Subsolidus phase relations in the ZnS-In2S3 system: 600 to 1080° C

Eleven ternary compounds were synthesised, of which nine are new. The seven most ZnS-rich compounds form a series. Each has a primitive hexagonal lattice witha=3.85±0.01 Å. Thec-dimension varies from 37.47 Å (39 ZnS8 In₂S₃) to 18.63 Å (18 ZnS8 In₂S₃) with a periodicity of 3.14 Å which can be related to a decrease in the ZnS content. One of the four remaining compounds (12 ZnS8 In₂S₃) is triclinic, but at least two polytypes are present. The more common variety has the parameters:a=3.86 Å,b=15.48 Å,c=3.54 Å,=90° 16,=120° 02 and=89° 47. The structure of the other polytype and of compounds 17 ZnS:8 In₂S₃ and 10 ZnS8 In₂S₃ have not been determined. Stoichiometric ZnIn₂S₄ has a rhombic hexagonal lattice (a=3.86 Å,c=36.95 Å). This ternary compound which has been synthesised previously is the only one to show appreciable solid solution. Approximately 2 mole % ZnS is soluble in ZnIn₂S₄ at 600° C; this increases to 8 mole % at 1080° C.     

Singular Band Behavior of the Extended Emery Model for the Superconducting Cuprates

Mean field slave-boson approximation is performed on the extended Emery model for the CuO₂ conducting plane. The model is parameterized by Cu–O charge transfer energy _pd , copper–oxygen overlap t ₀, oxygen–oxygen overlap t', and Coulomb interaction U on the copper site taken as infinite. Special emphasis is placed on the role of t in the renormalization trends of the effective band parameters _pf and t, replacing _pd and t ₀, at small doping . It is shown that small, negative t expands the range of stability of the metallic phase, changing, in the second order of the perturbation theory, the nature of the metal–insulator transition point. In the nonperturbative limit, t modifies strongly the renormalization of _pf , making it saturate at the value of 4t. Finite doping suppresses the insulating state approximately symmetrically with respect to its sign. The regime _pf 4t fits very well the ARPES spectra of Y123, Bi2212, and LSCO and also explans, in the latter case, the evolution of the FS with doping accompanied by the spectral weight-transfer from the oxygen to the resonant band.     

Effect of pH on 980 °C spinel phase-mullite formation of Al2O3-SiO2 gels

Different reaction paths of mullite formation via sol-gel processing techniques are reviewed. These variations are due to differences in hydrolysis/gelation behaviours of the silica and alumina components used. Variations of pH during processing without altering other variables follow three different routes of mullite formation. In the highly acidic region(pH 1), the gel does not exhibit a 980 °C exotherm but forms -Al₂O₃. Mullite forms at high temperature by diminution of -Al₂O₃ and -cristobalite, respectively. In the pH range of 3–4.5, gels exhibit a 980 °C exotherm and develop only mullite. In the highly alkaline region (pH 14), the gel produces a Si-Al spinel phase at the 980 °C exotherm and mullite formation at the 1330 °C exotherm takes place from the intermediate Si-Al spinel phase.     

Polymorphism of the Ag8S4O4 and Ag6S3O4 compounds

Experimental results of the reaction occurring between aqueous solutions of AgNO₃ and Na₂S₂O₃ were presented in this work. It has been demonstrated that these reagents form two phases to which the summary formulas: -Ag₈S₄O₄ and -Ag₆S₃O₄ were ascribed. The -Ag₈S₄O₄ phase crystallizes in a tetragonal system and has the following parameters of the unit cell: a = b = 0.72052 nm, c = 0.51140 nm, V = 0.26550 nm³, Z = 1, d_x = 6.60 g/cm³. At 223°C -Ag₈S₄O₄ undergoes an irreversible, endothermic polymorphic transition. The heat of this transition amounts 8.03 × 10^–3 J/mol. A high-temperature polymorphic form -Ag₈S₄O₄ melts at 400°C. The -Ag₆S₃O₄ phase crystallizes in a monoclinic system and has the following parameters of the unit cell:a = 2.09616 nm, b = 0.53118 nm, c = 1.49885 nm, = 102.78°, V = 1.62753 nm³, Z = 8, d_x = 6.59 g/cm³. -Ag₆S₃O₄ also undergoes an irreversible, endothermic polymorphic transition at 221°C. The heat of this transition amounts 7.65 × 10^–3 J/mol. A high-temperature form, -Ag₆S₃O₄, melts at 390°C.     

