Air oxidation of Beypazari lignite at 50°C, 100°C and 150°C

Structural changes occurred in Beypazari lignite while oxidizing at 50°C, 100°C and 150°C in a dynamic air atmosphere was detected by infrared spectroscopy. Relative changes in the amount of various oxygenated functional groups with respect to time were determined by curve resolution procedures. Investigation of the spectra of the samples at different oxidation conditions reveal that as the duration and temperature of the oxidation reactions increased the absorption in the aliphatic C–H stretching region decreased, while the absorption in the C=O stretching region presented a significant increase.     

Direct Electroplating of Lithium Cobalt Oxide Film on Platinum Substrate in 100°–200°C Aqueous Solution

Well crystallized, shape-formed, and electrochemically active lithium cobalt oxide (LiCoO₂) films are electroplated directly on an electron-conducting substrate in an aqueous solution using a soft solution processing that is economical, consumes less energy consuming, and is environmentally friendly. Although LiCoO₂ films are easily and economically prepared without any post-synthesis heat treatment, the estimated film properties show a possibility of using the deposited films as a cathode film for lithium rechargeable microbatteries. In addition, the soft solution processing reveals that an exact understanding of chemical reactions and the proper combination of the chemical reactions can create an advanced synthetic procedure.     

The System Magnesia-Silica-Water Below 300°C.: I, Low-Temperature Phases from 100° to 300°C. and Their Properties

Reactions in the ternary system MgO-SiO₂-H₂O were studied over the temperature range 100° to 300° C. and were found to produce only two phases. Under conditions of 100° to 200° C. and atmospheric pressure up to 20,000 lb. per sq. in., and regardless of the initial MgO/SiO₂ ratio, the predominant magnesium silicate product was found to have a MgO/SiO₂ ratio of 1.5. In the range 200° to 300°C. at 210 to 20,000 lb. per sq. in. two stable phases, 3MgO.2SiO₂.2H₂O (I) and 3MgO.4SiO₂.H₂O (II), were observed. In this case the phase that was favored was determined by the molar ratio of MgO/SiO₂ of the reaction mixture at the start of the run. The physical and chemical properties of phases (I) and (II) resembled those of the natural minerals serpentine and talc respectively. Two different morphologies were observed in electron micrographs of phase (I). From 100° to 160°C. it occurred as crumpled foils, and at 170°C. fibrous crystallites, which resembled the natural asbestos mineral chrysotile, appeared at the expense of some of the foils.     

Chemistry and Morphology of Hydrogarnets Formed in Cement-Based CASH Hydroceramics Cured at 200° to 350°C

We have studied the chemistry and the morphology of hydrogarnet crystals produced in cement-based hydroceramic materials at elevated temperatures (200°–350°C) with silica and alumina additions. Such materials lie within the hydrothermal CaO–Al₂O₃–SiO₂–H₂O (CASH) system. Hydrogarnet Ca₃Al₂(SiO₄)_{3− y} (OH)_{4 y} is the dominant aluminum bearing phase formed and its composition is influenced mainly by the curing temperature and to a lesser degree by the addition of silica. The composition parameter y was estimated by Rietveld refinement of X-ray diffraction (XRD) data. Electron probe microanalysis (EPMA) shows that the hydrogarnets incorporate minor elements such as Fe, Mg, and S. EPMA data confirmed the hydrogarnet composition estimated from XRD. Both octahedral and icositetrahedral forms are observed. The icositetrahedral form is associated with higher minor element content.     

Reaction of UC with Nitrogen from 1475° to 1700°C

The reaction of nitrogen with arc-melted UC was studied from 1475° to 1700°C at an N₂ pressure of about 400 torr. The reaction appeared to proceed toward equilibrium in distinct stages: (1) UC and N₂ reacted to form UC₂ and UC_0.8N_0.2; (2) UC₂ reacted with N₂ to produce more UC_0.3N_0.2 and free C; and (3) this U(C,N) reacted with N₂ to produce more free C and the equilibrium product, a high-N U(C,N). These results are consistent with known thermodynamic and phase behavior in the U-C-N system.     

Kinetics of the Graphite-Uranium Dioxide Reaction from 1400° to 1756°C

The reaction of microspheres of UO_z with graphite was studied from 1400° to 1756°C. When a spherically symmetrical layer of carbide was produced around the UO₂ core, only UC₂ was formed, and the diffusion of oxygen through this layer was rate-controlling. The Arrhenius relation for this system is k_D= 21.0 exp(−90, 000/ RT ) cm²/s
The reaction of a geometrically nonsymmetrical configuration of UO₂ and UC₂ was also studied. Comparison of the reactions in the symmetrical and nonsymmetrical systems demonstrated that the kinetic behavior of the two systems is quite different and that the conversion in the nonsymmetrical system was 2 to 5 times faster. The importance of these observations to kinetic results reported in the literature for analogous systems is discussed.     

