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.     

A thermally-stimulated-current study of ethylene-methyl-methacrylate copolymers

Two ethylene-methyl methacrylate (EMMA) copolymers were studied by infrared (i.r.) spectroscopy, Differential Scanning Calorimetry (DSC) and Thermally Stimulated Currents (TSCs). The critical temperature, T _c, relaxation time, _c (measured in seconds), and Degree of Disorder (DOD) of the polymers were also measured by Relaxation Map Analysis (RMA). The i.r. and DSC results showed that these two copolymers are random copolymers with a melting point, T _m, at 82.6 °C for a sample containing 18 wt % methyl methacrylate (EMMA18), and at 53.0 °C for a sample having a MMA content of 38 wt % (EMMA38). From TSC spectra analysis, EMMA18 showed a glass-transition point, T _g, at – 41.0 °C, whereas EMMA38 had a T _g at – 36.0 °C together with a -relaxation at – 120 °C. Both samples have their space charges in the range 5–30 °C. The RMA showed that EMMA18 has T _c = – 5.74 °C, log _c = – 2.90 ( in seconds) and DOD = 54.65. EMMA38, however, has T _c = – 1 5.71 °C, log _c = – 1.59 and DOD = 48.69. The absence of -relaxation in EMMA18 was explained by evidence that it has a higher DOD. While the low mechanical properties of EMMA38 were mainly attributed to the contribution from the short chain segments, demonstrated by its low T _c.     

Transport Properties of a Tl-2201 Film Close to the Critical Temperature: The Vortex Glass Transition

We have studied the I-V characteristics of a Tl-2201 film at zero field. In the regime in which flux creep is the dominant dissipation mechanism, the J _c -T curve is divided into two parts at a temperature T _g (about 82 K), close to the critical temperature (84 K). The I-V characteristics around T _g are well described using a flux creep model. For T>T _g , J _c /J _c (0) =0.445x(l-0.525t-0.5t ² ); for T _g , J _c /J _c (0) = 0.9x(1-0.595t-0.44t ² ). Differential resistance (dV/dI) as a function of the measuring current shows a change in curvature close to T _g . The I-V curves collapsed nicely into two branches by plotting (V/I)/|T–T _g | ^v(z-1) vs. (I/T)/|T _g –T| ^2v , indicating a current–reduced vortex glass transition.     

Investigation of the temperature dependence of electron and phonon Raman scattering in single crystal YBa2Cu3O6.952

Detailed Raman-scattering measurements have been performed on high-quality YBa₂Cu₃O_6.952 single crystal (T _c =93 K, T _c =0.3 K). A sharp (FWHM 7.2 cm^–1 at 70 K and 10.0 cm^–1 at 110 K) 340 cm^–1phonon mode has been observed inB _1g polarization. An electronic scattering peak at 500 cm^–1 in theB _1g polarization extends down to 250 cm^–1. These FWHM values determine the upper limit of the homogeneous linewidth of the phonon and electronic excitations. The start of the electronic spectral function renormalization and of the 340 cm^–1 mode anomalies (frequency softening, linewidth sharpening, and intensity increase) have been observed to occur approximately 40 K aboveT _c . The 340 cm^–1 mode Fano shape analysis has been performed and the temperature dependences of the Fano shape parameters have been estimated. All 340 cm^–1 mode anomalies have been explained by the electronic spectral function renormalization.This work was supported by Swedish Natural Sciences Research Council (G.B. and L.B.) and by the National Science Foundation (DMR 91-20000) through the Science and Technology Center for Superconductivity (G.B. and M.V.K.).     

Thermally induced transformations in Ge x AsyTe100−x−y glasses

Chalcogenide glasses with compositions Ge_7.5As _y Te_92.5–y (y=20, 40, 45, 47.5, 50, 52.5, 55) and Ge₁₀As _y Te_90–y (y=15, 20, 22.5, 35, 40, 45, 50) have been prepared by the melt-quenching technique. The amorphous nature of these glasses has been confirmed by X-ray powder diffractometry. The thermal stability of these glasses has been studied using differential scanning calorimetry (DSC). The compositional dependence of the glass transition temperature,T _g, the crystallization temperatures,T _c1 andT _c2, and the melting temperature,T _m, are reported. The glass-forming tendency,K _gl, and the activation energy of crystallization,E, are calculated. The activation energy decreases with increasing tellurium content for both sets of glasses.     

