Knudsen Cell Studies of the Vaporization of Samarium Dicarbide

The vapor pressure of SmC₂ in equilibrium with graphite was measured by the Mnudsen effusion technique. Rates of weight loss from the cells were measured with an automatic recording balance. The apparent pressures varied with orifice size, and equilibrium pressures were calculated by extrapolation to zero orifice area. This work was combined with other studies to obtain log₁₀ P(atm) = - 13.869 × 10³/T + 3.752 (1300°-2050°K) for the Sm vapor pressure above SmC₂-C. Estimates of S°₂₉₈ and c_p were made for SmC₂, and δH°₂₉₈ was calculated to be 72.0 ± 2 kcal/mol for the reaction SmC₂(s) = Sm(g) + 2C(s). This value combined with δH°_{v, 298}= 48.6 kcal/mol for Sm gives a δ°_f298 for SmC₂ of 23.4 ± 2 kcal/mol.     

Knudsen Effusion Study of the Vaporization of CdO(s)

Knudsen effusion studies were conducted on CdO(s) in fused-SiO₂ Knudsen cells from 889 to 1110 K. The orifice size was varied; the largest cell orifice yielded a calculated pressure significantly lower than could be accounted for by experimental error. Average-slope- and average-free-energy-function-based values of Δ H ^c₂₉₈ were 86.9 ± 1.1 and 88.8 ± 0.5 kcal/mol, respectively. The data from the present work were combined with those of previous investigators using a method discussed by McCreary and Thorn; a Δ H ^c₂₉₈ of 88.2 ± 0.2 kcal/mol was obtained. Using this combined value for Δ H ^c₂₉₈ and ΔS^c₂₉₈= 51.40 ± 0.1 eu (obtained from the literature), the equilibrium constant for the reaction CdO(s)= Cd(g)+ 1/2 O₂(g) at 900 to 1380 K was calculated to be log K = 10.6-18,800(I/T).     

Knudsen and Langmuir Evaporation Studies of Stannic Oxide

A Knudsen study of the reaction SnO₂( c ) = SnO( g ) +½O₂( g ) yielded for the enthalpy of dissociation of SnO( g ), D°₀= 5.44 × 0.1 ev/molecule, which is in agreement with literature values (average 5.5 × 0.1 ev/molecule) obtained by various methods. Langmuir studies indicated an evaporation coefficient upper limit of 0.1 for the measured reaction. The Langmuir enthalpy of activation (Δ H *₂₉₈= 134 × 2.9 kcal/mole), as calculated by the second-law method, is less than that measured for the equilibrium reaction (Δ H *₂₉₈= 143.4 × 1.1 kcal/mole), but probably should be considered to agree within experimental error. Electron micrographs showed evidence of stepwise evaporation that depends on the crystallographic orientation of the polycrystalline grains.     

Thermal desorption in the pressure gap and at high pressure using a Micro Knudsen Effusion Cell

A new method and apparatus are described for performing thermal desorption at relatively high adsorbate pressure over single crystals and over porous adsorbents and catalysts. The apparatus, a Micro Knudsen Effusion Cell, encloses a sample in a small chamber or cell and allows only a limited pumping speed from the cell. Adsorbate pressures from ultrahigh vacuum to industrial reaction pressures are attainable depending on the ratio of surface area to pumping speed.     

A Direct Ammonia Fuel Cell Using Barium Cerate Proton Conducting Electrolyte Doped With Gadolinium and Praseodymium

The performance characteristics of fuel cells based on proton conducting BaCeO₃ solid electrolyte doubly doped with gadolinium and praseodymium are reported. The amount of praseodymium doping is systematically varied in order to optimize the fuel cell performance. Fuel cells incorporating the optimum amount of praseodymium exhibit power density levels enhanced by a factor of three, compared to those incorporating undoped BaCeO₃. The performance of the fuel cell is essentially the same irrespective of the fuel used. However, the performance of the fuel cell is slightly better in hydrogen than in ammonia. Nevertheless, fuel cells operated in ammonia show a greater decrease in peak power density with decreasing temperature than those operated in hydrogen. This behaviour suggests that alternative anode materials need to be utilized at lower operating temperatures.     

Vaporization of Rare-Earth Oxides

A study of the vaporization of La₂O₃, Nd₂O₃, and Y₂O₃ at high temperatures has been carried out by the Knudsen effusion technique. The methods employed, the results of the measurements, and the low-temperature heat capacities of these materials, which were determined for the purpose of calculating thermal functions of the oxides, are discussed. All the foregoing oxides react with tantalum, forming gaseous tantalummonoxide as a principal product. Cubic tantalates of lanthanum and neodymium are also formed in these reactions.     

Ceramic Properties of Samarium Oxide and Gadolinium Oxide; X-Ray Studies of Other Rare-Earth Oxides and Some Compounds

Samarium oxide forms a ceramic of medium strength and density (6.0) when it is fired at 1300°C. When it is fired at 1500°C., its density is increased to 7.4 but it loses its stability toward boiling water. X-ray data are given concerning the structure of the oxide as received and at 1300°C. and of the reaction product of Sm203 with water. Gadolinium oxide forms a ceramic of somewhat higher density (7.0) at 1300° than does Sm₂O₃; at 1500°C. its density is slightly higher (7.6) whereas its stability toward water is unchanged. Data concerning both oxides include density, shrinkage, moduli of elasticity and of rupture, linear thermal expansion, differential thermal analysis, and specific heat as well as X-ray data. Results are given of a brief survey of the reaction products obtained when Sm₂O₃ and Gd₂O₃ are heated at 1500°C. in equimolecular quantities with Al₂O₃, BaO, CaO, CdO, Fe₂O₃, HfO₂, MgO, SO₂, SrO, ThO₂, and ZrO₂. X-ray data for the following rare-earth oxides as received and after calcination at 1400°C. are given: ceria, praseodymia, neodymia, europia, dysprosia, holmia, erbia, thulia, and ytterbia.     

