Journal of Research of the National Bureau of Standards Vol. 58, No.5, May 1957 Research Paper 2760 Thermal Conductivity of Nitrogen from 500 to 500°C and 1 to 100 Atmospheres* R. 1. Nuttall! and D. C. Ginnings A new apparatus has been constructed for measurements of the thermal conductivity of gases up to 500°C and 100-atmosphere p ressure. The parallel-plate m ethod was used with a spacing of about 0.5 millimeter between the hotplate and coldplate. A capacitance method was used to measure the effective spacing and area of t he plates under the conditions of the experim ent. No solid material was used bet,,'ecn the plates. The e ffect of radiation was minimized by use of polished s ilver parts a nd I\"aS accounted for by experiments with the conductivity cell evacuatcd. ~1ea s ure m e llt s on nit roge n were made at J, 50, and 100 atmospheres, and from 50° to 500°C. It is beli el'ed that. the accuracy of the r esul ts is about 0.5 percent, except at the highest gas densities. 1. Introduction T he correction for heat transfer by radiation between t he hotplate and the coldplate was evalu ated Accurate data on the thermal conductivity of gases by an experiment with the condu ct i v i t ~ · cell evacu­ are needed for two reasons. First, they arc needed ated. Of course, this assumes t hat no appreciable to check present theories of heat conduction in gases. part of t he radi ation is absorbed b.\- tll e gas in the Experimental measurements at very hi gh tempera­ co nduetivi ty experimen t. III all Lhe experiments, it tures and pressures arc ext r e m el~T diffi cult, so that was neC'essary to know t he efrective spaC'i ng between theoretical means of predicting thermal co ndu ctiv­ t he plates. Frequently, solid spaC'ers (of known ities in this range arc needed. Second, accurate data dimel1 sions) are used between t he hotplate and cold­ are needed on at least one gas so t1tat engin eering plate. Ko su ch spacers were used in this apparatus data on other gases can be obtained with relatively because it was bel ieved that the heat transfer simple apparatus by a comparison method. The t hrough t he contaC't area of t he spacers and plates present apparatus was co nstructed primarily to fur­ might be d ifferent with gas present than with the nish very accurate data for use as standards by C'ell evacuated. The radiation correction experi· others making thermal-co nductivity measurements mrll t would thu s be partially invalidated. The on gases. Nitrogen was chosen as the first gas to effeC'Live spacing was m easured by a capaci tance be measured for several reasons. ~l ore measure­ method under the actual cond itions of the experi · ments have been made on nitrogen than 0 11 any other ment. T hi s method has several advantages over the pure gas. It is readily available in a state of high usual method with measurecl spacers. F irst, the purit~- and is entirely sui table as a standard reference above-m entioned uncertainty in t he effect of contact gas for use in calibrating apparatus for relative is eliminated. Second, t he capacitanC'e method measuremen ts. au tornatically accot! nts for an ~- change in dimensions. Third, because the direct capacitance between the 2. Method hotplate and. coldplate is measured, nonli near hrat fl ow at the C' irC' umferrnce of the hotplate is accounted. ~Iea s uremen ts of thermal co nductivi ty of gases for. in a steady state have been made by two general The thermal C'ol1ductivity, k (w C'm - 1 deg C-l) , for methods, radial heat flow and linear heat flow. ll eat flow between parallel plates is given by the ~Io st measurements have been made with radial equation heat flow, either from a hot wire or between coaxial cylinders. In principle, these measurements are S llS­ (1) ceptible to co nvection errors. Elimination of these errors aL the high pressures requires extremely small dimensions, which may be difficult to determine. where Q is the rate of heat (walts) flowing only b~­ The linear heat flow method is free from co nvection C'onduction from the hotplate throug'h the gas to lilr if the heat flow is downward . For this reason, the colclplate, t:..t is t he temperature . difference ( de ~ C) parallel-plate method was chosell for these measure­ between t he ho t pla.te and C'oldplate, t:..X is the ments up to 100 atm. The spacing between the effective distance (cm) betweell the two plates, 9,nd plates was made small to minimize the errol' caused A is the d l' ective area (cm 2) of t he ho tplate. The by radial heat transfer. In addition , the small spac­ fa.ctor t:..X/A may be considered as the constant of ing reduces the possibility of convection in case the t he "C'ollducLiv ity cell" determined b.