Ionic Conductivity of the System Si-Al-O-N from 850° to 1400°C

Electrical conductivity was measured from 850° to 1400°C for β-sialon and pure X phase as well as for the sintered system Si₃N₄-Al₂O₃, containing β-sialon, X phase, β-Si₃N₄, and glassy phase. Ionic conductivity was measured at >1000°C. The charge carriers were identified by electrolysis. The results showed that pure β-sialon is ionically conducting because of Si⁴⁺ migration for the temperature range studied. Pure X phase shows ionic conduction by Si⁴⁺ above 1000°; below 1000°C, it shows electronic conduction because of impurities. The conductivity of the sintered system Si₃N₄-Al₂O₃ containing β-sialon, β-Si₃N₄ X phase, and glassy phase changes as the relative quantities of β -sialon and X phase change. The apparent activation energies for the ionic and electronic conductivities are 45 and 20 kcal/mol, respectively.     

Experimental Determination and Thermodynamic Calculation of the Zirconia–Calcia–Magnesia System at 1600°, 1700°, and 1750°C

Phase relations in the ternary system ZrO₂–CaO–MgO were experimentally established at 1600°, 1700°, and 1750°C. The investigation was based on powder X-ray diffractometry, scanning electron microscopy–energy dispersive spectroscopy, and electron probe microanalysis, on 24 ternary compositions. The compositions were prepared using attrition milling of respective oxides and carbonates as raw materials. The results obtained allowed construction of the corresponding isothermal sections, which verified the existence of the cubic-ZrO₂–CaZrO₃ phase compatibility field at the three temperatures. Finally, experimental results also were compared with the thermodynamic assessment previously reported of the system ZrO₂–CaO–MgO.     

Oxidation of Hafnium Carbide in the Temperature Range 1400° to 2060°C

After hafnium carbide has been oxidized at temperatures in the range of 1400° to 2060°C, three distinct layers are present in the film cross section: (a) a residual carbide layer with dissolved oxygen in the lattice, (b) a dense-appearing oxide interlayer containing carbon, and (c) a porous outer layer of hafnium oxide. Experimental measurements of layer thicknesses and oxygen concentrations are combined with an extended formulation of moving-boundary diffusion theory to obtain the diffusion constants of oxygen in each of the three layers. The results indicate that the oxide interlayer is a better diffusion barrier for oxygen than either of the other layers. Based on X-ray microanalysis, X-ray diffraction, and resistance measurements, the interlayer is an oxygen-deficient oxide of hafnium with a carbon impurity. The interlayer hardness equals that of the residual carbide layer.     

Effect of Additives on the Pressureless Sintering of Aluminum Nitride between 1500° and 1800°C

Combined oxide additives (Y₂O₃, CaO, La₂O₃, CeO₂, SiO₂, TiO₂, and Fe₂O₃) were investigated as AIN sintering aids. AIN can be fully sintered at 1600°C to substantial thermal conductivity (92 W/(m·K)) using a multiple sintering aid of Y₂O₃, CaO, SiO₂, La₂O₃, and CeO₂. This lowtemperature material has small grain size (1 to 3 μm).     

Factors Affecting Strength of Clays in the Temperature Range 110° to 800°C

Using a carefully established procedure for the preparation of test bars and for the measurement of the modulus of rupture, the effect of firing temperature on the transverse strength of a Georgia kaolin, Kentucky ball clay, and Missouri fire clay was determined. The kaolin showed the greatest temperature vs. strength sensitivity (-19% to +50% of the dry strength); it therefore was used in a study of the effect of aging, electrodialysis, and ion exchange on strength-temperature relationships. Kaolin treated with H₂O₂ and HC1, and electrodialyzed kaolin with NaCl and CaCl₂, was weaker than the untreated clay after being fired at temperatures below 650°C. but was as much as 131% stronger at 800° firings.     

Single-Crystal Elastic Constants of Yttria-Stabilized Zirconia in the Range 20° to 700°C

Elastic constants of single crystals of yttria-stabilized zirconia were determined through the temperature range 20° to 700°C. Crystals containing 8.1, 11.1,12.1, 15.5, and 17.9 mol% Y₂0₃were measured. The elastic constant C₁₁ was found to decrease and C₁₂ and C₄₄ to increase with increasing Y₂O₃ content; this appears to be due to decreasing coulombic interaction as Y³⁺ replaces Zr⁴⁺. Except for the 8.1 mol% Y₂O₃ crystal, the conventional elastic constants all showed normal monotonic decreases with increasing temperature. In the case of the 8.1 mol% Y₂O₃ crystal, measurements as a function of temperature were not reproducible, and it is likely that this composition at room temperature is below the composition limit of thermodynamic stability of the cubic fluorite phase.     

