Solid-state wetting transitions at grain boundaries

A previously developed model of the dependence of grain boundary (GB) segregation on GB character has been exercised to investigate solid-state wetting transitions at GBs, and their anisotropy. In the case of binary systems displaying a solid-state miscibility gap, it is shown that the wetting transition temperature for precipitates at a GB is anisotropic, and is inversely related to GB energy. The model also allows calculation of prewetting transitions and associated excursions in adsorption off phase coexistence. These transitions are first order below a prewetting critical point (T_PWC), and higher order at temperatures above T_PWC. Investigation of the prewetting behavior provides the means for construction of the two-dimensional phase diagram of a GB.     

Analysis of the melting thermograms of thin refractory metal plates heated by laser radiation with harmonically modulated intensity

The results of experimental and theoretical investigations show the possibility of identifying the solid-liquid phase transition in a refractory metal plate heated by laser radiation with harmonically modulated intensity. Thermograms measured in a single-phase region of a heated sample surface display a harmonic component with a constant amplitude and frequency, which exhibits degeneracy in a two-phase state when the melting takes place. At temperatures in the vicinity of the melting point T _m, the thermograms exhibit lower (T < T _m) and upper (T > T _m) half-waves of variable amplitudes, with intermediate short intervals corresponding to T = T _m. The time periods featuring the lower and upper half-waves are separated by a longer interval corresponding to a two-phase state (T = T _m). The results of analytical calculations qualitatively agree with experimental data.     

Creep properties of sintered copper steel

The creep properties of sintered steel can be described by the normal creep laws. In particular, the activation energy for creep can apparently be identified with that for self diffusion even at temperatures far below 0.5 T _m. As might be expected the creep rupture strength of sintered steel is only a small percentage of its ultimate tensile strength. The creep rupture strength depends largely on the ductility and hence better strengths can be expected from alloys with larger ductilities.     

Superplastic flow in nanocrystalline and sub-microcrystalline yttria-stabilized tetragonal zirconia

Tensile deformation of nanocrystalline ZrO₂ + 5 mol% Y₂O₃ at temperatures in the range of 1283–1403 K is described. It is demonstrated (a) that steady state flow is possible at temperatures of the order of 0.42 T _m, where T _m is the absolute melting point, (b) that 70% engineering strain could be obtained at 1403 K (0.46 T _m), and (c) that significant grain boundary sliding was present during deformation. Static and dynamic grain growth as also a decrease in the relative density of the specimen with deformation could be observed. The present results as well as those of Owen and Chokshi concerning superplastic flow in sub-microcrystalline materials taken from literature could be accounted for quantitatively using the grain boundary sliding controlled flow model of Padmanabhan and Schlipf, originally proposed for microcrystalline superplastic alloys.     

Effect of the Consolute Point of Isobutyric Acid+Water on the Rate of an SN1 Hydrolysis Reaction

The rate constant for the S _N 1 hydrolysis of 2-chloro-2-methylbutane was measured near the consolute point of the liquid mixture isobutyric acid+water. At temperatures far from the upper critical solution temperature, T _c, of this mixture, the rate constant obeyed the Arrhenius equation. In the one-phase region just above T _c, however, the rate constant decreased below the Arrhenius background, while in the two-phase region below T _c, it increased. This combination of slowing-down with speeding-up appears to be beyond the scope of explanation of the current theories of dynamic critical slowing-down.     

Deformation behaviour of single crystals of InP in uniaxial compression

The deformation characteristics of indium phosphide (InP) single crystals under uniaxial compression have been examined as a function of strain rate, temperature and orientation. It has been shown that at temperatures below 0.55T _m (T _m=melting point; 1335 K) the material fractures in a brittle manner whereas at higher temperatures, within the range 0.55 to 0.71T _m, plastic deformation occurs by both slip and deformation twinning; above 0.71T _m, slip alone is the operative deformation mechanism. The observed operative slip systems are of the type {1 1 1} 0 1 1 which are characteristic of most Group IIIb-Group Vb compounds. Deformation twinning occurs predominantly on {1 1 1} planes but some activity is also observed on planes of the type {3 4 5}.     

