Narrow Results

We consider Wong's equations for a particle with a continuous mass spectrum in a random Yang–Mills field approximating the quantum field at finite temperature. We show that particle time evolution can be approximated by a relativistic diffusion. Kubo's generator of the relativistic diffusion is defined as an expectation value of the square of the Liouville operator.

We study the magnetic moment of leptons in extremely hot universe and superdense media of stars at high temperatures. Anomalous magnetic moment of charged leptons is inversely proportional to its mass, whereas the induced dipole moment of neutral leptons is directly proportional to their mass. Neutral massive point particles exert nonzero magnetic moment as a higher order effect, which is smaller than the anomalous magnetic moment of a charged particle of their same flavor partner. All leptons acquire some extra mass due to their interaction with the medium and affect the magnetic moment accordingly. We compare the contribution to the magnetic moment of various leptons due to their temperature and chemical potential dependent masses. It is shown that the magnetic moment contributions are nonignorable for lighter leptons and heavy neutrinos. These calculations are very important to study the particle propagation in the early universe and in superdense stellar media.

The influence of partial substitution of Nd by Gd on the temperature dependence of the lattice parameters of the optimal doped Nd_1.85Ce_0.15CuO₄n-type superconductor (maximum value of T_c) was studied by using X-ray diffraction measurements function of temperature.

The transition from the structural normal phase (I4/mmm) to the distortion phase (Acam) was evidenced above y = 0.65Gd substitution level in (Nd_1-yGd_y)_1.85Ce_0.15CuO₄. The structural phase transition temperature T_I-A from undistorted phase to the distorted phase changes function of Gd concentration.

The structural phase diagram for thermal controlled distortion by Gd ions, T_I-A = f(y) was obtained, and compared by the superconducting phase diagram T_c(y).

High pressure-high temperature electrical resistivity study on composition-controlled Nd_0.9Ca_0.1Ba₂Cu₃O_7-δ high T_c superconductor (HTSC) is carried out by a four-probe technique using Bridgman anvils. A simple heating coil arrangement is used for heating the samples. Electrical resistivity behavior under pressure (up to a maximum of 8 GPa) at various temperatures (up to a maximum of 523 K) were studied and reported in this paper. Simulation of energy dispersive X-ray diffraction confirms substitution of calcium at the Nd site of Nd-123. Variation of the electrical resistivity under pressure is compared with that of the structural changes and the bulk modulus was determined.

In this study, creep behavior of a CFRP laminate subjected to a constant stress is analyzed based on the time-dependent homogenization theory developed by the present authors. The laminate is a unidirectional carbon fiber/epoxy laminate T800H/#3631 manufactured by Toray Industries, Inc. Two kinds of creep analyses are performed. First, 45° off-axis creep deformation of the laminate at high temperature (100°C) is analyzed with three kinds of creep stress levels, respectively. It is shown that the present theory accurately predicts macroscopic creep behavior of the unidirectional CFRP laminate observed in experiments. Then, high temperature creep deformations at a constant creep stress are simulated with seven kinds of off-axis angles, i.e., θ = 0°, 10°, 30°, 45°, 60°, 75°, 90°. It is shown that the laminate has marked in-plane anisotropy with respect to the creep behavior.

The high-temperature deformation and damage behavior of ultrafine-grained (UFG) Cu produced by equal channel angular pressing (ECAP) were investigated and compared at two strain rates of 10^-2 s^-1 and 10^-3 s^-1. It is found that the strain rate has an obvious effect on the deformation and damage behavior of UFG Cu, especially at the testing temperatures near and above recrystallization. In general, the lower the strain rate, more universally grain coarsening takes place, the lower the yield stress and steady flow stress are, and more aggravated the degree of deformation damage becomes.

