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 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’.     

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.     

Small polaron conduction in V2O5P2O5 glasses

Dc conductivity measurements have been made between 90 and 520 K on three bulk samples of V₂O₅P₂O₅ glass. Heat treatment is found to result in a reduction of the activation energy at a given temperature and this is most noticeable at low temperatures. The behaviour at low temperatures can be described using Mott's variable range hopping arguments, and at high temperatures by non-adiabatic small polaron hopping between nearest neighbours. At intermediate temperatures a simple model is used in which excitations by optical and acoustic phonons are considered to make independent contributions to the jump frequency. Mott's theory is extended to the polaron case for $\text{T>}\text{1}\text{4}\text{?}$ and is shown to be in good agreement with results. Values for $\text{r}{}_{\mathrm{<mtext>p</mtext>}}\text{(～2.8}\text{A}\text{?}\text{)}$ the polaron radius and $\text{α(～3.5}\text{A}\text{?}{}^{\mathrm{?1}}\text{)}$ the electron decay constant are shown to be consistent with the model for small polarons. A method is suggested for obtaining α and N(E_F) from the ac conductivity and the slope of 1nσ versus $\text{1}\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ at low temperatures. Values of N(E) are obtained which correlate with those obtained by the previous analysis. This implies that the disorder energy separating adjacent sites Δ₀ is large (～0.4 eV) in these materials.     

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.     

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.     

Theory of molten zone shape and stability

The Laplace-Young capillary equation for the shape of an axisymmetric liquid-vapor interface has been solved numerically for boundary conditions relevant to a model of the floating zone process. The stability of these solutions with respect to axisymmetric and asymmetric perturbations which conserve volume has been determined via the conjugate point criterion of the calculus of variations. The liquid zone shape is governed by five dimensionless parameters: $\text{R}{}_{\mathrm{m}}\text{R}{}_{\mathrm{f}}$, $\text{L}\text{Rf}$, $\text{V}\text{φR}{}^2{}_{\mathrm{f}}\text{L}$, $\text{? = ρg}\text{R}{}^2{}_{\mathrm{f}}\text{γ}$, and $\text{?}{}_{\mathrm{R}}\text{= ρΩ}{}^2\text{R}{}^3{}_{\mathrm{f}}\text{γ}$, where R_m and R_f are the radii of the melting and freezing solids, respectively, L is zone length , V is the zone volume, ρ is the density difference between liquid vapor, g is the gravitational acceleration, γ is the liquid-vapor surface tension, and Ω is the constant angular velocity of the uniformly rotating zone. For growth of constant diameter crystals, the angle ø_f, measured between the meniscus and the growth axis at the freezing interface, is constant. For R_m = R_f, ?_R = 0, and ø_f = 0, the maxi mum value of ? for which a stable liquid zone exists has been calculated for various values of L/R_f. For some values of ?, two different stable liquid zones with different volumes (but all other parameters identical) give the same value of ø_f.     

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.     

Radiative and non-radiative transition probabilities and quantum yields for excited states of Er3+ in germanate and tellurite glasses

Absorption and fluorescence spectra of Er³⁺ in germanate and tellurite glasses were obtained. Spontaneous transition probabilities of the ${}^4\text{S}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and ${}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ to all terminal levels of Er³⁺ were calculated using the Judd-Ofelt theory and intensity parameters obtained from measured intergrated absorption coefficients. Quantum efficiencies of the ${}^4\text{S}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and ${}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ fluorescences were measured by the comparative method and by the use of measured decay times. Multiphonon relaxation rates for ${}^4\text{S}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ and ${}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ were calculated using the experimental data. The average rate for ${}^4\text{S}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{→}{}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ in germanate is 1.16 × 10⁵ sec^?1 and in tellurite is 1.60 × 10⁴ sec^?1, and the rate for ${}^4\text{F}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{→}{}^4\text{I}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{is}\text{2.85 × 10}{}^5\text{sec}{}^{\mathrm{?1}}$ in germanate and 2.33 × 10⁵ sec^?1 in tellurite. The higher rates in germanate glasses are explained by the stronger interaction of the glass phonons with the electronic states of Er³⁺ in germanate than in tellurite glasses. This also explains the higher quantum yield of the visible fluorescence of Er³⁺ in tellurite glasses compared to other glasses.     

