Silver sulfide based glasses (I). Glass forming regions,structure and ionic conduction of glasses in GeS2Ag2S and GeS2Ag2SAgI systems

Ag₂S forms with GeS₂ stable glasses over a wide range of compositions (0–55% Ag₂S mol%). In the same system, more complex glasses obtained by dissolving silver iodide have been synthesized with up to 50 mol% AgI.Raman spectra are presented and a vibrational assignment in terms of bridging and non-bridging sulfur has been made. The electrical conductivity of these glasses has been measured over a temperature range (?50°C? + 50°C) and for various compositions by the complex impedance diagram method. At 25°C, the conductivity reached a maximum value of 6 × 10^?3 Ω^?1 cm^?1. Whatever the glass used, the same limit value of conductivity $\text{(σ ? 10 su?2 Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$) and activation energy ($\text{E}{}_{\mathrm{\sigma }}\text{? 0.25}\text{eV}$) are obtained for the highest content of silver iodide. A conduction mechanism is proposed.     

Study of amorphous Ge–SiO2–P+Si tunnel junctions

A new method to determine ac conductivity of amorphous Ge using Al-amorphous Ge–SiO₂–P⁺Si tunnel junctions is presented. Frequency dependence of ac conductivity is found to satisfy the power law in the frequency range between 1 and 50 kHz and the density of localized states at the Fermi level is estimated to be ～ 1.7 × 10²⁰ cm^?3 eV^?1 which decreases to ～ 4.5 × 10¹⁹ cm^?3 eV^?1 after annealing at 175°C.Temperature dependence of tunneling conductance of Al-amorphous Ge–SiO₂–P⁺Si junctions is appreciable only near zero bias. Zero bias conductance of the junctions obeys the $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$ law of Mott; the density of localized states obtained from the $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$ law is one order of magnitude smaller than that obtained by ac conductivity measurements, being insensitive to annealing. This behavior of the tunnel junctions differes in many respects from those of Al–Al₂O₃-amorphous Ge tunnel juntions.     

Electrical,structural and optical properties of amorphous carbon

The planar and transverse electrical resistivity of amorphous carbon (a-C) films getter-sputtered at low temperature (77–95 K) is well-fitted by the expression $\text{? = ?}{}_0\text{exp}\text{(T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ The exponent T₀ being approximately the same in both cases (≈ 7 × 10⁷ K) suggests that the amorphous films are isotropic. Films thinner than 600 Å display a two-dimensional hopping conductivity from which one deduces a density of states N(E_F) at the Fermi level of 10¹⁸ eV^?1 cm^?3 and a radius of the localized wave functions (a) of 12 Å. Tunneling experiments and optical absorption measurements are consistent with a pseudogap of approximately 0.8 eV. Electron diffraction experiments indicate that a-C films consist of a mixture of diamond and graphite bonds; this fact taken in the light of the other experiments would suggest that the graphite bonds act as the localized conduction states.     

Nucleation kinetics of sodium disilicate

The kinetics of crystal nucleation in Na₂O · 2SiO₂ have been determined over the range of undercoolings between 173 and 373°C. The plot of log(I_v?) versus $\text{1}\text{ΔT}{}^2{}_{\mathrm{r}}\text{T}{}^3{}_{\mathrm{r}}$ is a straight line of negative slope over some 13 orders of magnitude in I_v. The slope of this relation indicates a nucleation barrier of about 45 kT at ΔT_r = 0.2, and the intercept at $\text{1}\text{ΔT}{}^2{}_{\mathrm{r}}\text{T}{}^3{}_{\mathrm{r}}\text{= 0}$ is 10²⁶ cm^-3 sec^-1. poise. The results are in good agreement with predictions of the theory of homogeneous nucleation, even in the pre-exponential factor.     