With the promotion of fuel economy policy and automobile lightweight concept, ferritic stainless steels applied in vehicles’ exhaust hot end systems have been developed. This paper simulated the high-temperature environment at which the automobile exhaust system serviced in for high-temperature corrosion. Kinetic curves were conducted in isothermal environments at 1000${}^{\circ }$C. X-ray diffraction, scanning electron microscope and energy dispersive spectrometer were used to study the oxidation behavior of ferritic stainless steels and the effects of tungsten (W) addition. The results show that, with increasing oxidation time, the rate of weight gains increase and the main failure is spalling of surface oxide layer. The addition of W has a complicated effect on the oxidation behavior of ferritic stainless steel weldment.

From femtosecond spectroscopy (fs-spectroscopy) of metals, electrons and phonons reequilibrate nearly independently, which contrasts with models of heat transfer at ordinary temperatures ($T>100$ K). These electronic transfer models only agree with thermal conductivity $(k)$ data at a single temperature, but do not agree with thermal diffusivity $(D)$ data. To address the discrepancies, which are important to problems in solid state physics, we separately measured electronic (ele) and phononic (lat) components of $D$ in many metals and alloys over $\sim$290–1100 K by varying measurement duration and sample length in laser-flash experiments. These mechanisms produce distinct diffusive responses in temperature versus time acquisitions because carrier speeds $(u)$ and heat capacities $(C)$ differ greatly. Electronic transport of heat only operates for a brief time after heat is applied because $u$ is high. High $D_{\mathrm{\text{ele}}}$ is associated with moderate $T$, long lengths, low electrical resistivity, and loss of ferromagnetism. Relationships of $D_{\mathrm{\text{ele}}}$ and $D_{\mathrm{\text{lat}}}$ with physical properties support our assignments. Although $k_{\mathrm{\text{ele}}}$ reaches $\sim 20\times k_{\mathrm{\text{lat}}}$ near 470 K, it is transient. Combining previous data on $u$ with each $D$ provides mean free paths and lifetimes that are consistent with $\sim 298$ K fs-spectroscopy, and new values at high $T$. Our findings are consistent with nearly-free electrons absorbing and transmitting a small fraction of the incoming heat, whereas phonons absorb and transmit the majority. We model time-dependent, parallel heat transfer under adiabatic conditions which is one-dimensional in solids, as required by thermodynamic law. For noninteracting mechanisms, $k\cong \mathrm{\Sigma }{C}_i{k}_i\mathrm{\Sigma }{C}_i/(\mathrm{\Sigma }{C}_i^2)$. For metals, this reduces to $k=k_{\mathrm{\text{lat}}}$ above $\sim$20 K, consistent with our measurements, and shows that Meissner’s equation $(k\cong k_{\mathrm{\text{lat}}}+k_{\mathrm{\text{ele}}})$ is invalid above $\sim$20 K. For one mechanism with multiple, interacting carriers, $k\cong \mathrm{\Sigma }{C}_i{k}_i/(\mathrm{\Sigma }{C}_i)$. Thus, certain dynamic behaviors of electrons and phonons in metals have been misunderstood. Implications for theoretical models and technological advancements are briefly discussed.

The compound Cu₂Se has been synthesized and its crystal structure has been studied. Structural studies were carried out by angle dispersive XRD method. Structural phase transition was detected by analyzing the spectra obtained in the temperature range $T=290$–1000K. It was found that, under normal conditions, this compound has an orthorhombic crystal structure. At a temperature $T=407$K, a phase transition occurs from orthorhombic to cubic with the formation of a highly symmetrical phase. The values of crystallographic parameters were determined for each phase.

To avoid the delamination of bimorph actuator and enhance the performance of the room-temperature type functionally graded (RTFG) piezoelectric bending actuator, the high temperature type FG (HTFG) piezoelectric bending actuator was designed and fabricated. The material compositions with different dielectric and piezoelectric constants were selected from the Pb(Ni_1/3N_b2/3)O₃-PbZrO₃-PbTiO₃ (PNN-PZ-PT) family, and used as the five layers in the HTFG piezoelectric actuator. Compared with the FG actuator, the HTFG actuator has advantages for applications at high temperature. The durability of the fabricated HTFG piezoelectric actuators was measured in a vibration test and compared with that of the bimorph actuator to evaluate the improvement of performance. The results show that the durability of the HTFG piezoelectric actuators is much higher than that of the bimorph actuator.