Charged defects-controlled conductivity in Ge-ln-Se glasses

The variation of the d.c. electrical conductivity, , with composition and temperature was investigated for glasses of the Ge-In-Se system. The results indicate a decrease in the activation energy for electrical conductivity, E, and an increase in on introduction of indium into Ge-Se glasses. The changes in E and with composition (selenium content in the glasses) are identical for the Ge_x In₅ Se_95–x and Ge_x In₈Se_92–x families. The results have been traced to the conduction controlled by charged defects in these chalcogenide glasses. The changes in E and have been explained by a shift in the Fermi level, being brought by the introduction of indium.     

Phase decomposition in extruded Zn-Al based alloy

Ageing characteristics of an extruded eutectoid Zn-Al based alloy were investigated using X-ray diffraction and scanning electron microscopy techniques. The extruded alloy consisted of Al rich phase and Zn rich _E and phases. The original cast eutectoid Zn-Al alloy was extruded at 250 °C. Both supersaturated _s and _s phase decomposed during extrusion and appeared as fine and coarse lamellar structures. The _E and phases particles formed in the original interdendritic region. It was found that two Zn rich phases _E and decomposed sequentially during ageing at 170, 140 °C. The decomposition of the _E phase occurred as a discontinuous precipitation in the early stage of ageing and the decomposition of the phase took place in a four phase transformation: + T + in the prolonged ageing. Two typical morphologies of the decomposition of the Zn rich phases _E and were distinctive in back-scattered scanning electron microscopy.     

Investigations of the copper isotope effect in oxygen-deficient YBa2Cu3O7−δ

We present data on the copper isotope effect (⁶³Cu-⁶⁵Cu), C_u =-nT_c/nm_Cu, for two isotopic pairs of oxygen-deficient YBa₂Cu₃O_7–, where varies between 0.06 and 0.52. _Cu is below 0.01 at =0.06 (fully oxygenated), it takes values between –0.14 and –0.34 in the 60 K plateau. Larger negative values of _Cu are observed away from the plateau. The dependence of _Cu is similar to that of the pressure effect dnT_c/dP.     

Dielectric properties of chemically vapour-deposited Si3N4

The dielectric properties of chemically vapour-deposited (CVD) amorphous and crystalline Si₃N₄ were measured in the temperature range from room temperature to 800° C. The a.c. conductivity ( _a.c.) of the amorphous CVD-Si₃N₄ was found to be less than that of the crystalline CVD-Si₃N₄ below 500° C, but became greater than that of the crystalline CVD-Si₃N₄ over 500° C due to the contribution of d.c. conductivity ( _d.c.). The measured loss factor () and dielectric constant () of the amorphous CVD-Si₃N₄ are smaller than those of the crystalline CVD-Si₃N₄ in all of the temperature and frequency ranges examined. The relationships of ^n-1, (- ) ^n-1 and/(- ) = cot (n/2) (were observed for the amorphous and crystalline specimens, where is angular frequency andn is a constant. The values ofn of amorphous and crystalline CVD-Si₃N₄ were 0.8 to 0.9 and 0.6 to 0.8, respectively. These results may indicate that the a.c. conduction observed for both of the above specimens is caused by hopping carriers. The values of loss tangent (tan) increased with increasing temperature. The relationship of log (tan) T was observed. The value of tan for the amorphous CVD-Si₃N₄ was smaller than that of the crystalline CVD-Si₃N₄.     

Methanation of carbon deposited directly from CO2 on rhodium-bearing activated magnetite

Methanation reactivity was studied for the surface carbon deposited from CO₂ on the surface of Rh-bearing activated magnetite. The most active material (Rh=0.83 wt %) for methanation was prepared by the impregnation method at 60°C and showed 98% conversion at 300°C. The surface carbon was composed of elemental carbon (-carbon) and polymerized carbon (-carbon), the proportion being dependent on the density of carbon deposited. In temperature-programmed surface reaction, the extent of conversion of the - and -carbon to CH₄ was 0.34 (-carbon) and 0.53 (-carbon), respectively, and the total conversion was 0.87. This result indicates that not only elemental carbon but polymerized carbon (-carbon) could be converted to CH₄ on the Rh-bearing activated (-carbon) magnetite, whereas -carbon is not hydrogenated on activated magnetite.     