Electrical conductivity and photoconductivity of As2S3PbS glasses

Measurements of dc electrical conductivity and photoconductivity of various glassy compositions (x = 0.1?0.625) in (As₂S₃)_1?x(PbS)_x have been made. Experimental results of the temperature dependence of dc conductivity from room temperature to 200°C (which includes the glass transition temperature) are reported. All the compositions exhibit intrinsic conduction in the measured temperature range. Thermal activation energy, glass transition temperature and σ₀ for the compositions studied, were determined from the experimental data. The low value of $\text{σ}{}_0\text{(10?10}{}^{\mathrm{?2}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$) in these semiconducting glasses is attributed to the greater participation of localized states in the conduction process.In the measurements of photoconductivity, the variation of photocurrent with temperature, photon energy, light intensity and electric field is observed. The recombination model has been involved to explain the results of photoconductivity. Both electrical and photoconductivity data support the presence of higher density of localized states in the x = 0.1 composition than in others.     

Effect of thermal history on conductivity and electrical relaxation in alkali silicate glasses

Electrical conductivity σ₀ and electric field relaxation measurements have been carried out as a function of thermal history for two alkali silicate glasses, Na₂O3SiO₂ and K₂O3SiO₂. Specimens of each glass with three different thermal histories, two of the anneal-and-quench type and one of the rate-cool type, were studied. The average structural or fictive temperature T_f of each of the specimens was characterized by measuring their indices of refraction. Effects of thermal history on σ₀ and its activation enthalpy $\text{H}{}_{\mathrm{\sigma }}{}^1$ were in accord with results of previous investigators. That is, for a given type of thermal history σ₀ was lower and $\text{H}{}_{\mathrm{\sigma }}{}^1$ higher the lower T_f. In addition it was found that for two specimens with the same T_f or index of refraction but different thermal histories the rate-cooled specimen exhibited a lower conductivity than the annealed-and-quenched specimen, in accord with the results of Ritland. The distribution of relaxation times τ_σ for decay of the electric field due to ionic migration was found to be due primarily to a distribution in the pre-exponential term ln $\text{τ}{}_{\mathrm{\sigma }}{}^1$ in the equation $\text{ln}\text{τ}{}_{\mathrm{\sigma }}\text{=}\text{ln}\text{τ}{}_{\mathrm{\sigma }}{}^1\text{+ H}{}^1\text{/RT}$; the distribution in $\text{H}{}^1$ was extremely narrow. Differences in thermal history caused small differences in the distribution of τ_σ, but no difference in the average activation enthalpy $\text{〈H}{}^1\text{〉}$ for τ_σ. From this result it appeared that the dependence of the conductivity activation enthalpy $\text{H}{}_{\mathrm{\sigma }}{}^1$ on thermal history was due to the effect of thermal history on the temperature dependence of the distribution in τ_σ.     

Prediction of glass-forming ability for metallic systems

The relative glass-forming ability (GFA) of metallic alloys is considered in terms of a parameter $\text{ΔT}{}^1\text{= (T}{}_{\mathrm{<mtext>liq</mtext>}}{}^{\mathrm{<mtext>mix</mtext>}}\text{? T}{}_{\mathrm{<mtext>liq</mtext>}}\text{)/T}{}_{\mathrm{<mtext>liq</mtext>}}{}^{\mathrm{mix}}$, which represents the departure of the alloy liquids temperature, T_liq, from that of the simple rule of mixtures liquids temperature, T_liq^mix. For values of $\text{ΔT}{}^1\text{> 0.20}$ a metallic system is likely to form a glass by melt-quenching in useful thicknesses (i.e. > 20 μm) at a cooling rate of 10⁵?10⁷ K s^?1. Hence, a rapid assessment of the GFA of novel compositions may in general be obtained simply from a knowledge of the melting points of the pure components and the liquidust emperatures of the alloys.     