\- lhe capfLci­ plates are not quite horizontal. tanC'e method, so that *rphi s work was supported in part by t he National Advisory Committee for 0.0885516Q A l1fonau tics. k (2) I Pn'Sf'llt address },ryonnc Nationa l Laboratory, Lemon t, ]11 . Ct:..t ' 271 wh ere C is the direc t capaci tance (micl'omicrofarads) between the hotplate and t be cold plate, assuming the material between the plates to have a dielec tric cO llstant of unity corresponding to a vacuum. "Vith ~ Aluminum the gas between the plates, a small correction must be made for its di electric constant. The conduction ~ Stainle ss Steel equation (2) would apply, of course, to an~~ two surfaces, as well as to parallel plates, provided only BlSilver that the temperature and electric fielcls arr geo­ metrically similar. 3. Experimental Procedure 3.1. Apparatus 1W0flf.- --- C The general assembl~~ of the tllermal-condu<:tivit.," cell, the pressure vessel, and the surrounding furnace is shown in figure 1. The hotplate (H ), coldplatr (J), guard (F), and auxiliary guard (E) are showll schematicall.Y . The pressure vessel (G) is made of stainless steel and is sealed with a Monel gasket (D). ~§~~;;J_F This vessel is surrounded by a furnace (K) and H~~~~~H furna,ce "neel,," (C), which are made of aluminum and are equipped wit h electric heaters ill numerous K porcelain tubes in the aluminum. The furnace temperature was au to maticall~~ controlled b~T means of a plati num resistallce thermometer n.nd bridge circuit. The furnace neck was controlled r elative to the fmnace b.,- means of a thermocouple. The pressure vessel extends upward out of the furnace region so that t he electric leads can be brought out of t he pressure vessel in a cold r egion. The cooling o 2 4 INCHES coil (B) dissipates the heat from the furnace, so that I I I t he top is cool. The eleetric leads (all No. 32 gold F I G(;RE 1. Thermal-conductivity apparatus. wire) ar e brought up from the a uxiliary guard A, Pressure seal for electric loads; B , cooli ng co il ; OJ furnace neck; D, wIonel through t hree 1n conel tubes, which serve as electro­ g~s kc t; E, auxiliary guard; F, guard; G, pressure vessel; H , hotplate; J, cold p late; static shi elds, as described later. There are 29 of ] ..... , fu rnacc. th ese leads, which are brought out of the pressure vessel through a pressure seal, A. These leads go A out radially between two " Kel-F" (polychlorotri­ fluorocthylene) disks, which are pressed together for the seal. A number of difficulties were encountered before obtaining a successful seal at A. At first, the material T eflon (polytetl'afluOl'o ethylene) was used, o --t<:~~+--.... but it was found to flow excessively at the high E ---+;>~' pressures necessary for th e seal. The method finallv used for this seal was to mold Kel-F around 0 the gold wires at about 200 C. G The vital parts of the the rm al- co ndu ct ivit~ - eell al'e shown in fi gure 2. The hotplate (':'1 ), coldplate (0 ), guard (E), and auxiliar:v guard (B ) are all made of silver to minimize temperature gradients and h eat transfer by radiation. The hot plate is made of three parts, silver-soldered together. The hotplate heater (L) is located between the lower parts and consists of about 55 ohms (at 25 0 0 ) of (0.05-mm diameter) o 0.5 1.0 INCHE S platinum wire insulated with mica. Gold leads I from this h eater are brought out through the tem­ FIGURE 2. Thermal-wnductivity cell. pering region located at H between the upper two A, C, V', H , P, '"!"'hermal tic·downs for electric leads; B, a l1 xiliar~.r guard; D. Q. silve], pieces of the hotplate and then to the therma.l platinum resistance thermometers; E , guard; OJ silver screw; J . quartz supports for hotplate; K, aluminum insert; L, hotplate heater; ::V[ , hotplate; X, quartz tie-down, F. The purpose of this thermal tic-d own spacers; 0, coldplate. 272 is to bring the leads as dose as possible to the tem­ hotplate b:," m eall S of the differential tllCrmopile pre­ peratUl'e of the silver. This type of t ie-down IS used viously m ent ioned.