Electrical properties of glassy AsxGe10Te90−x alloys

This paper analyses the electrical properties of glassy alloys of As_xGe₁₀Te_90?x, while reporting the conductivity and dielectric constant of As₅Ge₁₀Te₈₅ and As₁₅Ge₁₀Te₇₅ compositions in the temperature range 77–383 K and the frequency range from dc to 5 MHz. The dc conductivity has been shown to be of the form

The ratio σ₀₁/σ₀₂ is of the order of 10⁶. ΔE₁, the higher temperature activation energy, is dependent on the composition, while ΔE₂, the lower temperature activation energy, is less dependent on the composition. The dielectric constant has been found to be independent of temperature and frequency up to about 253 K. However, at higher temperatures, it becomes activated and proportional to log ω.Some common features of As_xGe₁₀Te_90?x are a kink in dc conductivity, a ω^0.8 relationship for ac conductivity, no evidence of variable-range hopping at low temperatures, field-dependent conductivity and memory switching. The data can be interpreted in terms of the dangling-bond theory of Mott and his collaborators. A high density of states of the order of 10²⁰eV^?1 cm^?3 near the Fermi level may be expected.     

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.     

Partial correlations in Ni60Nb40 metallic glass

Combined X-ray and neutron diffraction experiments were performed on the Ni₆₀Nb₄₀ metallic glass samples prepared by rapid quenching from the melt with natural Ni^nat and isotope ⁵⁸Ni, respectively. The partial structure factors were separated for the three kinds of atomic pairs: NiNi, NiNb and NbNb. The partial distribution functions were calculated by means of Fourier transformation and the following atomic distances were obtained: $\text{r}{}_{\mathrm{<mtext>NiNi</mtext>}}\text{=2.52}\text{A}\text{?}\text{, r}{}_{\mathrm{<mtext>NiNb</mtext>}}\text{=2.72}\text{A}\text{?}$ and $\text{r}{}_{\mathrm{<mtext>NbNb</mtext>}}\text{=2.70}\text{A}\text{?}$. The values n_NiNi=7.3, n_NiNb=4.5, n_NbNi=6.8 and n_NbNb=5.4 were obtained for the number of nearest neighbours.     

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.     

Alloying effect on the viscous flow in metallic glasses

The temperature dependence of viscosities near the glass transition is measured from the rates of thermal transformation for metallic glasses PtNiP, PdNiP, NiPBA1 and (Fe, Co)PBA1. Alloying among metallic elements which lowers the glass transition temperature T_g lowers the ideal glass transition temperature T₀, but raises the residual configurational entropy S_g and the activation energies for “diffusive” rearrangement, $\text{Δμ}{}^1$, of the alloying glasses, while compositional ordering associated with the addition of metalloids raises the T_g and T₀ and lowers the S_g and $\text{Δμ}{}^1$. Results are correlated to the atomic ordering and stability of the glasses. The extracted free volume and the critical diffusive volume are much smaller, by a factor of 4, for metallic glasses than for many other glasses.     

On mechanisms of structural relaxation in a Pd48Ni32P20 glass

Structural relaxation processes are investigated calorimetrically for a pre-conditioned Pd₄₈Ni₃₂P₂₀ glass over a wide temperature range from well below to just above the glass transition. The low temperature anneals further stabilize the glassy structure. Upon heating, the annealed sample shows an excess endothermic specific heat ΔC_p above the annealing temperature and completely recovers the initial enthalpy before any manifestation of glass transition, T_g. Significantly the ΔC_p peak evolves in a continuous manner with annealing time. A physically reasonable activation energy spectrum $\text{N}{}_0{}^1\text{(Q)}$ is obtained with the proper choice of coupling constants which are dependent on annealing temperature. Results suggest the existence of localized relaxation modes which do not contribute to macroscopic flow. A concept of distribution in glass transition temperatures H(T_g,m) is conceived to account for the reversible relaxation with temperature. A model glass transition based on percolation theory is proposed and is found to reproduce the calorimetric relaxation phenomena well.     

