1.Molar heat capacities of standardized materials The molar heat capacities of silver, iron highly pure silicon, fused quartz and pure chromium were measured to examine the equipment, and the results obtained for chromium, fused quartz and silicon are described in this report. The molar heat capacities of chromium(99.9% purity; 0,0.09%;N,0.0037%;H,trace;arc-melted after electrolysis using chromic acid and heat treatment in hydrogen), transparent fused quartz of 99.99% purity and highly pure silicon(P type;resistivity,1000～1700 Ω-cm;life time,600～800μ-sec;impllrity conoentra- tion.,9.1×1012atoms/cc;refined hy the floating zone method)are shown in Figs.4～6. Fig.6 Molar heat capacity of silicon. Fused quartz:Cp=13.36+0.00421T -316100/T2(room temp .～300℃) Silicon:Cp=6.61+0.000288T -158400/T2(room temp .～400℃). In general,excessively fast or slowheating caused dif- ficultyin the adiabaticcontrol and deviationsin the requiredtime. Itis therefore necessary to selecta pro- per heatingrate for each sample. The properheating rate for the present experiment was found to be in the range of 1～3℃/min. Moreover, the atmosphere within the outer shell was atmospheric air or argon. 2.Molar heat capacity, heat of fusion and melting point of silicon tetraiodide Fig.4 Molar heat capacityof chromium. The molar heat capacity of silicon tetraiodide obtained Fig.5 Molarheat capacity offused quartz. Fig.7 Molarheat capacity ofsilicon tetraiodide.

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