Effect of gadolinium content on the thermal volume change of Ba4Na2Nb10O30 − Ba3NaGdNb10O30 solid solutions at ferroelectric phase transition temperature

For sintered samples of Ba₃₊ x Na_1+xGd_1–xNb₁₀O₃₀ (1 x 0), the effect of the addition of gadolinium to barium sodium niobate (BNN,x = 1, tungsten bronze type) was examined by differential scanning calorimetry (DSC) and X-ray powder diffractometry. The volume change (i.e. especially the change ofc-axis length) at the ferroelectric phase transition temperature (T _c< 590 dg C) decreased with the increasing gadolinium content. This result suggests that the addition of gadolinium or other lanthanide elements to BNN is possibly effective to obtain an uncracked single crystal with the Czochralski technique. The DSC indicator method is useful to determine the relative magnitude of volume change atT _c for the same type of sample series, a method which is much easier than the dilatometer method. The relation between the DSC peak area and the volume change of unit cell atT _c is discussed from the thermodynamic viewpoint.     

Preparation and study of Ca1 − xMgx(PO3)2 glassy and crystalline phases

The structure of Ca_{1 – x} Mg _x (PO₃)₂ crystalline and glassy samples was investigated in the whole concentration region of x = 0–1.0. From X-ray diffraction data it was found that, in the crystalline samples, solid solutions are formed for x < 0.3 on the calcium-side of the system, with the structure of -Ca(PO₃)₂, and for x > 0.6 on its magnesium-side, with the structure of Mg₂P₄O₁₂. Similar results were obtained from the study of their infrared and Raman spectra. In the glassy state, homogeneous glasses were formed within the whole concentration region. The values of their transformation temperatures, T _g, and crystallization temperature, T _c, change slightly with the composition and lie within the region of T _g = 529–544 °C and T _c = 631–677 °C.     

The high-temperature thermal expansion of BiPbSrCaCuO superconductor and the oxide components (Bi2O3, PbO,CaO, CuO)

The thermal expansion of superconducting Bi_1.6Pb_0.4Sr₂Ca₂Cu₃Ox (BiPbSrCaCuO) and its oxide components Bi₂O₃, PbO, CaO and CuO have been studied by high-temperature dilatometric measurements (30–800°C). The thermal expansion coefficient for the BiPbSrCaCuO superconductor in the range 150–830°C is =6.4×10^–6K^–1. The temperature dependences of L/L of pressed Bi₂O₃ reveals sharp changes of length on heating (T ₁=712°C), and on cooling (T ₂=637°C and T ₃=577°C), caused by the phase transition monoclinic-cubic (T ₁) and by reverse transitions via a metastable phase (T ₂ and T ₃). By thermal expansion measurements of melted Bi₂O₃ it is shown that hysteresis in the forward and the reverse phase transitions may be partly caused by grain boundary effect in pressed Bi₂O₃. The thermal expansion of red PbO reveals a sharp decrease in L/L, on heating (T ₁=490°C), related with the phase transition of tetragonal (red, a=0.3962 nm, c=0.5025 nm)-orthorhombic (yellow, a=0.5489 nm, b=0.4756 nm, c=0.5895 nm). The possible causes of irreversibility of the phase transition in PbO are discussed. In the range 50–740°C the coefficient of thermal expansion of pressed Bi₂O₃ (_m=3.6 × 10^–6 and _c=16.6×10^–6K^–1 for monoclinic and cubic Bi₂O₃ respectively), the melted Bi₂O₃ (_m=7.6×10^–6 and _c=11.5×10^–6K^–1), PbO (_t=9.4×10⁶ and _or=3.3×10^–6K^–1 for tetragonal and orthorhombic PbO respectively), CaO (=6.1×10^–6K^–1) and CuO (=4.3×10^–6K^–1) are presented.     

Barium potassium bismuth oxide: A review

Ba_1–x K _x BiO₃(BKBO) has aT _c (onset) of 34 K. It is the highest-temperature oxide superconductor which is cubic, with a coherence length of 30–60 Å. The basic properties of this compound are reviewed.     

Pyroelectric and dielectric properties of some heavy rare-earth orthochromites

The measurement of pyroelectric coefficient (p) and dielectric constant (K) of rare-earth orthochromites RCrO₃, where R = Tb, Dy, Ho, Er and Yb are reported for the temperature range 300–600 K. Pyroelectric data show that all the studied orthochromites have ferroelectric phase in this temperature range. The dielectric data in some cases support this conclusion. Spontaneous polarization (P _s) and Curie temperature (T _c) have also been evaluated. The maximum value of P _s varies from 0.041 Cm^–2 to 0.30 Cm^–2 in the studied orthochromites, which is very small.     

