The electrical conductivity and Hall effect of glassy carbon

The electrical conductivity and Hall effect of glassy carbon heat treated for three hours between 1200 and 2700°C was measured at temperatures from 3 to 300 K in magnetic fields up to 5 tesla.The electrical conductivity, of the order of 200 (ohm cm)^?1 at room temperature, can be empirically written $\text{σ = A + B}\text{exp}\text{(?CT}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}\text{)-DT}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$, where the first term is a strongly scattering metallic component, the second term is attributed to variable range hopping, and the third and new term is negative correction to the metallic conductivity associated with one-dimensionality. All of the constants A, B and C were insensitive to heat-treatment temperature; the constant D decreased with increasing temperature until it disappeared at about 2200°C.The Hall coefficient was independent of magnetic field, insensitive to temperature, but was a strong function of heat-treatment temperature, crossing over from negative to positive at about 1700°C and ranging from ?0.048 to 0.126 cm³/coul.The idea of one-dimensional filaments in glassy carbon suggested by the electrical conductivity is compatible with the present consensus view of the microstructure constructed through such means as lattice imaging in transmission electron microscopy, and X-ray diffraction and small angle scattering.     

Thermal conductivity of ZnSe by molecular dynamics simulation

Molecular dynamics simulation using the new fit of the empirical Tersoff Potential is applied to study the thermal properties of ZnSe at and below the room temperature. The resulting diffusivity and thermal conductivity are compared with the photoacoustic measurements on the grown ZnSe crystal. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal conductivity and specific heat of tellerium-based chalcogenide glasses

The specific heat and thermal conductivity of tellerium-based chalcogenide glasses are reported, together with their glass transition and synthesizing temperatures. The thermal conductivity has been measured in the temperature range between 100 and 500 K while the specific heat was determined at temperatures between 373 and 600 K. Below the glass transition temperature, both physical parameters were observed to be temperature independent.     

The effect of copper and silver on the conductivity and photoelectric properties of glassy arsenic selenide

A study was made of the effect of copper and silver on the electric properties, photoconductivity and its kinetics in glassy arsenic selenide.     

Ac conductivity of Ag-doped bulk,glassy As2S3

Measurements of the electrical conductivity of Ag-doped bulk As₂S₃ glasses have been made as functions of temperature, pressure, frequency and Ag doping level. A Debye-like loss peak was observed near 10⁴ Hz. The frequency of the loss peak is dependent on temperature, pressure and doping level, but these dependences are different from those of the dc conductivity. The ac loss is attributed to the Maxwell-Wagner losses characteristic of inhomogeneous materials. The materials are presumed to be inhomogeneous mixtures of As₂S₃ and Ag₂S. We have also searched unsuccessfully for ac conductance in several bulk chalcogenide glasses.     

Thermal conductivity of mixed alkali silicate glasses at low temperature

Thermal conductivity of glass is one of the fundamental properties of it. However, that has not been studied enough. That of only less than 20 compositions has been measured below the room temperature. In this study, we measured the thermal conductivity of xNa₂O · (100 ? x)SiO₂ and (33 ? y)Na₂O · yCs₂O · 67SiO₂ glasses by a transient heating method in the temperature range from about 150 K to room temperature. The conductivity of xNa₂O · (100 ? x)SiO₂ is found to decrease with the increase in alkali content. The dominant factor of this behavior is the decrease in phonon mean free path, which is due to the increase of non-bridge oxygen. Thermal conductivity of (33 ? y)Cs₂O · yNa₂O · 67SiO₂ is decreased with the increase in Cs₂O/(Na₂O + Cs₂O) ratio. The dominant factor of this behavior is the decrease of sound velocity. However, composition dependence of the phonon mean free path also affects the thermal conductivity. Phonon mean free path of 33Cs₂O · 67SiO₂ glass is longer than that of 33Na₂O · 67SiO₂ glass, and should be related to the change in distribution of structural unit in glass. In addition, phonon mean free path of mixed alkali glasses are shorter than that of single alkali glasses.     

Thermal conductivity and refractive indices of Nd:GdVO4 crystals

The thermal conductivities of Nd:YAG, M(Y,Gd)VO₄ crystals were measured at 298 K. The value of Nd:GdVO₄ crystal along <001> direction was 11.4 W/mK, which was higher than that of YAG crystal measured to be 10.7 W/mK. The principal refractive indices of Nd:GdVO₄ crystal in the temperature range from 20 °C to 170 °C were determined by auto‐collimation method. Based on the measured values of refractive indices, the Sellmeier equation and expression of temperature dependence of refractive indices have been obtained. The measured results show that the birefringence Δn is 0.22007 at 20 °C and temperature coefficient of birefringence is 4.33 × 10⁻⁶/°C for 1.064 μm. These results prove that the GdVO₄ crystal is a laser crystal with excellent thermal and birefringence properties. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal conductivity of Tb2(MoO4)3 crystals at low temperatures

Thermal conductivity of the improper ferroic Tb₂(MoO₄)₃ was measured in the range 0.3 K–100 K by the method of the stationary heat flow. An additional maximum in thermal conductivity observed at 0.45 K is due to the anomaly of specific heat corresponding to the antiferromagnetic phase transition. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ductility improvement of Zr-Cu-Ni-Al glassy alloy