Intensity parameters and laser analysis of Pr and Dy in oxide glasses

Intensity parameters ($\text{Ω}{}_{\mathrm{\Lambda }}$) of Pr³⁺ and Dy³⁺ have been obtained in tellurite, borate and phosphate glasses. It has been found that $\text{Ω}{}_{\mathrm{\Lambda }}$ calculated by exlusion of hypersensitive transitions, gives a better fit between measured and calculated lifetimes and branching ratios than those including hypersensitive transitions. Using these parameters and calculated matrix elements U^(Λ), radiative transition probabilities, branching ratios and integrated cross sections for stimulated emission were calculated for ³P₀, ³P₁ and ¹D₂ excited states of Pr³⁺ and ${}^4\text{F}\text{9}\text{2}$ excited state of Dy³⁺. Potential transitions are indicated.     

Temperature variation of the mobility gap in non-polar amorphous semiconductors

For a non-polar amorphous semiconductor such as a-Si, we derive an explixit formula for (?E_g/?T)_V, the derivative with temperature of the mobility gap E_g at constant volume V. Within the framework of second-order perturbation theory for the electron-phonon (eφ) interaction, many of our physical assumptions are fundamentally different from those that apply to the crystal phase. The principal ingredients of our model are: (1) the random-phase-model (RPM); (2) the principle of non-conservation of particle momentum in the eφ interaction; and (3) the deformation potential approximation. Narrowing of E_g is found with increasing values of the temperature T. At very low T, we have $\text{(?E}{}_{\mathrm{<mtext>g</mtext>}}\text{/?T)}{}_{\mathrm{<mtext>V</mtext>}}\text{≌ ? ¢A · c}{}_{\mathrm{V}}\text{(T)}$, where c_V(T) is the average lattice specific heat per mode at constant volume and ¢A is a positive dimensionless quantity in the model. By contrast with low-temperature behavior of the crystal, this result implies that the mobility gap at constant volume dynamically responds to the phonomic “gas” of the disordered lattice. The high-T limit yields behavior quite similar to that of the crystal phase. We find $\text{(?E}{}_{\mathrm{<mtext>g</mtext>}}\text{/?T)}{}_{\mathrm{V}}\text{≌ ? x ¢A · k}{}_{\mathrm{<mtext>B</mtext>}}$, where k_B is Boltzmann's constant and the parameter x, expected to be confined to the interval $\text{1}\text{2}\text{? x ? 1}$, measures the admixture of the optical-phonon and acoustical-phonon coupling strengths.     

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.     

Theoretical analysis of equilibrium adsorption layers in CVD systems (Si-H-Cl,Ga-As-H-Cl)

Adsorption of Cl, GaCl, As, As₂, As₄, Ga and AsGaCl species on GaAs(111) in the temperature range of 973–1223 K, and Cl, H, Si, SiCl and SiCl₂ on Si(111) in the range of 1425–1525 K, has been considered for different gas phase compositions. All the faces are covered mainly with Cl and H atoms. The fraction of vacancies on GaAs(111)Ga varies in the range of 4–13% reaching ?50% on GaAs(111)As. On the Si(111) the typical fraction of the vacancies is ?1%. The main adsorbed molecules containing Ga and As atoms are GaCl and As₄. For example, at and T = 1073 K, the surface concentration of these molecules reaches ?4 and 2% on GaAs(111)Ga and ?26 and 6% on the GaAs(111)As faces, respectively. Under these conditions, AsGaCl complexes occupy only ?10^-4?10^-5 adsorption sites on both faces. The set of other data is presented. Surface diffusion in the dense adsorption layer is seriously hindered by the lack of vacancies. A rough estimate gives a value of the order of 10^-8 cm²/sec for the diffusion coefficient and diffusion length of several tens of distances for AsGaCl or As₄ complexes on the GaAs(111)Ga face. The local electric field caused by adatoms on the interface may seriously influence the reaction rate.     

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.