Studies of the optical properties of sodium copolyphosphate glasses

A number of samples of glassy sodium polyphosphate and copolyphosphates of sodium — cobalt, sodium — copper, sodium — nickel, sodium-manganese and sodium — calcium were prepared and their optical properties were investigated. The ultraviolet and visible spectra of these glasses were recorded at the room temperature. It was found that the fundamental absorption edge of these glasses usually occurs in the ultraviolet — visible region. The optical absorption edges (E₀) were calculated by using the relation h ν = B (hν − E₀)² where B is constant. The linear variation of (hν)^1/2 with hν where is the absorption coefficient and hν is the incident photon energy, is taken as evidence of non-direct interband transitions. The E₀ values obtained for copolyphosphate glasses appeared to depend on the size of the counter cation. The infrared spectra of all these glasses appeared to be almost the same, indicating the presence of characteristic P --- O --- P linkages of linear polyphosphate chains and two non bridging oxygen atoms bonded to phosphorus atoms O --- P --- O (PO₂) units in the polyphosphate tetrahedra.     

B NMR investigations in xV2O5–B2O3 and xV2O5–B2O3–PbO glasses

11B (I=3/2) MAS NMR in the binary glass system xV₂O₅–B₂O₃ (x=0.053, 0.43) and the ternary glass system xV₂O₅–B₂O₃–PbO (0.1x1.5) has been investigated at room temperature. In the xV₂O₅–B₂O₃ glasses, one NMR line due to BO₃ unit was observed. Meanwhile in the xV₂O₅–B₂O₃–PbO, two NMR lines which arise from BO₃ and BO₄ units were detected, where the appearance of BO₄ units is produced by the presence of PbO. From the computer-simulation of the ¹¹B NMR central transition line (m=−1/2↔1/2), the quadrupole parameters (e²qQ/h and η) for BO₃ units in xV₂O₅–B₂O₃, and those for BO₃ and BO₄ units in xV₂O₅–B₂O₃–PbO were obtained as a function of x. As the V₂O₅ content increases in xV₂O₅–B₂O₃–PbO, the e²qQ/h and η values of the BO₃⁻ associated resonance are found to slightly decrease and increase, respectively. Meanwhile, the e²qQ/h and η values of BO₄⁻ associated resonance in xV₂O₅–B₂O₃–PbO are found to slightly increase and decrease, respectively. By comparing the intensities of the total transitions (m=−3/2↔−1/2,m=−1/2↔1/2, and 1/2↔3/2) for the ¹¹B NMR line of BO₃ and BO₄ units contained in xV₂O₅–B₂O₃–PbO with those of respective standard samples of 0.053V₂O₅–B₂O₃ and NaBH₄, the quantitative fractions of BO₃ and BO₄ in xV₂O₅–B₂O₃–PbO were obtained as a function of x.     

TiO2-SiO2 monolithic glass formation from sol-gel

This study demonstrates that hydrolysis should be carried out in a step manner in gel synthesis. The key to the increase in the amount of water added is the control of the hydrolysis rate of Ti(OC₄H₉)₄. The hydrolysis of Si(OC₂H₅)₄ can be carried out at about 75°C. The amount of added water (γ_WI), which varied with TiO₂ content (in mol%), was about 64–88% of the total amount of added water. The hydrolysis reaction should be performed at room temperature while Ti(OC₄H₉)₄ is added. The total amount of added water (γ_W) is related to the amount of solvent (R). For example, if TiO₂ is 40mol%, γ_W will vary from 3.2 to 8.0 when R varies from 0.8 to 2.0. The amount of added water was affected by the distribution of solvent in the metal alkoxides. The amount of added water can be increased when R_Si(OC2H5)4 = 1, R_Ti(OC4H9)4 > 1. The rate of rise in temperature of the thermolysis of the dried gel should be less than 10°C per hour, and the heat treatment temperatyre is related to the TiO₂ content (in mol%). Gel glasses without devitrification can only be obtained by thermolysis at 600°C from the gel with no less than 20 mol% TiO₂.     

Absorption spectra of GaN: film characterization by Urbach spectral tail and the effect of electric field

The absorption tail of undoped and Si-doped GaN films was investigated at different temperatures and under applied electric field. It was found that the spectral dependence of logarithm of absorption coefficient is combined of two linear functions: ln[(hν)]=C₁+(hν−E_g)/U₁ for hν<3.42 eV and ln[(hν)]=C₂+(hν−E_g)/U₂ for 3.44<hν<3.5 eV with Urbach energies U₁=400–470 meV and U₂=10–20 meV. The influence of an electric field effect on the absorption spectra follows the Dow and Redfield theory. It was shown that the intrinsic electric field about 10⁵ V/cm exists in our samples. The implemented analysis of the absorption spectra gives the qualitative method of film characterization.     

Charge transfer in CdTe at 200 and 300 K

Using a perfect single crystal sample of CdTe grown using PVT method, the electronic charge transfer in the II–VI compound semiconductor CdTe at 200 and 300 K has been evaluated using two different approaches: (1) by solving a quadratic equation involving the observed structure factors of h+k+l=4n+2 type reflections; and (2) by a graphical approach in which the observed and calculated atomic form factors are extrapolated to sinθ/λ=0, to determine the transferred charge. Precise X-ray structure factors collected using MoK radiation have been used for the analysis. The results obtained are reasonable and clearly indicate the ionicity by which charge is transferred from Cd to Te in CdTe.     