Hydrogen permeation in the metallic glass Fe40Ni40P14B6

Hydrogen permeation from the gas phase through the metallic glass Fe₄₀Ni₄₀P₁₄B₆ (Metglas 2826) was measured with the electrochemical technique over 313 to 353K and 10⁵ to 10³ Pa. Specimen surfaces were coated with a thin layer of electrodeposited palladium to eleminate surface impedances to hydrogen entry and exit. Sievert's law behavior was observed over the temperature and pressure ranges studied. An excellent fit to the permeation transients as obtained from Fick's diffusion theory. Diffusivities derived from least squares fitting of permeation trasients were in good agreement with those obtained from time lag measurements. Within the concentration range studied, diffusivity was found to be independent of input hydrogen concentration. The expressions for the permeability coefficient, 2.24 × 10¹⁶ exp{[(48.99 ± 2.93) kJ/mol]/RT} atom H/ms $\text{Pa}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, the diffusivity 8.52 × 10^?7 exp{[(?47.07 ± 1.63) kJ/mol]/RT} m²/s, and the solubility, 2.62 × 10²²exp{[(?1.92 ± 4.56) kJ/mol]/RT} atom H/m³$\text{Pa}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, were determined.An annealing treatment at 573K for one hour decreases bot the permeability and diffusivity of hydrogen in the metallic glass.     

Electrical and optical properties of amorphous boron and amorphous concept for ß-rhombohedral boron

Measurements of the conductivity (σ), thermoelectric power (S) and thermal conductivity (κ) of amorphous boron are made over wide temperature ranges (T = 77–850 K for σ, T = 300–850 K for S and T = 80–1100 K for κ). The room temperature spectral dependencies of the reflection (R) and absorption (α) coefficients are determined for the wavelength intervals 2–25 μm and 1.3–25 μm respectively. The I–V characteristics are also studied and shown to be consistent with the Poole-Frenkel law.The value obtained for the thermal energy gap of amorphous boron (1.3 eV) is slightly smaller than that of crystal ß-rhombohedral boron (1.4 eV). The temperature dependence of the electrical conductivity can be satisfactorily described by the Mott law $\text{ln}\text{σ ≈ ?(T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$, where $\text{T}{}_0\text{? 10}{}^8\text{K}$. This gives an estimate, N ≈ 10¹⁸ cm^?3, for the concentration of trapping levels responsible for the hopping conduction. The value $\text{?}{}_0\text{? 9}$ is found from the spectral dependence of R while α has Urbach-like character $\text{? α ≈}\text{exp}\text{(}\text{h}\text{?}\text{ω/Δ)}$, where $\text{Δ ? 0.19}\text{eV}$.A comparison is made between amorphous boron and crystalline ß-rhombohedral boron. Because of the very complex crystal structure and the large dimensions of the unit cell of ß-boron, some of its physical properties could be qualitatively described on the basis of the so-called ‘amorphous concept’.     

Properties of heavily doped GDSi with low resistivity

Significant decrease in resistivity has been observed in glow-discharge-produced silicon (GDSi) containing 10¹⁹?10²¹ cm^?3 phosphorous atoms. At the highest doping level a resistivity of 0.01 ω cm at room temperature was obtained. The temperature dependence of the resistivity follows the form, $\text{? = ?}{}_0\text{exp}\text{(T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$, over a temperature range from 80–400 K. Optical absorption, which increased with wavelength and was roughly proportional to the conductivity, was observed in the longer wavelength side of the intrinsic absorption edge and it was ascribed to mobile charge carriers. Hall effect measurements have shown that μ_H of phosphorus doped GDSi is about 1 cm² V^?1s^?1 and has a normal (negative) sign.     

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.     

LPE growth kinetics of CaGe-Substituted EuTm2Fe5O12 garnet films

The diffusion-reaction growth melts fluxed with PbO-B₂O₃-Fe₂O₃, has been extended to include a thermally-activated diffusion term. The fluxed melt had a lower PbO : B₂O₃ ratio (11 : 1) than that generally used (≈16 : 1) for garnet LPE films. Measured growth rates for LPE films, produced from melts with four different concentrations of CaGe-Substituted EuTm₂Fe₅O₂ garnet, fit the model with: diffusivity, $\text{D (cm}{}^2\text{/s) = 0.06 exp(}\text{-1.0 eV}\text{kT}\text{)}$; reactivity, $\text{K (cm/s) = 8000 exp(}\text{-1.61 eV}\text{kT}\text{)}$; viscosity, $\text{v (}\text{cm}{}^2\text{s}\text{) = 0.01}$; and concentration, $\text{c}{}_{\mathrm{e}}\text{(}\text{g}\text{cm}{}^3\text{) = 4220 exp ?}\text{1.0678 eV}\text{kT}$ for rotation rates ω (rpm) = 36 to 196 in the growth temperature range 890 to 965°C. It is shown that the garnet melt concentration terms are the most critical ones in determining the growth rate, hence the activation energy (heat of solution) for the equilibruim concentration is reported to five significant figures. Evaluating the D and the K effects by both a numerical (results given above) and an analytic method demonstrates that these parameters are least critical in determining the growth rate in this case.     