Nonexponential Behavior Near the Critical Point of an Ionic Micellar System

The coexistence curves of the system dodecylammonium chloride+water+KCl have been obtained for different salt concentrations. We found that the asymptotic behavior of the order parameter can be describe using extended scaling and the usual Ising value for the =0.325 exponent. The static light scattering data for the critical composition are also compatible with the Ising value v=0.63. Dynamic light scattering results have been obtained near the liquid–liquid critical point. The correlation functions have been found to be single-exponential for temperatures well above critical, while a second decay process at longer times becomes evident for T–T _c<4K. These correlation functions can be fitted to the sum of two exponential functions. The diffusion coefficient associated with concentration fluctuations has been calculated from both relaxation modes. It has been analyzed in terms of the predictions of the mode-coupling theory. These results, together with correlation length and viscosity data, are in good agreement with the calculations.     

Deposition rates of titanium nitride plates prepared by chemical vapour deposition of TiCl4+NH3 system

Titanium nitride plates (TiN_x,x = 0.74–1.0, about 2 mm thick maximum) were prepared by chemical vapour deposition (CVD) using TiCI₄, NH₃ and H₂ as source gases. The effects of CVD conditions, i.e. gas molar ratio (m _N/Ti = NH₃/TiCI₄) and deposition temperature (T_dep), on deposition rates and surface morphology were examined, and the deposition mechanism of the CVD-TiN_x plates was discussed. The relationship between m_N/Ti and deposition rates showed a maximum peak at certainm _N/Ti, and this maximum peak shifted to lowerm _N/Ti with increasing_{T dep}. The activation energy for the formation of CVD-TiN_x plates was about 80 kJ mol^–1 in the lower temperature range. The decomposition reaction of NH₃ gas could be associated with the rate-controlling step. At higher temperatures, the diffusion process may be the rate-controlling step, and a large amount of powder (mainly NH₄Cl) was formed in the gas phase. The highest deposition rate obtained in the present work was 1.06×10^–7 ms^–1 (0.38 mmh^–1) atT _dep = 1773 K andm _N/Ti = 0.87.     

Anomalous creep behaviour of a Ni-22 at % Cu alloy and the miscibility gap

The purpose of this study is to understand the anomalous creep behaviour of Ni-22 at % Cu alloy at the suggested critical miscibility gap temperature, below 598 K (0.36T _m). The Cu-Ni system is classified as a class II solid solution at temperatures above 0.4T _m, and it is also experimentally verified by the authors that the characteristic creep behaviour of the alloy used for this work is that for a class II solid solution. However, at low temperatures, this particular alloy shows different creep behaviours, with small stress increment in the steady state, sigmodial creep deformation is observed while with large stress increases normal primary creep occurs. When unloading the stress during creep and ageing at the test temperature, no softening due to recovery is observed but the same creep rate is achieved. The activation energy of the creep for the quenched and aged specimen is anomalously high, 326 kJ mol^–1, however, for the annealed specimen it was 167 kJ mol^–1 which is the same for that of pipe diffusion. On the basis of the observed experimental results and proper analysis, it is hypothesized that, at the test temperature, the possible formation of the solute clustering is responsible for the high activation energy and stress exponent for the creep deformation. Using the mechanical testing, creep test, it is experimentally verified that Cu-Ni system has a miscibility gap at low temperature.     

Resistivity of Lightly Doped Ferromagnetic Semiconductors

In metallic magnets with a low carrier density, scattering from magnetic fluctuations above and near the transition temperature T _c provides a large contribution to the electrical resistance. Because the fluctuations can be suppressed by a magnetic field, a large negative magnetoresistance ensues. In a simple model, we find the low field magnetoresistance scales with the ratio of field induced magnetization m(H) to the saturation magnetization m _sat: /=((T, 0)–(T, H))/(T, 0)C(m/m _sat)². At very low carrier densities magnetic polarons should form in a range of temperatures above T _c. The CMR perovskite manganites cannot be explained without strong coupling of the magnetic order to lattice distortions (of the Jahn–Teller type) above T _c.     

Mixed state thermoelectric power of vanadium substituted 2223 (Bi, Pb) superconducting polycrystalline cuprates

Measurement of the mixed state thermoelectric power of pure and vanadium substituted 2223 pellets of the type (Bi_0.6Pb_0.2–y V _y )₂Sr₂Ca₂Cu₃O _x are presented for temperatures between 77 and 120 K. Mixed state Seebeck voltages for both pure and vanadium substituted samples show almost similar features. Our data forT>0.8T _c could be explained, attributing the observed dissipation to the normal quasiparticle excitations. For temperatures below 0.8T _c , we argue that the dominant contribution to the mixed state Seebeck voltage may originate from the dissipation induced by the fluctuations of the order parameter across the internal weak links.     