Regularities of the Vaporization of Oxygen-Containing Acid Salts

The vaporization of oxygen-containing acid salts is analyzed as a function of different factors affecting the qualitative and quantitative vapor compositions. It is shown that the character of vaporization of these salts can be predicted with a high accuracy only from the complete set of a number of criteria such as the difference between the acid–base properties of oxides forming the salt, the difference in their volatilities, the position of cation- and anion-forming elements in the periodic table, and the evaporation conditions.     

Disproportionation and Vaporization of Solid Silicon Monoxide

The vaporization rate of solid SiO was studied in the temperature range 1005° to 1475°C. It was concluded that ( a ) in an argon atmosphere the vaporization rate is external gas diffusion controlled; ( b ) only the upper layers of the charge contribute to the vaporization, so that a zero-order process is observed; ( c ) the heat of vaporization of SiO is 72.5 kcal/mole. Solid SiO disproportionates when heated in the temperature range 1000° to 1440°C, but the reaction does not go to completion. The reaction in the solid state is irreversible. The product silicon inhibits the reaction.     

Vaporization coefficients of SiO2 and MgO

The vaporization coefficients are measured for both SiO₂ (cristobalite) and the (100) face of MgO. These oxides vaporize congruently and are studied with free surface (Langmuir) vaporization using a vacuum thermogravimetric apparatus. The measured fluxes are compared to the calculated equilibrium fluxes to determine vaporization coefficients. Post-vaporization microstructures are also examined and discussed in regard to the vaporization process.     

Mechanosynthesis of Gadolinium Iron Garnet

We investigated the mechanosynthesis of gadolinium iron garnet by high-energy ball milling stoichiometry amounts of α-Fe₂O₃ and Gd₂O₃, followed by short thermal annealings conducted at moderate temperatures. X-ray diffraction and Mössbauer spectroscopy results revealed, for the as-milled samples, the formation of the GdFeO₃ perovskite phase, in relative amounts that depend on the milling time. The formation of the Gd₃Fe₅O₁₂ garnet phase was observed in 1000°C/2 h or 1100°C/3 h as-annealed samples. The occurrence of a milling time was verified in which the relative amount of garnet phase formed by further annealing is maximized.     

直接微生物燃料电池的构建及初步研究

利用Geobacter metallireducens能够以Fe(OH)3固体作电子受体进行呼吸的特性,用其构建直接微生物燃料电池,初步考察了产电情况和产电原理. 实验证明,Geobacter metallireducens直接微生物燃料电池的电能产出主要依赖于吸附在电极上的细菌. 燃料醋酸钠可以完全氧化至CO2,反应结束后其浓度低于检测下限(<10 mmol/L). 电子回收率达80%,电流密度达704.4 mA/m2.     

Chemistry of Vaporization of Refractory Materials

Most recently the practical importance of high temperature vaporization reactions has appeared in the synthesis of pure materials through vapor transport reactions and in the study of the movement of materials in nuclear power reactor experiments. The most recent basic development is the establishment, at temperature, of the range of nonstoichiometry in oxides. Long-standing applications of vaporization studies are in the measurement of thermodynamic properties and the discovery of new species. A discussion is given of the principles of physical chemistry important in vaporization studies, notably the concepts of equilibrium, phase behavior, thermodynamics, solid solution, and kinetics. The important factors influencing equilibrium vaporization phenomena are discussed and illustrated. A proper course of a vaporization study consisting of 9 stages is proposed. The important experimental techniques of Knudsen effusion, Langmuir vaporization and mass spectrometry are discussed. The principles, the factors, the course of a study and the experimental techniques and procedures are illustrated by recent work on the Ti-O system.     

Knudsen diffusion in powders. Part I Critical examination of a gas diffusion relationship used in Knudsen flow permeametry

B. V. Derjaguin's treatment of gas diffusion in a porous medium under conditions of Knudsen flow is discussed. A semi-theoretical value obtained for the constant β in the general equation is tested experimentally. In opposition to previous conclusions, molecular diffusion seems best interpreted in terms of elastic collisions against the pore walls. The structure of the pore system is shown to be the controlling parameter of the gas flow, and the novelty of the present treatment consists in ways to take the pore structure into account.     

Vaporization of Lead Zirconate-Lead Titanate Materials

A thermogravimetric investigation of the vaporization of cold-pressed Pb(Zr_0.65Ti_0.35)O₃ in vacuum from 690° to 1130°C shows that the vaporization occurs in two steps. An initial loss of 1 to 7 wt% proceeds by a mechanism with a logarithmic time dependence and represents the vaporization of the unreacted PbO that remains after the initial calcining. The second step proceeds by a slower, diffusion-controlled mechanism with a parabolic time dependence. It is shown that this rate-determining step is bulk diffusion across a thickening lead-depleted layer in the cold-pressed pellet, despite pore volumes of 24 to 42%. The rate constant, g/cm² sec½, for the parabolic weight loss process is expressed by: log K = (3.37 ± 0.14) - (8.46 ± 0.16) (10³/ T _K). The activation energy is 38.7 ± 2.0 kcal/mole.     

纯物质汽化热的预测

在纯物质常沸点汽化热的剩余函数预测法的基础上,结合并改进W-S方程,推导出预测纯物质在任一温度下的汽化热的新方程。由最基本的物性数据T_b,T_c,P_c,预测245种各类物质7574个温度点的汽化热,与实验值相比较的总绝对平均误差为1.77％,优于原剩余函数法。