Ageing characteristics of rapidly solidified Al-Li-Ti alloys at 473 K

The influence of titanium on the precipitate free zones (PFZs) and on the stable phase is investigated in four melt-spun Al-Li-Ti alloys with 2 mass% lithium and 0.10–0.35 mass% titanium. Rapid solidification gives rise to a homogeneous distribution of titanium on the wheel side of the ribbons, and to a cellular distribution of solute atoms in intermediate regions and on the gas side. Heat treatments of up to 1000 h at 473 K do not modify titanium distribution, which remains in solid solution. During ageing, preferential coarsening of particles occurs on cell walls. The growth rate of the PFZs is lower than in other Al-Li alloys and is mainly controlled by the titanium concentration of the alloys. The nucleation and growth of the stable phase is also delayed by the presence of titanium. These results indicate that the effect of titanium in solid solution is to retain vacancies thus reducing lithium diffusion rate.     

Mechanism of β1 formation in conventional dental amalgams

This study reexamined the mechanism of ₁ formation in conventional dental amalgams in the light of a previously unrecognized intergranular structure, the SnHg-₂ phase. Two amalgams, A (45. 5% Hg) and B (65. 0% Hg), were made from a Zn-free alloy of the following composition (wt%): Ag-70, Sn-26, Cu-4. After storage at 37°C for 1 day, both amalgams were heat treated at 70°C for up to 16 days. The structural changes in the two amalgams as a function of their heat treatment were characterized by differential scanning calorimetry and X-ray diffraction techniques. On the basis of the experimental data, we have proposed a new mechanism on ₁ formation. This mechanism involves a series of interrelated reactions. The primary one is a reaction between AgSn- and ₂ that leads to the nucleation of ₁ at boundaries. Simultaneous operation of several secondary and tertiary reactions result in (1) further growth of ₁ and its continuing change in composition, (2) loss of Sn from the ₁ matrix, (3) conversion of CuHg into , and (4) a possible transformation of ₂ into Sn-rich ₁ or ₁.     

Nitriding of iron boride to hexagonal boron nitride

A process for preparing hexagonal boron nitride (h-BN) from iron boride (FeB) has been described for the first time. FeB powder was nitrided in ammonia at 700 K for 24 hours yielding non-crystalline BN and -Fe₂N. Subsequent annealing in ammonia for 1 hour at 1273 K resulted in a mixture of crystalline h-BN together with -Fe, -Fe₃N and -Fe₄N. The morphology of the product grains is described. This comprises an iron nitride core loosely encapsulated in a foliated BN layer. The open nature of the product layer may account for the relatively low temperature (700 K) required for the nitriding reaction. These iron containing phases were removed with dilute mineral acid leaving h-BN and trace amounts of FeB₄₉.     

A Method for Calculating Thermal Radiation Properties of Multilayer Films from Optical Constants

A calculation method for the incident angle dependence of the solar absorptance _S and the temperature dependence of the total hemispherical emittance _H of multilayer films is proposed. The method is based on calculation of _S and _H from optical constants in the wavelength region from 0.25 to 100 m for thin polymer films and deposited metal. In this paper we provide values of _S in the incident angle region from 0 to 90° and _H in the temperature range from 173.15 to 373.15 K for two-layer samples of aluminum-deposited polyimide film. The results obtained for _S and _H by the present method are compared with experimental results measured by both spectroscopic and calorimetric methods. The calculated results of _S and _H agree well with the experimental results.     

Relationships between structure and hardness developed during the high temperature ageing of a smart Cu-based alloy

Smart shape memory alloys are used industrially due to their novel properties. Copperbased shape memory alloys are strongly influenced by ageing treatments which involve microstructural changes. The present work has been carried out on a CuZnAlCo alloy which has been aged at 400, 500 and 600 °C for time durations ranging from 5 min to 12 h. Hardness measurements have been performed and the microstructural evolution has been determined by electron microscopy. The results show that microstructural changes are strongly dependent on the ageing temperature and time. A maximum hardness of 300 Hv _0.2 has been obtained after ageing treatments. Equilibrium and phase and non-equilibrium phases ( or martensite) are present in the alloy depending on the ageing temperature and the ageing time. The maximum hardness has been associated with the presence of the and phases.     