Pinning Effects in Vortex States of High-T c Superconductors: Monte Carlo Simulations

Large scale Monte Carlo simulations are performed in order to clarify thermodynamic properties of interlayer Josephson vortices induced by a magnetic field parallel to the ab plane of high-T _c superconductors. It is revealed that there is a tricritical point in the B–T phase diagram, so that for magnetic field above (below) this point the normal to superconductivity transition is continuous (discontinuous). The critical point is described by the product of the magnetic field and the c-axis anisotropy parameter. The origin of the change in the nature of phase transition is the intrinsic pinning effect of the layer structure.     

Morphology and preferred orientation of titanium nitride plates prepared by chemical vapour deposition

Thick titanium nitride (TiN _x ; x = 0.74–1.0) plates (up to 2 mm thick) were prepared by chemical vapour deposition using TiCl₄, NH₃ and H₂ as source gases at a total gas pressure, P _tot, of 4 kPa, deposition temperatures, T _dep, from 1373–1873 K, and NH₃/TiCl₄, m _N/Ti, gas molar ratio from 0.17–1.74. The effects of deposition conditions on morphology, preferred orientation and composition of CVD-TiN _x plates were investigated. Surface morphology changed from faceted to nodular texture with increasing m _N/Ti and T _dep. The faceted and nodular deposits showed columnar and shell-like fracture cross-sections, respectively. The composition (x = N/Ti) increased with increasing m _N/Ti and T _dep below m _N/Ti = 1.0, and was constant above m _N/Ti = 1.0. Three kinds of preferred orientations were observed: (100) orientation at low T _dep, (110) orientation at intermediate T _dep and low m _N/Ti, and (111) orientation at high T _dep and high m _N/Ti. This tendency is discussed thermodynamically, and explained as being due to changes in the degree of supersaturation in the gas phase.     

Residual crystallinity of vapor-deposited tin films

Using transmission electron microscopy and selected-area electron diffraction techniques, the residual crystallinity of vapor-deposited tin thin films has been investigated. At substrate temperatures above about 0.8T_m, liquid-like particles were observed to be single crystals, with no evidence of grain boundaries or related crystallographic inhomogeneities. A significant portion of the single-crystal particles exhibited epitaxial features, and they appeared to have solidified by a liquid-solid phase transformation beginning at the particle-substrate interface and propagating in the [001] direction, with the (00$\text{1}$) plane facing the (001) NaCl substrate. The observation of crystal facets supports this model. For substrate temperatures of approximately 0.7T_m or below, the tin films were composed of island structures. An examination of the crystallographic and morphological features of the tin films suggests a coalescence mechanism consistent with that outlined by Pocza. For substrate temperatures greater than 0.8T_m, coalescence is dominated by liquid drops with liquid drops. Below 0.8T_m, the mechanism shifts from one of liquid drops with crystallites to one dominated primarily by the coalescence of crystallites with crystallites below 0.6T_m.     

Texturing by cooling a metallic melt in a magnetic field

Abstract

Processing in a magnetic field leads to the texturing of materials along an easy-magnetization axis when a minimum anisotropy energy exists at the processing temperature; the magnetic field can be applied to a particle assembly embedded into a liquid, or to a solid at a high diffusion temperature close to the melting temperature or between the liquidus and the solidus temperatures in a region of partial melting. It has been shown in many experiments that texturing is easy to achieve in congruent and noncongruent compounds by applying the field above the melting temperature T_m or above the liquidus temperature of alloys. Texturing from a melt is successful when the overheating temperature is just a few degrees above T_m and fails when the processing time above T_m is too long or when the overheating temperature is too high; these observations indicate the presence of unmelted crystals above T_m with a size depending on these two variables that act as growth nuclei. A recent model that predicts the existence of unmelted crystals above the melting temperature is used to calculate their radius in a bismuth melt.     