Investigation of recombination and trapping processes in 3As2Se32Sb2Se3 amorphous films by photoconductivity method

Photoconductivity experiment has been performed on high resistivity ($\text{π ? 10}{}^9\text{Ω-}\text{cm}$) 3As₂Se₃2Sb₂Se₃ amorphous films in the temperature range from 278°K to 308°K, as a function of light intensity, I₀. The results are that at intermediate light levels the photocurrent varies as I₀^0.7 and at high light levels the photocurrent is directly proportional to I₀. A simple recombination and trapping model is developed to interpret the observed photoconductivity data. From the temperature dependence of the photocurrent, an effective trap level located 0.3 eV below the band edge is deduced.     

Nonlinear ultrasonic attenuation in glasses

Measurements of the attenuation of longitudinal sound waves of frequencies between 730 MHz and 1400 MHz were made on a borosilicate glass at temperatures from 0.4 K to 2 K. For high acoustic intensity J the absorption decreases with decreasing temperature. At low temperatures the attenuation is strongly dependent on the acoustic input power and was found to be proportional to $\text{J}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$ indicating a saturation of the absorption. For low acoustic intensities (J ? 2 × 10^?7 W/cm²) an increase of the attenuation with decreasing temperature below T < 0.6 K was observed. The experimental results are compared with recent data in vitreous silica and can qualitatively be described by current theories based on a tunneling model.     

Pressure and heating rate dependence of the crystallization temperature for amorphous (Fe65Ni35)75P16B6Al3 and Ti50Be40Zr10

The crystallization temperature, T_x, was determined at constant heating rate, $\text{R =}\text{T}\text{?}\text{? 7}\text{K min}{}^{\mathrm{?1}}$, by monitoring the electrical resistance. Such experiments were carried out under pressures up to 2.5 GPa, and the resulting dT_x/dP was 15.9 K GPa^?1 for (Fe₆₅Ni₃₅)₇₅P₁₆B₆Al₃ and 8.7 K GPa^?1, 8.1 K GPa^?1 for the two crystallization processes in Ti₅₀Be₄₀Zr₁₀. The activation energies of crystallization under atmospheric pressure were obtained from measurements of T_x at rates from 0.05 K min^?1 ?55 K min^?1, analysed by plotting ln(T_x²R^?1) versus T_x^?1.     

Frequency dependence of conduction and polarization in conductive polymers

We report the results of the measurement and analysis of the complex conductivities of two high polymers over the frequency range 10²–10⁶ Hz, and temperature range 70–300 K. Giant polarization of the nomadic type is observed, with dielectric constants ranging from about 50 to 6000 in these aromatic hydrocarbon polymers. The complex conductivities resemble power law behavior, σ_ac = Aω^s (with s in the range 0.7–1.0) in some temperature ranges, and deviates from this in others. The dc conductivity and the real part of the ac conductivity at various frequencies follow a $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$ law. The dielectric constant varies as expected for nomadic polarization in long-chain molecules. An attempt is made to develop an understanding of the observed dependences of the complex conduction or polarization on temperature and frequency in terms of interchain and intrachain transport processes.     

Optical and lasing properties of fluorophosphate glass

Neodymium-doped fluorophosphate glass is a laser material newly-developed for use in high power laser fusion systems. The low refractive index (n_d ～ 1.45) and low dispersion (Abbe number ～90) of fluorophosphate glasses give them the properties of low nonlinear refractive indices and long Nd³⁺ fluorescence lifetimes, which are desirable for the high power laser applications. We have measured the intensity gain of 1.052 and 1.064 nm laser light produced by flashlamp-pumped fluorophosphate glass amplifiers, varying in size from 4–34 clear aperture. The measured gains are compared with those measured in other laser glass types and with those predicted from the spectroscopic properties of Nd³⁺. We estimate that the peak cross section for the ${}^4\text{F}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>11</mtext><mtext>12</mtext>}}$ transition in commercial fluorophosphate laser glasses is ～2.2 × 10^?20 cm².     