Behavior of a screw dislocation near a partially bonded bimetallic interface

This paper, based upon the isotropic elastic continuum approximation, deals with the forces of interaction between a stationary screw dislocation and a partially bonded bimetallic interface. The complex variable method is used throughout and the closed form solution is obtained. The result indicates that if the material on the other side of the interface is more rigid than that in which the dislocation is present, the bonded region will act as a barrier to the dislocation located in a certain region near the interface and that there is no stable equilibrium position of single dislocation line even in the aforementioned case since the free surface operates an attractive force on it.

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Zusammenfassung Diese Arbeit behandelt die Kraefte der Wechselwirkung zwischen einer stationaeren Schraubenverschiebung und einer teilweisen unfreien zwei-metallischen Zwischenflaeche. Sie beruht auf dem isotropischen, elastischen Kontinuumnaeherungswert. Die Methode der komplexen Veraenderlichen wurde benutzt und die Loesung der geschlossenen Form wurde erhalten.Das Ergebnis zeigt, dass, sollte das Material auf der anderen Seite der Zwischenflaeche haerter sein, als jenes in welchem die Verschiebung stattfindet, das unfreie Gebiet als Barriere zu der Verschiebung, die sich in einem bestimmten Gebiet in der Nahe der Zwischenflaeche befindet, auftritt. Ferner gibt es keine stabile Gleichgewichtsposition von einer einzelnen Verschiebungslinie, selbst in dem erwaehnten Falle nicht, da die frei Oberflaeche Zugkraft ausuebt.

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Résumé Se basant sur l'approximation d'un milieu continu élastique et isotrope, le mémoire est relatif aux forces d'interaction entre une dislocation-vis et l'interface de deux corps métalliques partiellement solidarisés. On utilise la méthode des variables complexes, et l'on obtient une solution particulière et limitée.Les résultats indiquent que si le mátériau qui se trouve de l'autre côte de l'interface est plus rigide que celui dans lequel se trouve la dislocation, la zone de liaison fera office de barrière pour toute dislocation située à une certaine distance de cet interface. Par ailleurs, il n'y a pas de position d'équilibre stable pour une ligne simple de dislocations se trouvent dans le cas susmentionné, puisque la surface libre opère une force d'attraction sur cette ligne.

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Notation a, h location of screw dislocation - b magnitude of Burgers vector of screw dislocation - force acting on the dislocation - F_y y component of - i –1 - i, i unit vectors in x and y directions - half length of bonded segment - L, L bonded segment and traction free one on real axis - S⁺, S^– upper and lower half-planes in z plane - u, v, w carte sian displacement components - x, y, t cartesian coordinates - X(z) (z+)^–1/2(z–)^–1/2 - z x+iy - h+ia - eeⁱ z– - tan^–1 [a/(+h)] - tan^–1 [a/(–h)] - µ^* shear modulus - - _* - _xt, _yt shear stress components - _xt ⁽ⁱ⁾ _yt(¹) shear stress components at the location of dislocation due to the interaction with the interface - (z) complex displacement potential - (z) potential as defined in text On leave from Department of Mechanical Engineering, Tohoku University, Sendai, Japan.     

Thermal expansion and isothermal compressibility of solid nitrogen

The coefficients of linear thermal expansion and of isothermal compressibility have been measured for solid nitrogen in the temperature intervals from 4.2° to 40° K and 8° to 40° K, respectively. Length changes of the sample were detected with a resolution of 10 ^–9 cm using a dilatometer of the three-terminal capacitor variety contained inside a pressure vessel. Apparatus and experimental technique are described in detail. The thermal expansion and compressibility of -N₂ increase rapidly with increasing temperature above 20° K, and in the former case the length increase is exponential in 1/T above 24° K. A reproducible value for the volume discontinuity at the - crystallographic transformation at 35.6° K was obtained only if many hours were taken to effect the transformation. The data are discussed in relation to recent experimental and theoretical information obtained from studies of Raman and infrared spectra, nuclear quadrupole resonance, and specific heat. The anomalous thermal expansion is attributed to excited librational and translational states of large amplitude and appreciable anharmonic content. The adiabatic compressibility calculated from the experimental data agrees well with published values from sound velocity experiments. The Grüneisen parameter of -N ₂ is found to be strongly temperature dependent.Research supported by National Science Foundation Grants GP-7772 and GP-15324.     

