Oxidation-reduction reactions of CeMO4+x (M = Ta or Nb) phases

Phase equilibrium methods, single crystal and powder x-ray diffraction analyses, thermogravimetric analysis and magnetic susceptibility measurements were utilized to define subsolidus phase relations $\text{in air}$ for the systems cerium oxide - Ta₂O₅ and cerium oxide - Nb₂O₅. Stoichiometric CeTaO₄ is stable in air (P_O2 = 0.21 atm) only above 1265°C. At 1265°C, the reversible reaction, $\text{3}\text{CeTaO}{}_4\text{+}\text{1}\text{2}\text{?}$CeTa₃O₉ + 2CeO₂ is established. If CeTaO₄ is quenched to room temperature and $\text{reheated}$ below 1000°C, or, if the material is rapidly cooled from above 1265°C to below 1000°C it absorbs oxygen according to, . The x parameter is variable and temperature dependent. Using a thermal microbalance, three distinct complex reaction series involving a homogeneity range in x were established, (a) 0.50 ≥ × ≥ 0.48 (<350°–600°C), (b) 0.17 ≥ × ≥ 0.06 (600°–950°C), (c) 0.40 ≥ × ≥ 0.34 (950°-room temperature). CeNbO₄ (not isostructural with CeTaO₄) also absorbs oxygen below ≈700°C in air to yield CeNbO_4+x materials.     

Approximate Mode II velocity correction factors for 90° unidirectional fiber composites

A static Mode III isotropic/Mode II orthotropic analogy is developed for 90° highly orthotropic fiber composites. This static analogy is extended to the dynamic regime and the velocity correction factors for the dynamic energy release rate and the dynamic stress intensity factor are obtained for the Mode II case. The complete dynamic analogies are given. The approximate Mode II 90° velocity correction factors $\text{W}{}_{\mathrm{II(90°)}}$ and $\text{S}{}_{\mathrm{II((90°)}}$ are of the same general form as the approximate Mode I velocity correction factors, $\text{W}{}_{\mathrm{I(0°)}}$, $\text{W}{}_{\mathrm{I(90°)}}$, $\text{S}{}_{\mathrm{I(0°)}}$ and $\text{S}{}_{\mathrm{I(90°)}}$ Whereas the four Mode I velocity correction factors approach zero as the crack-tip velocity approaches the composite shear wave speed, $\text{w}{}_{\mathrm{II(90°)}}$ and $\text{s}{}_{\mathrm{II(90°)}}$ approach zero as the crack-tip velocity approaches the larger extensional wave speed.     

Carburization and decarburization kinetics of iron in CH4 — H2 mixtures between 1000 – 1100 °C

In this study the rate constants of the methane decomposition reaction on iron surfaces were determined in the 1000–1100°C temperature range, by grav? metric methods. Earlier works showed that the reaction velocity was given by The results indicate that the constant values vary from 2.72 × 10^?6 to $\text{16.74 × 10}{}^{\mathrm{?6}}\text{mol C/cm}{}^2\text{/sec/atm}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ for k and 2.61 × 10^?8 to $\text{8.62 × 10}{}^{\mathrm{?8}}\text{mol C/cm}{}^2\text{/sec/atm}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ for k′ between 1000 and 1100°C.     

Crystal growth and characterization of In23PS3

Crystals of $\text{In}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{PS}{}_3$ were grown by chemical vapor transport of the elements from a charge zone maintained at 630°C. and a growth zone at 560°C. The crystals were characterized by chemical, x-ray, and densitometric techniques. $\text{In}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{PS}{}_3$ is structurally related to the M^IIPS₃ thiophosphides. The compound is an insulator and has an optical absorption edge of 3.1(1) e.V.     

A new method of sulfur vapor pressure control and its application to the V-S system

A new apparatus for obtaining partial pressure of sulfur was devised by using liquid sulfur and carrier gas of nitrogen. This successfully works in the range of ～10^?4 to ～1 atm of Ps₂ at any temperature higher than about 400°C up to about 1000°C. As an application of it, the equilibrium study of V-S system was done at 800°C. Two phases V₃S₄ and V₅S₈, each having homogeneity range, were found while no existence of the V₂S₃ phase could be detected thermodynamically and X-ray crystallogrpahically at this temperature. The standard free energy of following reaction, $\text{VS}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{+}\text{2}\text{15}$$\text{S}{}_2\text{=}\text{VS}\text{8}\text{5}$, was calculated by considering the transition-Ps₂ and the activities and · ΔG° (1073K) = ?1150 cal·mole^?1was obtained with an estimated uncertainty of ± 100 cal to the above reaction.     