Vacancy Assisted Motion of Charges in Quantum Crystals

From comparison of results of measurements of diffusion coefficients D(T) of the positively and negatively charged complexes (charges), created under irradiation in perfect crystals, grown from pure helium or hydrogen at small pressures, with diffusion coefficients of the isotopic impurities or the self-diffusion coefficients known from NMR studies one can conclude that motion of the more mobile charges through these crystals (of positive charges in HCP ⁴ He, ³ He and D ₂ samples and of negative charges in BCC ⁴ He and ³ He and also in HCP crystals, grown from pure p–H ₂ ) is vacancy assisted. Thus strong departures of the temperature dependencies of diffusion coefficients of the positive charges D ₊(T) in HCP ⁴ He samples and of the negative charges D _–(T) in p–H ₂ samples from the simple Arrhenius type of behavior D= D ₀ exp[–G/T] at low temperatures can be attributed to the change of the mechanism of diffusion of thermal vacancies: from classical thermally activated hopping of the localized vacancies near T _melt to the band motion of delocalized vacancions at T< T _melt/2. To explain the nature of the maxima on the D ₊(T) curves observed in perfect ⁴ He crystals, it may be assumed that the flow conditions of the vacancion gas around a positive charge (a probe particle with an effective radius of a few lattice constants) can change significantly with lowering the temperature: from a hydrodynamic flow of the viscous gas round the probe at the transition temperatures to a kinetic flow of the rarefied vacancion gas at low temperatures. In this case the bandwidth of the vacancions in studied ⁴ He samples is near Q _V 10^–4 K.     

Molecular Dynamics Calculation of Critical Point of Nickel

The critical point of nickel and the phase diagram near the critical point are numerically evaluated using molecular dynamics (MD) computations. Thermodynamic states on the phase diagram are calculated for a homogeneous material at equilibrium states. Isothermal lines on p–v diagrams are constructed at temperatures below and above the critical temperature, and the liquid-gas coexistence lines and regimes are obtained. The critical point of nickel is obtained as T _c = 9460± 20 K, ρ _c = 2560± 100 kg· m⁻³, and p _c = 1.08± 0.01 GPa. The method used in this work can be used to estimate thermodynamic properties of other materials at high temperature/pressure.     

The melting points of ultralong paraffins and their homologues

Because of the recent availability of the melting points of several ultralong normal paraffins, the melting behavior of normal paraffins has been investigated. Taking the melting point of polyethylene to represent the melting point of an ultralong paraffin, a new function has been established to represent the melting points of alkanes from the carbon number 32 onwards. Adopting the same value for the limiting melting point of an ultralong paraffin, equations are derived for the melting points of several homologous series.Nomenclature a A constant to be determined - b A constant to be determined - m Number of methylene groups in the molecule - n Number of carbon atoms in the molecule - n ^* Number of carbon atom above which Eq. (6) is applicable - T ₀ Temperature constant, K - T _c Critical temperature, K - T _c Critical temperature of an ultralong normal paraffin, K - T _b Normal boiling point, K - T _b Normal boiling point of an ultralong normal paraffin, K - T _m Melting point, K - T _m Melting point of an ultralong paraffin, K - Standard deviation {[(T _m (obsd)–T _m (calc)]²/(No. points–No. parameters)}^0.5     

Anomalous temperature dependence ofD andT 2 for dilute solutions of3He in solid4He

Results of measurements of the spin diffusion coefficientD and NMR relaxation timesT ₁,T ₂, and T_1p are presented for a range of fractional³He concentrations 1 × 10^–4x ₃2.5 × 10^–3 in solid⁴He at molar volumes 19.85V _m21.0 cm³ and temperatures 0.4<T<2 K in both hcp and bcc phases. We observe a minimumD(T) atx ₃=5×10^–4, which is interpreted in terms of a transition from coherent impuriton motion to thermally activated diffusion. ForT<0.8 K, (lnD)/(lnV _m)=60±8. TheT ₂ measurements show a minimum as a function of temperature forx ₃10^–3. TheT ₂ (T) andT ₁ (T) results yield values for activation energy and tunneling frequency of vacancies in these dilute solutions. Forx ₃=5×10^–4 andT 0.5 K,T ₂ (V _m) is anomalous.T ₁ measurements at the same concentration indicate there is an important contribution to the spectral density of dipole field fluctuations in the kHz region.Financial support provided for apparatus, materials, and a research studentship (ARA) by the Science Research Council.     

Devitrification and vitrification of tellurite glasses

The crystallization of pure tellurite glass during various heating rates was studied. The activation energy for crystallization was 115 × 10²² eV mol^–1. The glass transformation, T _g, starting crystallization, T _x, crystallization, T _c and melting temperatures, T _m, have been reported for binary tellurite glasses of the form (1 – x) TeO₂–xA_nO_m [A_nO_m = MnO₂, Co₃O₄ and MoO₃]. Among many different parameters of the glass forming potential the two-thirds rule, T _g/T _m, the glass stabilization range, T= T _x – T _g, and the glass forming tendency, K _g= (T _c– T _g)/(T _m – T _c), are reported for the first time for tellurite glasses.