Temperature dependence of fracture toughness of epoxy resins cured with diamines

The fracture toughness (critical stress intensity factor, K _Ic) of epoxy resins cured with four diamines has been measured as a function of temperature over the range from –35° C to above T _g. It was found that K _Ic for each epoxy-diamine system did not vary below room temperature, and in the higher temperature range K _Ic increased with increasing temperature to a maximum and then decreased. The temperature which maximized K _Ic, agreed with the temperature at which the flexural modulus of the epoxy resins abruptly dropped. This temperature was therefore considered as T _g. This temperature was found to be about 20° C lower than the heat deflection temperature under load (1.82 M Pa) of the resins.     

Thermal induced transformations in glassy chalcogenides SixTe60−xAs30Ge10

Temperature-induced transformations are considered to be interesting characteristic properties of amorphous materials including the Si _x Te_60–x As₃₀Ge₁₀ system, withx=5, 10, 12 and 20. Density (), X-ray diffraction and differential thermal analysis (DTA) were used to characterize the compositions. DTA traces of each glass composition at different heating rates from 5 to 30° C min^–1 were obtained and interpreted. Fast and slow cooling cycles were used to determine the rate of structure formation. Cycling studies of materials show no memory effect but only ovonic switching action. The compositional dependence of the crystallization activation energy (E) and the coefficient of glass-forming tendency (K _gl) have been calculated. The thermal transition temperatures and associated changes in specific heat have been examined as a function of the Te/Si ratio by differential scanning calorimetry. It was found that andE increase linearly with increasing tellurium content, while the heat capacity (c _p) andK _gl, decrease with increasing tellurium content.E=1.54 eV andc _p=0.246 J g^–1 K^–1 forx=20 whileE=2.74eV andc _p=0.22 J g^–1 K^–1 forx=5.     

Non-isothermal kinetic studies for binary TeO2-P2O5 glass system

A study of TeO₂–P₂O₅ glass system has been carried out by Differential Thermal Analysis (DTA) to elucidate the kinetics of crystallization for these glassy samples. The results of DTA performed at different heating rates are discussed. The values of the glass transition temperature, T _g , as well as the glass crystallization temperature, T _c , are found to be dependent upon the heating rate. From this dependence, the values of activation energy for both the glass transition and crystallization are evaluated and discussed     

Glass transition temperature-structural studies of tungstate tellurite glasses

Raman, IR and DSC studies have been carried on the (100 − x)TeO₂–xWO₃ (TW) glasses with 10 ≤ x ≤ 40 mol%. The Raman, IR spectra of these samples show that glass network consists of [TeO₃]/[TeO₃₊₁], [TeO₄], [WO₄] and [WO₆] groups as basic structural units. The W ion coordination state changes from 4 to 6 when WO₃ concentration increases beyond 30 mol%. Addition of WO₃ oxide to the TW glasses increases the amount of lower coordination of [TeO₃]/[TeO₃₊₁] units and decreases the higher coordination [TeO₄] units, Te–O–Te chains. From DSC thermogram, thermal properties such as the glass transition temperature (T_g), onset crystallization (T_o) of the glass systems were calculated. The compositional variation of glass transition temperature (T_g) is found to linear with an increase in WO₃ content.     

Thermomechanical properties and stability of mixed strontium-zinc borophosphate glasses

Borophosphate glasses of the SrO-ZnO-B₂O₃-P₂O₅ system were prepared in 3 compositional series of 25ZnO-25SrO-xB₂O₃-(50 – x)P₂O₅, (58.3 – x) ZnO-xSrO-16.6B₂O₃-33.33P₂O₅ and (50 – x)ZnO-20B₂O₃-30P₂O₅ and the changes in their thermomechanical and thermal properties with changes in their composition were investigated. The thermal expansion coefficient decreases with increasing B₂O₃ content, whereas the values of glass transformation temperature T _g and dilatation softening temperature T _d increase. The replacement of ZnO by SrO increases both T _g and T _d. From the DSC results the value of the the Hruby's criterion K _gl was calculated and changes in the glass-forming tendency with changes in glass composition were discussed.     

Internal friction in Se-Sb glasses

The internal friction,O ^–1, as a function of temperature has been investigated for Se_100–x Sb _x systems, wherex=7.5, 12.5 and 17.5 at%, using the magnetostrictive pulse-echo method in the range of vibrational frequencies 50–100 kHz. Two well-defined peaks appeared, characterizing this glass. The first peak covers the temperature range 320–330 K, where the peak position has been shifted to the higher temperature as Sb content increases. This peak indicates the glass transition temperature,T _g, of the tested glassy samples. The second peak was detected in the temperature range 360–378 K. The position of this peak has been shifted to a lower temperature as Sb content increases. This peak is attributed to the crystallization temperature,T _c, of each glassy sample tested. The peak height of both of the above peaks decreased as the Sb content increased. Also, the appearance of these two peaks was affected by annealing.     