Electrodeposition of silicon carbide

Thin layers of SiC were electrodeposited from $\text{Li}{}_2\text{CO}{}_3\text{SiO}{}_2$ melts onto α-SiC substrates at 1000°–1050°C using potential differences around ?0.5V versus polycrystalline SiC anodes. The layers appear to be epitaxial. Although conditions were found under which the melt exhibited good long term stability, attempts to grow bulk SiC crystals were handicapped by the delamination of the seed crystals.     

A magnesium oxide defect structure of hexagonal symmetry

When brucite, Mg(OH)₂, is carefully heated in vacuum beyond the dehydration temperature (250–270°C) the (001) x-ray intensity decreases rapidly but does not vanish. The (100) reflection persists up to 480°C. The lattice contracts by 0.2% in $\text{c}$-direction. The thermal expansion in $\text{a}$-direction slows down. This indicates the formation of a nearly anhydrous high defect MgO which retains the brucite structure: $\text{Mg(OH}\text{)}{}_{\mathrm{2?x}}\text{O}{}_{\mathrm{<mtext>x</mtext><mtext>2</mtext>}}\text{□}{}_{\mathrm{<mtext>x</mtext><mtext>2</mtext>}}$ with x→2 (□ unoccupied anion site). This relict structure collapses progressively to the cubic MgO.     

Stability of the perovskite phase LaBO3 (B = V,Cr, Mn,Fe, Co,Ni) in reducing atmosphere I. Experimental results

The chemical stability of perovskites LaBO₃ where B = V, Cr, Mn, Fe, Co, Ni was studied by thermogravimetry at 1000°C in gas mixtures of $\text{CO}{}_2\text{H}{}_2$, $\text{O}{}_2\text{CO}{}_2$ and $\text{O}{}_2\text{Ar}$ at 1 bar.The stability limits of the perovskite phases expressed in terms of -log Po₂★ (Po₂★ = critical oxygen partial pressure in bar) were for LaCrO₃ and LaVO₃ (greater than 21.1), LaFeO₃ (16.95), LaMnO₃ (15.05), LaCoO₃ (7.0) and for LaNiO₃ (～0.6). The changes in standard enthalpy ΔH° and entropy ΔS° of the following reactions were obtained.$\text{LaVO}{}_4\text{=}\text{LaVO}{}_3\text{+}\text{1}\text{20}{}_2\text{δ=328}\text{kJ}\text{mol}\text{°=135}\text{J}\text{mol·deg.}$,$\text{LaMnO}{}_3\text{=}\text{1}\text{2}\text{La}{}_2\text{O}{}_3\text{+}\text{MnO}$     

Fracture toughness and fatigue crack propagation in high strength steel from room temperature to −180°c

Fracture toughness under tensile test and fatigue test on high strength steel at temperature ranging from room temperature to ?180°C were experimentally studied. The value of fracture toughness under fatigue test is considerably tower than that obtained under tensile test.Within the range from room temperature to ?100°C the following results were obtained: the power coefficient δ of the fatigue crack propagation rate $\text{[(dc)/(dN)] = AΔK}{}^5$ is related with [(1)/($\text{T}$)] as: $\text{δ = b}{}_1\text{+ [(a}{}_1\text{)/(kT)]}$. $\text{[(dc)/(dN)]}$ shows Arrhenius type, and, however, different equation from usual stress dependent rate process equation. The trend is in good agreement with the dislocation dynamics theory of fatigue crack propagation.     

A correlation between dynamic initiation toughness and crack arrest toughness

Dynamic initiation toughness values were obtained from testing small 3-point bend specimens at ?50, ?25, 0, +23°C. Two specimen orientations were tested which showed no marked difference in critical dynamic initiation toughness. The obtained $\text{K}{}_{\mathrm{Id}}$ data were correlated with the crack arrest toughness $\text{K}{}_{\mathrm{Im}}$ and $\text{K}{}_{\mathrm{Ia}}$. The value of $\text{K}{}_{\mathrm{Id}}$ is in the range 0.93 to 1.29 times the corresponding fast fracture toughness, $\text{k}{}_{\mathrm{im}}$ while the ratio $\text{K}{}_{\mathrm{Id}}\text{/K}{}_{\mathrm{Ia}}$ varies between 1.27 and 1.60.     