Alumina thin films deposited by electron beam (EB) evaporation are investigated with regard to their performance in high-temperature electrical insulators. The most important application is high-temperature sensors. The leakage behavior of EB-evaporated alumina thin films is investigated by analyzing the temperature dependence of the I–V characteristics of alumina thin films deposited on Pt/n-Si(100) substrates. The temperature is extending in the range from 300 K up to 1273 K. The results show that ln(J) increases linearly with the increasing electric field at high-temperature range, the trap depth of $q{\varphi }_B$ is 280 meV, the conductivity increases with the increasing temperature, while the resistivity decreases with the increasing temperature.

The temperature range of $T$ = 77–770 K in the system alloys: Holl coefficient $(R)$, thermo-emf $(S)$, electric conductivity $(\sigma )$, measured $d$-density of components and analyzed. It has been established that samarium additive atoms contain donor-type properties and the effectiveness increases with the temperature increase: up to 40% proportional to $T\le 440$ K in $p$-type specimens, whereas in $n$-type samples this increase is higher and covers the contents of pH varying from $p$ to $n$. An electrical conductivity of compounds increased due to the carrier activation with further increase of temperature. The activation energy of carriers at low temperatures ($T\le 400$ K) is $\mathrm{\Delta }{E}_g\approx 0.37$ eV for $x=0.05$ mol.% and $x=0.10$ mol.% compounds at $T$ = 77–320 K and for $x=1.00$ mol.% and $x=3$ mol.% compounds are $\mathrm{\Delta }{E}_g\approx 0.39$ eV. $\sigma (T)\approx$ const at $T=300$–400 K for $x=0.05$ mol.% and $x=0.10$ mol.% compounds, and passing with minimum increases at $T$ = 400–500 K.

${\mathrm{\text{BaFe}}}_{11.1}{\mathrm{\text{Sc}}}_{0.9}{\mathrm{\text{O}}}_{19}$ hexaferrite compound was synthesized, the crystal structure was studied by the X-ray diffraction method. Powder samples were prepared from the mark “analytical grade” oxides ${\mathrm{\text{Fe}}}_2{\mathrm{\text{O}}}_3$, ${\mathrm{\text{Sc}}}_2{\mathrm{\text{O}}}_3$ and carbonate BaCO₃. It was found that under normal conditions and at room temperatures, the crystal structure of this compound has hexagonal symmetry with the P6₃mmc space group. The lattice parameters correspond to the values: $a=5.8842\mathrm{\text{Å}}$ and $c=23.1774\mathrm{\text{Å}}$. Atomic dynamics of ${\mathrm{\text{BaFe}}}_{11.1}{\mathrm{\text{Sc}}}_{0.9}{\mathrm{\text{O}}}_{19}$ hexaferrite in the range of low temperatures of $-190^{\circ }\mathrm{\text{C}}\le T\le 20^{\circ }\mathrm{\text{C}}$ and high temperatures of $20^{\circ }\mathrm{\text{C}}\le T\le 490^{\circ }\mathrm{\text{C}}$ were studied. As a result of the analysis of the spectra obtained by the Raman spectroscopy method, it was determined that the structural phase transition does not occur in this compound over a wide temperature range $-190^{\circ }\mathrm{\text{C}}\le T\le 490^{\circ }\mathrm{\text{C}}$. At the room temperature, the obtained Raman modes are observed at both low and high temperatures.

The crystal structure and thermal properties of the ${\mathrm{\text{Ca}}}_{0.5}{\mathrm{\text{In}}}_{1.5}{\mathrm{\text{Se}}}_3$ compound have been investigated. Structural studies were performed by X-ray diffraction at room temperature. The crystal structure of this compound was found to correspond to the hexagonal symmetry of the space group P6₁. Thermal properties were studied using a differential scanning calorimetry (DSC). It was found in the temperature range $25^{\circ }\mathrm{\text{C}}\le T\le 950^{\circ }\mathrm{\text{C}}$ that thermal effects occur at temperatures $T_1=102^{\circ }\mathrm{\text{C}}$ and $T_2=848^{\circ }\mathrm{\text{C}}$. The thermodynamic parameters of these effects are calculated.