Non-contact measurements of thermophysical properties of niobium at high temperature

Four thermophysical properties of both solid and liquid niobium have been measured using the vacuum version of the electrostatic levitation furnace developed by the National Space Development Agency of Japan. These properties are the density, the thermal expansion coefficient, the constant pressure heat capacity, and the hemispherical total emissivity. For the first time, we report these thermophysical quantities of niobium in its solid as well as in liquid state over a wide temperature range, including the undercooled state. Over the 2340 K to 2900 K temperature span, the density of the liquid can be expressed as _L (T) = 7.95 × 10³ – 0.23 (T – T _m)(kg · m^–3) with T _m = 2742 K, yielding a volume expansion coefficient _L(T) = 2.89 × 10^–5 (K^–1). Similarly, over the 1500 K to 2740 K temperature range, the density of the solid can be expressed as _s(T) = 8.26 × 10³ – 0.14(T – T _m)(kg · m^–3), giving a volume expansion coefficient _s(T) = 1.69 × 10^–5 (K^–1). The constant pressure heat capacity of the liquid phase could be estimated as C _PL(T) = 40.6 + 1.45 × 10^–3 (T – T _m) (J · mol^–1 · K^–1) if the hemispherical total emissivity of the liquid phase remains constant at 0.25 over the temperature range. Over the 1500 K to 2740 K temperature span, the hemispherical total emissivity of the solid phase could be rendered as _TS(T) = 0.23 + 5.81 × 10^–5 (T – T _m). The enthalpy of fusion has also been calculated as 29.1 kJ · mol^–1.     

An assessment of expressions for the apparent thermal conductivity of cellular materials

Diverse expressions for the thermal conductivity of cellular materials are reviewed. Most expressions address only the conductive contribution to heat transfer; some expressions also consider the radiative contribution. Convection is considered to be negligible for cell diameters less than 4 mm. The predicted results are compared with measured conductivities for materials ranging from fine-pore foams to coarse packaging materials. The dependencies of the predicted conductivities on the material parameters which are most open to intervention are presented graphically for the various models.Nomenclature a Absorption coefficient - C _v (Jmol^–1 K^–1) Specific heat - E Emissivity - E _L Emissivity of hypothetical thin parallel layer - E ₀ Boundary surfaces emissivity - f Fraction of solid normal to heat flow - fics Fraction of total solid in struts of cell - K(m^–1) Mean extinction coefficient - k(W m^–1 K^–1) Effective thermal conductivity of foam - k _cd(W m^–1 K^–1) Conductive contribution - k _cr(W m^–1 K^–1) Convective contribution - k _g(W m^–1 K^–1) Thermal conductivity of cell gas - k _r(W m^–1 K^–1) Radiative contribution - k _s(W m^–1 K^–1) Thermal conductivity of solid - L(m) Thickness of sample - L _g(m) Diameter of cell - L _s(m) Cell-wall thickness - n Number of cell layers - r Reflection coefficient - t Transmission coefficient - T(K) Absolute temperature - T _m(K) Mean temperature - T _N Fraction of energy passing through cell wall - T ₁(K) Temperature of hot plate - T ₂(K) Temperature of cold plate - V _g Volume fraction of gas - V _w Volume fraction of total solid in the windows - w Refractive index - (m) Effective molecular diameter - (Pa s) Gas viscosity - Structural angle with respect to rise direction - (W m^–2 K^–4) Stefan constant     

Crack velocity dependence of longitudinal fracture in bone

An evaluation of the fracture characteristics of bovine tibia compact tension specimens associated with controlled crack propagation in the longitudinal direction has been made. The fracture mechanics parameters of critical strain energy release rate (G _c) and critical stress intensity factor (K _c) were determined for a range of crack velocities. A comparative fracture energy (W) was also evaluated from the area under the load-deflection curve. It was found that an increase in the average crack velocity from 1.75 to 23.6×10^–5 m sec^–1 produced increases in G _c (from 1736 to 2796 J m^–2), K _c (from 4.46 to 5.38 MN m^–3/2) and W. At crack velocities >23.6×10^–5 m sec^–1, W decreased appreciably. Microstructural observations indicated that, for crack velocities <23.6 m sec^–1, relatively rough fracture surfaces were produced by the passage of the crack around intersecting osteons (or lamellae), together with some osteon pull-out. In contrast, at a higher crack velocity, fracture was characterized by relatively smooth surfaces, as the crack moved indiscriminately through the microstructural constituents.