A unifying strain criterion for fracture of fibrous composite laminates

A general fracture toughness parameter $\text{Q}{}_{\mathrm{c}}$ which is independent of layup, was previously derived using a strain failure criterion for fibers in the principal load-carrying plies. It was verified to be a material constant for centrally cracked boron/aluminum composite specimens. The specimens were made with various proportions of 0° and $\text{±45°}$ plies. Moreover, a limited amount of data indicated that the ratio $\text{Q}{}_{\mathrm{c}}\text{/?}{}_{\mathrm{tuf}}$, where $\text{?}{}_{\mathrm{tuf}}$ is the ultimate tensile strain of the fibers, might be a constant for all composite laminates, regardless of material and layup. In that case, a single value of $\text{Q}{}_{\mathrm{c}}\text{/?}{}_{\mathrm{tuf}}$ could be used to predict the fracture toughness of all fibrous composite laminates from only the elastic constants and $\text{?}{}_{\mathrm{tuf}}$.To verify that $\text{Q}{}_{\mathrm{c}}\text{/?}{}_{\mathrm{tuf}}$ is indeed a constant, values of $\text{Q}{}_{\mathrm{c}}\text{/?}{}_{\mathrm{tuf}}$ are calculated and presented here for centrally cracked specimens made from graphite/polyimide, graphite/epoxy, E-glass/epoxy, boron/epoxy. and S-glass-graphite/epoxy materials with numerous $\text{[0}{}_{\mathrm{i}}\text{/±45}{}_{\mathrm{j}}\text{/90}{}_{\mathrm{k}}\text{]}$ layups. The data indicate that $\text{Q}{}_{\mathrm{c}}\text{/?}{}_{\mathrm{tuf}}$ is reasonably constant for all laminates that did not delaminate or split extensively in the 0° plies at the crack tips. But some elevation of $\text{Q}{}_{\mathrm{c}}\text{/?}{}_{\mathrm{tuf}}$ due to small-to-moderate crack-tip damage is evident.Using a single value of $\text{Q}{}_{\mathrm{c}}\text{/?}{}_{\mathrm{tuf}}$ for all the layups and materials, strengths were predicted for the test specimens. The predicted and test values agree well except, of course, for the laminates that delaminated or split extensively. Then, the predicted strengths are usually conservative. Layup affects the ratio of cracked to uncracked strengths more for the resin matrix specimens than the boron/aluminum specimens.     

Crystal growth of KCl in a reactive atmosphere

Reactive atmosphere processing (RAP) with CCl₄ is employed in the crystal growth of KCl (mp = 776°C). Two carrier gases, He and CO₂, are compared. A carbon deposit accumulates in the growth of KCl in $\text{CCl}{}_4\text{He}$, a drawback which is eliminated in crystal growth under $\text{CCl}{}_4\text{CO}{}_2$.     

Epitaxial films of the low temperature modification of the compound Cu2Se

Epitaxial films of the low temperature modification of Cu₂Se deposited onto NaCl single-crystal substrates at 200°C and 400°C have been investigated. The diffraction pattern of this phase was indexed on the basis of a hexagonal lattice with unit cell parameters a = 7.07 and $\text{c = 6.68}\text{A}\text{?}$. These parameters are related to the parameters of the f.c.c. phase by the relations $\text{a}{}_{\mathrm{<mtext>h</mtext>}}\text{= (}\text{√3}\text{√2}\text{) a}{}_{\mathrm{<mtext>c</mtext>}}$ and $\text{c}{}_{\mathrm{<mtext>h</mtext>}}\text{= (}\text{2}\text{√3}\text{) a}{}_{\mathrm{<mtext>c</mtext>}}$.     

Mixed mode stress intensity distributions for part circular surface flaws

Using the frozen stress method, mixed mode stress intensity factor (SIF) distributions were obtained for a series of low aspect ratio part circular surface flaws in flat plates over a depth to plate thickness range of 0.20–0.70. Mode I results were compared with Mode I solutions in order to sense the influence of the crack surface inclination to the free boundary. It was found that maximum SIF values did not necessarily occur at maximum flaw depth. A rationale is offered to explain this observation. It was also found that the effect of the boundary inclination was to reverse the relative magnitudes of $\text{K}{}_1\text{K}\text{?}{}_1$ and $\text{K}{}_2\text{K}\text{?}{}_2$ in going from a 30° to a 60° crack surface inclination with the boundary.     

Hydrothermal synthesis of zeolites from rhyolitic pumice of different geological origins

Twelve samples of rhyolitic pumice from widely different origins were subjected to zeolitization in standard hydrothermal conditions (solid/liquid ratio, $\text{1}\text{5}$; T, 95°C; reaction time, 5 h; alkaline solution 5 N NaOH). The $\text{Si}\text{Al}$ ratio of the starting material was of special importance in that zeolites formed with $\text{Si}\text{Al}$ ratios similar to or smaller than that of the parent material. Pumice with low $\text{SiO}{}_2\text{Al}{}_2\text{O}{}_3$, and high Fe content, preferentially gave rise to zeolites with lower $\text{SiO}{}_2\text{Al}{}_2\text{O}{}_3$ ratio than in the case of lower Fe and higher $\text{SiO}{}_2\text{Al}{}_2\text{O}{}_3$ ratio.     