Calcium carbonate whisker (CW) can work as a cost-effective and environment friendly micro-fiber in reinforcing cementitious composites. Influence of high temperature on micro-structure of CW reinforced cement paste by nanoindentation and mercury intrusion porosimetry test is studied in this research. Up to 500${}^{\circ }$C, the indentation depth, elasticity modulus, indentation hardness and interfacial transition zone (ITZ) width of CW reinforced cement paste are near or even better than that at room temperature, due to the coupling effect of CW transformation from aragonite to calcite and internal autoclaving. However, when the temperature is higher than 700${}^{\circ }$C, nano-mechanical properties of CW reinforced cement paste degenerated significantly, due to the decomposition of CW and hydration products. Similarly, with the increase of temperature up to 400${}^{\circ }$C, the porosity and pore size increase little or even decrease, while the fractal dimension of pore volume increases. With the introduction of CW, the pore parameters and fractal dimension are decreased up to 400${}^{\circ }$C, due to the filler effect of CW. When the temperature is higher than 700${}^{\circ }$C, the pore diameter and fractal dimension of CW reinforced cement paste are significant higher than that of pure cement paste, due to the decomposition of CW and hydration products. In CW reinforced cement paste, the fractal dimension was increased with the increased temperature and porosity in this research. There are negative correlations between the pore volume fractal dimensions and the strengths of CW reinforced cement paste. Fractal dimension is a useful tool to evaluate the change of pore structure at high temperature.

This study experimentally estimated the fractal pore size distribution of granites, which is then linked to the hydro-mechanical properties of rocks treated by temperatures that range from 25${}^{\circ }$C to 900${}^{\circ }$C. The mercury intrusion experiment was carried out to characterize the pore size distributions and the MTS815.02 triaxial testing system was used to investigate hydro-mechanical properties of rocks. Finally, the micro-CT scanning system and scanning electron microscope system were employed to exhibit the evolutions of microstructures of cracks that were then linked to the macroscopic hydro-mechanical properties. The results show that the pore size distribution of granites follows the fractal scaling law and the fractal dimension ranges from 2.45 to 2.94. The fractal dimension decreases significantly when the temperature increases from 25${}^{\circ }$C to 100${}^{\circ }$C and then holds a constant with continuously increasing the temperature to 400${}^{\circ }$C. The fractal dimension slightly increases as the temperature increases from 400${}^{\circ }$C to 500${}^{\circ }$C and decreases following a linear relationship until the temperature of 900${}^{\circ }$C. The porosity obtained by the mercury intrusion experiment shows an exponential relationship with the fractal dimension. Both the axial stain and peak total stain, as well as the initial permeability and the permeability at the peak axial stress, have quadratic functions with the fractal dimension. The mean aperture of fractures increases from 56.75$\mu$m to 76.10$\mu$m with increasing the temperature from 100${}^{\circ }$C to 900${}^{\circ }$C through micro-CT scanning. The scanning electron microscope test clearly shows that the fractures are generated when the temperature exceeds 300${}^{\circ }$C, which agrees well with the micro-CT scanning results. With increasing the temperature from 400${}^{\circ }$C to 900${}^{\circ }$C, both the number and aperture of fractures increase, which well interprets that both the strain and permeability increase as indicated by the triaxial stress-flow tests.

The present paper reviews various types of coatings, including platinum, platinum alloys, palladium, ruthenium, iridium, nickel, nickel alloys and composite coatings, on superalloy substrates using electrodeposition method. Attempts were carried out to represent an overall view of plating conditions and electrolyte and highlight the importance of the layer regarding to the performance of high-temperature coatings applied on superalloys, which is extensively used on gas-turbine components.