Barrier to crystal nucleation in anorthite

The kinetics of crystal nucleation in anorthite (CaAl₂Si₂O₈) have been determined over the range of undercooling between 475 and 600°C. The plot of In(I_vη) versus (ΔT_|staggered|r²T_|staggered|r³)^?1 is a straight line of negative slope over the temperature range investigated. The slope of this relation indicates a free energy barrier to crystal nucleation of about 82 kT1 (where $\text{T1 = 0.8T}{}_{\mathrm{E}}$). The intercept at (ΔT_|staggered|r²T_staggered|r³)^?1 = 0 is approximately 10²⁷ cm^?3 s^?1P. Samples of anorthite glass were also analyzed by differential thermal analysis, with the temperature of maximum crystallization rate being determined as a function of heating rate. From this variation, a nucleation barrier of about 80 kT1 was estimated. The combined results are in good agreement with predictions of the classical theory of homogeneous nucleation.     

Pressure-volume-temperature relations in liquid and glassy selenium

The PVT properties of amorphous selenium are studied experimentally and theoretically in the temperature range 0–70°C and for pressures up to 200 MPa. PVT surfaces are determined for the metastable liquid and for a glass formed by a pressurization and cooling procedure. Its liquid—glass intersection line $\text{T}{}_{\mathrm{<mtext>g</mtext>}}{}^2\text{(P)}$ is compared with the glass transition line T_g (P), here obtained by pressurizing the liquid isothermally at a rate of 2 MPa/min. Analytical expressions based on the PVT data are compared with the predictions of the Simha-Somcynsky hole theory originally formulated for open chainmolecular fluids. The agreement for the liquid, although satisfactory, is not as good as for amorphous organic polymers thus far studied, possibly because selenium contains both open-chain and Se₈ ring molecules. The theoretical scaling parameters for the best fit to experiment are compared with those obtained for polymers. A very high characteristic pressure and thus cohesive energy density are noted. The theoretical hole fraction is found to be nearly constant along the $\text{T}{}_{\mathrm{<mtext>g</mtext>}}{}^2\text{(P)}$ line for low pressures. For the glass, a theoretical equation of state obtained from that for the liquid by freezing the hole fraction, compares favorably with experiment. The Prigogine-Defray ratio ΔκΔC_p/TV(Δα)² at atmospheric pressure, calculated using literature values of ΔC_p, is found to be about 2.0.     

Relation between electrical and optical gaps of amorphous semiconductors in the SiAsTe system

Electrical and optical properties of semiconducting SiAsTe glasses have been investigated. Compositional dependences of the properties in the Si_xAs_yTe_z system are examined as a function of atomic percentage x (or y, z) of one element with parameters of constant atomic ratio y/z (or x/z, x/y) of the other two elements. A pre-exponential factor σ₀ in the dc conductivity formula is estimated to be $\text{(2.1 ± 0.6) × 10}{}^4\text{(Ω ·}\text{cm}\text{)}{}^{\mathrm{?1}}$, inependently of the compositions. A systematic relationship between the compositional changes in the electrical gap E_g(el) and optical gap E_g(op) has been found. The energy gaps increase linearly with increasing Si content and decreasing Te content, but are almost independent of As content. The relation between E_g(el) and E_g(op) is expressed by E_g(el) = 1.60 E_g(op) ? 0.15 in eV. On the other hand, the optical absorption coefficient α(Ω) near the band edge follows the empirical formula, $\text{α(Ω) = α}{}_0$ exp ($\text{h}\text{?}\text{Ω/E}{}_{\mathrm{<mtext>s</mtext>}}$). The experimentally determined factor E_s increases linearly with E_g(op) and is closely related to the energy difference between the two gaps. A tentative model to explain these experimental results is proposed by taking into account of the effect of the potential fluctuations in such disordered materials.