Bounds for traces in complete intersections and degrees in the Nullstellensatz

In this paper we obtain an effective Nullstellensatz using quantitative considerations of the classical duality theory in complete intersections. Letk be an infinite perfect field and let f₁,...,f n–rk[X₁,...,X_n] be a regular sequence with d:=max_j deg f_j. Denote byA the polynomial ringk [X₁,..., X_r] and byB the factor ring k[X₁,...,X_n]/(f₁,...,f_n r); assume that the canonical morphism AB is injective and integral and that the Jacobian determinant with respect to the variables X_r+1,...,X_n is not a zero divisor inB. Let finally B^*:=Hom_A(B, A) be the generator of B^* associated to the regular sequence.We show that for each polynomialf the inequality deg (¯f) dⁿ r(+1) holds (¯fdenotes the class off inB and is an upper bound for (n–r)d and degf). For the usual trace associated to the (free) extensionA B we obtain a somewhat more precise bound: deg Tr(¯f) dⁿ r degf. From these bounds and Bertini's theorem we deduce an elementary proof of the following effective Nullstellensatz: let f₁,..., f_s be polynomials in k[X₁,...,X_n] with degrees bounded by a constant d2; then 1 (f₁,..., f_s) if and only if there exist polynomials p₁,..., p_sk[X₁,..., X_n] with degrees bounded by 4n(d+ 1)ⁿ such that 1=_ip_if_i. in the particular cases when the characteristic of the base fieldk is zero ord=2 the sharper bound 4ndⁿ is obtained.Partially supported by UBACYT and CONICET (Argentina)     

Tests of Predictive Viscosity Models for Pure Liquids

It is of considerable importance to be able to predict accurately the viscosity of liquids over a wide range of conditions. In the present work, the ability of the three-parameter generalized corresponding states principle (GCSP) for the prediction of the viscosity of pure liquids is demonstrated. The viscosity of six different classes of pure liquids, viz., alkanes (19 compounds; 207 data points), cycloalkanes (6 compounds; 74 data points), alkenes (9 compounds; 146 data points), aromatics (4 compounds; 123 data points), alkanols (8 compounds; 89 data points), and esters (4 compounds; 28 data points) have been predicted over a wide range of temperatures using the three-parameter (T _c, P _c, ) GCSP. Five options for the third parameter () were studied, viz., Pitzer's acentric factor , molar mass M, characteristic viscosity *, critical compressibility factor Z _c, and modified acentric factor , in addition to groups Z _c and Z _c being treated as composite third parameters. Pressure effects were neglected. Good agreement between experimental and predicted values of viscosity was obtained, especially with either or * being used as the third parameter. Furthermore, the viscosities of alkanes predicted by the TRAPP method and an empirical, generalized one-parameter model for liquid hydrocarbons provide comparisons with the more accurate GCSP method. The GCSP provides a simple and yet a powerful technique for the correlation and prediction of viscosities of a variety of pure liquids over a wide range of temperatures.     

Growth of Mg2TiO4 single crystals by the floating zone method

Single crystals of the peritectic compound Mg₂TiO₄ up to 8 mm diameter and 80 mm long along the [1 0 0] and [1 1 1] directions, were grown by the travelling solvent float zone (TSFZ) technique using a halogen lamp image furnace. The optimum conditions were as follows: sintering temperature of the charge rod, 1500° C, the growth rate, 2 mm h^–1 and the composition of the charge rod in molar ratio, MgOTiO₂=21.01. The chemical composition of the solvent zone was determined by application of the EPMA technique and the peritectic nature of Mg₂TiO₄ was proved. The refractive index of the grown crystal was 2.05.     

Low-temperature synthesis,pyrolysis and crystallization of tantalum oxide gels

Tantalum oxide gels in the form of transparent monoliths and powders have been prepared from hydrolysis of tantalum pentaethoxide under controlled conditions using different mole ratios of Ta(OC₂H₅)₅C₂H₅OHH₂OHCl. Alcohol acts as the mutual solvent and HCl as the deflocculating agent. For a fixed alkoxide water HCl ratio, the time of gel formation increased with the alcohol to alkoxide molar ratio. Thermal evolution of the physical and structural changes in the gel has been monitored by differential thermal analysis, thermogravimetric analysis, X-ray diffraction, and infrared spectroscopy. On heating to 400 °C, the amorphous gel crystallized into the low-temperature orthorhombic phase -Ta₂O₅, which transformed into the high-temperature tetragonal phase -Ta₂O₅ when further heated to 1450 °C. The volume fraction of the crystalline phase increased with the firing temperature. The -Ta₂O₅ converted back into the low-temperature phase, -Ta₂O₅, on slow cooling through the transformation temperature of 1360 °C, indicating a slow but reversible transformation.