The present work investigates the effects of heat treatment on friction and wear behavior of electroless Ni–B coatings at elevated temperatures. Coating is deposited on AISI 1040 steel specimens and subjected to heat treatments at 350${}^{\circ }$C, 400${}^{\circ }$C and 450${}^{\circ }$C. Coating characterization is done using scanning electron microscope, energy dispersive X-Ray analysis and X-Ray diffraction analysis. Improvement in microhardness is observed for the heat treated deposits. Further, the effect of heat treatment on the tribological behavior of the coatings at room temperature, 100${}^{\circ }$C, 300${}^{\circ }$C and 500${}^{\circ }$C are analyzed on a pin-on-disc setup. Heat treatment at 350${}^{\circ }$C causes a significant improvement in the tribological behavior at elevated temperatures. Higher heat treatment temperatures cause deterioration in the wear resistance and coefficient of friction. The wear mechanism at 100${}^{\circ }$C is observed to be predominantly adhesive along with abrasion. While at 300${}^{\circ }$C, abrasive wear is seen to be the governing wear phenomenon. Formation of mechanically mixed layers is noticed at both the test temperatures of 100${}^{\circ }$C and 300${}^{\circ }$C for the coatings heat treated at 400${}^{\circ }$C and 450${}^{\circ }$C test temperature. The predominant wear mechanisms at 500${}^{\circ }$C are abrasive and fatigue for as-deposited and heat treated coatings, respectively.

This study considers high temperature tribological behavior of Al-TiB₂ aluminum matrix composites (AMCs) fabricated through stir cast method. Effect of operating temperature on wear and friction behavior is studied for four different weight percentages of reinforcements using a high-temperature tribo-tester over a temperature range of 50${}^{\circ }$C–250${}^{\circ }$C under normal load in the range of 25–75N. Surface morphology and wear debris are studied through scanning electron microscopy (SEM) images. Energy dispersive X-ray (EDAX) and X-ray diffraction (XRD) studies are performed to observe the wear mechanism at high temperature. Increase in the amount of reinforcement improves wear resistance of composites at all temperatures. Friction and rate of wear are found to increase with operating temperature. Formation of oxide layers and softening of the surface are found to play crucial role in controlling the tribological behavior of stir cast Al–TiB₂ composites at high operating temperatures.

Electroless nickel coatings containing Mo possess higher thermal stability in comparison with the binary alloy variants. In a quest to achieve enhanced thermal stability of Ni–B coatings, Mo is incorporated to obtain a ternary Ni–B–Mo coating. The coatings are deposited on AISI 1040 steel and characterized using energy dispersive X-ray (EDX) analysis, X-ray diffraction method and scanning electron microscope. The coatings are observed to lie in the mid-B range with amorphous structure in as-deposited condition. On heat treatment, precipitation of crystalline Ni and its borides is observed. The typical cauliflower-like surface morphology of the deposits could be observed in scanning electron micrographs. Microhardness measurements reveal the enhanced thermal stability of Ni–B–Mo coatings. Tribological behavior of Ni–B–Mo coatings at room and elevated temperatures (100^∘C, 300^∘C and 500^∘C) is observed on a pin-on-disc type tribo-tester by varying the applied normal load (10–50N) and rotational speed of the counterface disc (60–100rpm). The purpose of the present work is to observe the tribological behavior and associated tribo-mechanisms at different temperatures under dry sliding condition. In general, the wear of the coatings increases with an increase in applied normal load and speed at room temperature, 100^∘C and 300^∘C. At 500^∘C, the wear increases with load but with speed it first increases up to 80rpm and then decreases. The COF does not show a similar behavior like the wear with varying load and speed at different temperatures. Instead, it is controlled by the accompanying wear mechanisms, formation of oxide debris and oxide layers of Ni and Mo. The worn surface of the coatings is examined using scanning electron microscope and EDX analysis. Back scattered images of wear tracks of Ni–B–Mo coatings at the highest levels of load (50N) and speed (100rpm) at different temperatures further reveal the oxide formation and tribochemical reactions.