Mechanical properties and microstructure of high strength concrete containing polypropylene fibres exposed to temperatures up to 200 °C

High strength concrete has been used in situations where it may be exposed to elevated temperatures. Numerous authors have shown the significant contribution of polypropylene fibre to the spalling resistance of high strength concrete. This investigation develops some important data on the mechanical properties and microstructure of high strength concrete incorporating polypropylene fibre exposed to elevated temperature up to 200 °C. When polypropylene fibre high strength concrete is heated up to 170 °C, fibres readily melt and volatilise, creating additional porosity and small channels in the concrete. DSC and TG analysis showed the temperature ranges of the decomposition reactions in the high strength concrete. SEM analysis showed supplementary pores and small channels created in the concrete due to fibre melting. Mechanical tests showed small changes in compressive strength, modulus of elasticity and splitting tensile strength that could be due to polypropylene fibre melting.     

Oxidation Kinetics of Hafnium Carbide in the Temperature Range of 480° to 600°C

The isothermal oxidation of HfC powders was carried out at temperatures of 480° to 600°C at oxygen pressure of 4, 8, and 16 kPa, using an electromicrobalance. The oxidized product was identified by X-ray analysis, Raman spectroscopy, X-ray photoelectron spectroscopy, and electron diffraction, and the morphology of the oxidized grains was observed by scanning electron microscopy. Oxidation proceeds by two processes: a diffusion-controlled process operates up to about 50% oxidation and a phase-boundary-controlled process operates above about 50% oxidation. The activation energies for both processes are the same (197 ± 15 kJ.mol⁻¹). The change in the oxidation process is associated with the generation of cracks on the grains, resulting from the growth or expansion stress due to the formation of monoclinic HfO₂ microcrystallites less than 3 nm in size. In the latter process, the thickness of the diffusion layer is kept constant, being time-independent, which allows the process to apparently obey the phase-boundary-controlled reaction.     

Thermal Conductivity of Uranium Dioxide from -57° to 1100°C by a Radial Heat Flow Technique

A radial heat flow technique was used to measure the thermal conductivity, k , of polycrystalline UO₂in the range -57° to 1100°C. The technique yielded results with a probable accuracy of ±1.5% and a precision of ±0.1% in the range 50° to 1100°C. Meaningful measurements were limited to 1100°C by Pt-90 Pt10Rh thermocouple instability, although the apparatus was structurally sound to 1400°C. The thermal conductivity data up to 1000°C could be explained on the basis of heat transport by phonons. The thermal resistance, l/ k , exhibits a linear temperature dependence from 200° to 100°C, which is expected for an insulator well above the Debye temperature. The slope of the l/ k -temperature plot is 0.0223 cm w⁻¹ which is independent of impurity content and is associated with three phonon umklapp processes. The intercept is sensitive to impurity content as indicated by the fact that it was decreased by a decrease in the oxygen/uranium ratio. Between 1000° and 1100°C, there is a slight departure of l/ k from linearity which may be due to the onset of an electronic contribution. Near room temperature, UO₂ has a maximum in k which is apparently caused by the rapid decrease in specific heat below this temperature.     

Electrical Conductivity of Single and Polycrystalline Near-Stoichiometric Rutile in the Range 600° to 1400°C

The electrical conductivity of both single-crystal and sintered near-stoichiometric rutile was investigated over the range 600° to 1400°C and the oxygen pressure range 0.005 to 1.0 atm. Above 1200°C, the conductivity of single-crystal and sintered rutile obeys the relation σ∼ P o₂^−1/4. This can be explained on the basis of the excitation of the first electron from an oxygen ion vacancy-two trapped electron defect associated with a departure from stoichiometry. The attendant activation energy for this mechanism is 1.8 ± 0.1 ev as calculated from the In σ vs. 1 /T curves. The conductivity below about 900°C is independent of oxygen pressure and has been associated with impurity controlled defects. Activation energies of 1.0 ± 0.05 ev for the c direction and 1.5 ± 0.05 ev for the a direction and for sintered specimens were obtained for this mechanism. The electrical conductivity in the range 900° to 1200°C can be interpreted on the basis that the preceding two mechanisms act in parallel.