We studied the relationship between the phase diagrams and formation of crystalline inclusions in cast samples in order to improve the ductility of Zr-Cu-Ni-Al bulk amorphous alloy. A Zr₅₀Cu₄₀Al₁₀ bulk amorphous alloy with a composition close to the ternary eutectic point has no crystalline inclusions and possesses superior mechanical properties of tensile strength σ_B=2000 MPa, Young’s modulus E=107 GPa and Vickers hardness HV=580. Oxygen embrittlement of Zr-Cu-Ni-Al bulk amorphous alloys can be avoided in the Zr₅₀Cu₃₀Ni₁₀Al₁₀ bulk amorphous alloy, which exhibits superior ductility and resistance to oxidation. Furthermore, we tried to improve ductility by cold rolling by making use of the apparent work-softening phenomenon of Zr-Cu-Ni-Al bulk amorphous alloys. Preliminary studies indicated that cold rolling to produce a fine slip-band structure, which enabled uniform deformation and superior deformability, is an important procedure for improving the ductility of Zr-Cu-Ni-Al bulk amorphous alloys.     

Relaxation effects in glassy selenium

Thermal relaxation phenomena were measured in samples of both melt-quenched and evaporated selenium. On the basis of earlier studies, the two methods of preparation produced samples with polymeric chain concentrations of 30–50% and 0% respectively; the rest of the material was in the form of eight member rings. The relaxation peak occurred between 320 and 330 K, and had an activation energy of ≈22.5 kcal/mole for the melt-quenched sample. A decrease of 1 K in the glass relaxation temperature and 0.5 kcal (a at)^?1 in the activation energy was observed in samples which had been initially evaporated. No change was observed as a result of melting and quenching these evaporated samples.It is postulated that there was no change when the evaporated sample was melt-quenched be because rings and chains can interconvert and equilibrate relatively readily at temperatures as low as T_g (50° C), and consequently the samples all had the same structure after once being heated above T_g. The differences observed are probably due to accidental impurities in the evaporated samples which help catalyze the interconversion of rings and chains. Further, since the equilibrium concentration of polymeric chains is temperature dependent, it is postulated that the large relaxation effects seen at T < T_g are due to the equilibration of these chains with the rings. The observed activation energy for the relaxation supports this view. It is about the same energy as has been previously measured for Se₈ ring breaking in liquid selenium. Two consequences of this postulate are that the selenium analog to the sulfur polymerization transition must occur well below room temperature if it exists, and therefore, that the equilibrium ring-chain ratio can be smoothly extrapolated down to room temperature from the published high-temperature data.     

High-frequency excitations in glassy crystals

We present broadband dielectric loss spectra of the orientationally disordered (‘plastic’) crystal ortho-carborane, extending well into the infrared region and compare the results to the low-molecular weight glass former glycerol. Focusing on the high-frequency dynamics beyond 1 GHz, we find a loss minimum showing evidence for excess intensity, quite similar to the findings in canonical glass formers. In the THz region a strongly asymmetric peak shows up, which exhibits marked differences to the corresponding peaks found in glycerol and other glass-forming liquids. We discuss the relation of this feature to the boson peak detected, e.g., in light and neutron scattering experiments.     

Ion conducting phosphate glassy materials

Fast ion conducting (FIC) phosphate glasses have become very important due to a wide range of applications in solid-state devices. We present an overview on silver based fast ion conducting phosphate glasses. Silver phosphate glasses containing chlorides of some metals viz; Li, Na, Mg, Pb and Cu [Ag₂O–P₂O₅–xMCl_y, where x = 0, 1, 5, 10 and 15 wt% and y = 1 when M = Li or Na and y = 2 when M = Mg, Pb or Cu] have been synthesized by melt quenching technique. Studies on these glassy materials characterized by X-ray diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetric techniques and ion transport measurements are presented. The FT-IR studies support the formation of P–O–M linkages. The values of glass transition temperature (T_g) of the glassy materials containing lithium or sodium chloride have been found to decrease with increasing dopant concentrations indicating expansion of the glassy network. On the other hand, the T_g values increase with increasing magnesium, lead or copper chloride concentrations in silver phosphate glasses. This indicates an increase in cross–link density and enhanced chemical durability of these glassy materials. Ion transport studies suggest that the values of electrical conductivities of the metal chloride doped glassy materials are higher than those of the undoped ones and, at a particular dopant concentration, the following trend is observed.σ (–LiCl) ≥ σ (–NaCl) > σ (–MgCl₂) > σ (–PbCl₂) > σ (–CuCl₂)These results are supported by the experimental results of FT-IR spectral and thermal studies.     

Rigidity percolation in glassy structures

We review the new concept of rigidity percolation and show that if local flexible units are joined together to form a network, the composite consists of floppy and rigid regions. When the rigid regions percolate, the whole network becomes rigid and resists attempts at elastic deformation. These ideas are applied to network glasses. It is shown that in the floppy region there exist low-frequency modes that should show up in inelastic neutron scattering.