Some experimental observations on the Poisson's ratio of sea-ice

Measurements were made on the longitudinal and transverse strain of sea-ice beams loaded in flexure. The specimens were tested with stress rates varying from 10 to 600 kPa s^?1 and temperatures ranging from ?5°C to ?40°C. Under these conditions, the effective strain ratio μ increases with increasing temperature and decreasing stress rate. The strong influence of the stress rate, σ, suggests an empirical law of the form: μ = 0.24$\text{(}\text{σ}\text{?}\text{σ}\text{?}{}_1\text{)}{}^{\mathrm{?0.29}}\text{+ μ}{}_{\mathrm{D}}$ where σ is the stress rate (kPa s^?1), $\text{σ}\text{?}{}_1$ is a unit stress ratio (1 kPa s^?1) and μ_D is a dynamic value of Poisson's rate which depends on the temperature.     

Fatigue crack propagation from a crack inclined to the cyclic tensile axis

Cyclic stresses with stress ratio R = 0.65 were applied to sheet specimens of aluminium which have an initial crack inclined to the tensile axis at angles of 30°, 45°, 72° or 90°. The threshold condition for the non-propagation of the initial crack was found to be given by a quadratic form of the ranges of the stress intensity factors of modes I and II. The direction of fatigue crack extension from the inclined crack was roughly perpendicular to the tensile axis at stress ranges just above the threshold value for non-propagation. On the other hand, at stress ranges 1.6 times higher than the threshold values the crack grew in the direction of the initial crack. The rate of crack growth in the initial crack direction was found to be expressed by the following function of stress intensity factor ranges of mode I, K₁, and mode II, K₂: $\text{dc}\text{dN}\text{= C(K}{}_{\mathrm{eff}}\text{)sum}$, where $\text{K}{}_{\mathrm{eff}}\text{= [K}{}_1{}^4\text{+ 8K}{}_2{}^4\text{]}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$. This law was derived on the basis of the fatigue crack propagation model proposed by Weertman.     

Growth of Ca3Fe2Ge3O12 and Ca3Fe(Al,Cr) Ge3O12 garnets in molten bismuth germanate glasses

Additions of Fe₂O₃ to CaO·Bi₂O₃·2 GeO₂ cause Ca₃Fe₂Ge₃O₁₂ garnets to precipitate from the resultant melt at 1250°C. Garnets with the composition Ca₃Fe(Al, Cr) Ge₃O₁₂ are also precipitated by adding either $\text{Al}{}_2\text{O}{}_3\text{Fe}{}_2\text{O}{}_3$ or $\text{Cr}{}_2\text{O}{}_3\text{Fe}{}_2\text{O}{}_3$ mixtures. The well-formed crystals range from several to 100 μm in size and are obtained in 50 to 70% yields at $\text{Fe}\text{Bi}\text{= 0.4}$. Additions of Fe₂O₃ (up to $\text{Fe}\text{Bi}\text{= 1.0}$) to compositions containing ZnO, CdO, SrO, and BaO yield only dark glasses. The physical properties of these glasses suggest that Fe(III), in contrast to AL(III) & Ga(III), prefers octahedral coordination.     

Dynamic stress distribution around the tip of a running crack

The stress distribution is obtained around the tip of a crack running in a brittle material. The stresses are written as the sum of the associated static solution and the wave-effect terms which depend upon the crack speed. The results obtained clearly reduce to the associated static solutions if the crack speed vanishes.Near the tip of the crack, the dynamic stress-intensity factor for the circumferential stress, $\text{σ}{}_{\mathrm{\theta \theta }}$, is written as the product of the associated static stress-intensity factor and the dynamic correction factor which is a nondimensional function of the crack speed, $\text{V}$, the angle from the crack plane, θ, and Poisson's ratio, ν. The value of the correction factor is computed for various values of $\text{V}$ and θ at $\text{ν = 0.25}$. It is shown that the maximum tensile value of $\text{σ}{}_{\mathrm{\theta \theta }}$, occurs on the crack plane for $\text{V}$ less than 0.7 time shear wave speed, $\text{c}{}_2$, and suddenly shifts to the plane of $\text{θ = 55°}$ for $\text{V}$ slightly larger than 0.7 $\text{c}{}_2$. For $\text{V > 0.7}$$\text{c}{}_2$, the angle θ for the maximum $\text{σ}{}_{\mathrm{\theta \theta }}$, θ being larger than 55°, varies continuously with the crack speed, $\text{V}$. The results obtained are used to discuss the growth of branching crack.