Observation of the periodic fluctuant dendritic structure in an Al–38 wt% Cu hypereutectic alloy processed by ACRT-B method

The microstructure of periodic fluctuant dendritic θ in a matrix of +θ eutectic was obtained in an Al–38 wt% Cu alloy processed by ACRT-B method grown at growth velocities (V) ranging from 5 to 60 μm/s and crucible rotating in a trapezoidal way with maximum rotation rates (Ω_max) ranging from 100 to 400 rpm. Formation of this structure is explained by the influence of the periodical Ekman flow on the growth of dendritic θ during the spin-up and spin-down process. It was also observed that the +θ eutectic between the primary dendrites of θ (CuAl₂) is not periodic and fluctuant during ACRT-B process. This is quite different from our previous observation of periodic eutectic in Al–Cu eutectic processed by similar ACRT-B method.     

Investigations of g-value correlations associated with the g = 4.3 ESR signal of Fe in glass

The g = 4.3 ESR signal of Fe³⁺ in glass is usually attributed to a “fully rhombic” distortion characterized by E = D/3, where E and D are the splitting constants belonging to the second-order rhombic and axial terms, respectively, of the crystal-field interaction. An alternate statistical approach to the origin of this signal has been given by Peterson et al. on the basis of axial symmetry and broad overlapping distribution of g_| and g that are correlated in opposition (negatively) resulting in spiking at g 4.3. The present paper reports detailed computer studies of the distribution of effective g value produced by large variations in both D and E to test whether negative g-value correlations leading to spiking at g 4.3 are generable from site dependent fluctuations in the second-order crystal-field terms. It is found that the ESR line arising from the middle Kramer's doublet in the fully rhombic limit (E → D/3, |D| > hν) uniquely generates the negative g-value correlation required for spiking at g 4.3.     

EXAFS and XANES joint analyses for semiconducting vanadium phosphate glasses

Vanadium EXAFS spectra of 50V₂O₅-50P₂O₅ glasses with different C = V⁺⁴/V_tot have been measured. The V-O distances increase by ΔR₁ = (0.03 ± 0.02) Å to ΔR₂ = (0.07 ± 0.03) Å going from a glass with C = 0.64 to C = 0.84 and from C = 0.51 to C = 0.84, respectively. The EXAFS data show a basically similar structure of the vanadium sites for both the V⁴⁺ and V⁵⁺ ionic states. The density of the glasses increases with C from the value d₁ = 2.81 g/cm³ for C = 0.51 to d₂ = 2.92 g/cm³ for C = 0.84 indicating a more random packing of glass units.     

Li conductivity in lithium niobate: silica glasses

Glasses of composition 65 mol% LiNbO₃:: 35 mol% SiO₂ have been shown to be Li⁺ ion conductors with a conductivity at 200°C > 1 × 10⁻⁵ (η cm)⁻¹ and an activation energy of 0.54 eV. The addition of approximately 0.1 mol% Fe₂O₃ leads to an enhancement of conductivity to ≈10⁻³ (η cm)⁻¹ at 200°C and an activation energy of 0.67 eV. The effect of Fe is shown to be in the control of microstructure in the glass, with Fe₂O₃ concentrations < 1 mol% acting as a grain growth inhibitors and larger concentrations acting as a nucleating agents. A model for this process based on the expected stoichiometry of the melt and the effect of Fe²⁺ and Fe³⁺ in charge compensation is in excellent agreement with experimental data from electron spin resonance.     

SIMS study of the concentration of dopants in LPCVD silicon thin films

Measurements of solid phase dopant concentration (S) of LPCVD Si thin films as a function of substrate temperature (T_s = 500−640 ° C) and gas phase doping ratio (R = 1 × 10⁻⁵ −4 × 10⁻²) by SIMS indicate different behaviors of P and B in the films. A linear relation S = b(T)R is observed for B-doped film with b(T) varying from 4 to 50 depending on T_s. Boron-doped microcrystalline film has a higher doping efficiency than that of P-doped ones.     

Structural distribution and rigidity of the glass network determined by EPR of Cu

The linewidth-broadening of the EPR spectra of Cu²⁺ in silicate, borate and phosphate glasses was analyzed in terms of the distribution of g_| and A_| (δ_| and δA_|) and related to the distribution of the rigidity of the network structure. X- and K-band spectra were measured for the glasses doped with ⁶³Cu²⁺ (93% abundance). The linewidth of the HFS shoulders with parallel orientation to H increased linearly with increasing m or microwave frequency. δg_| and δA_| showed a marked dependence on glass composition. For example, in Na₂O---B₂O₃ glasses, on going from x (mol% of Na₂O) being small through intermediate to large, δg_| varied from small through large to negligibly small. In contrast to these glasses δg_| was extremely large for 75PbO · 25B₂O₃ glass. The large δg_| for the Na₂O---B₂O₃ glassesof intermediate x was attributed to the coexistence of various borate groups competitively coordinating to Cu²⁺. Negligibly small δg_| for 70Na₂O · 30B₂O₃ glass and extremely large δg_| for 75PbO ·25B₂O₃ glass, both with a narrower structural distribution, reflect regidity of the glass network. The Pb---O bonding is strong enough to distort the coordination of Cu²⁺-complex. The situation is the reverse in Na₂O---B₂O₃ glasses.     

Small-angle X-ray scattering studies of a-Si1−xGex: H alloys

Small-angle X-ray scattering (SAXS) is used to study a-Si:H, a-Ge:H, and a-Si_1−xGe_x: H alloys prepared by plasma-enhanced CVD methods. These amorphous semiconductors show various types of scattering characteristics related to their composition, deposition method, and deposition conditions. For poor quality alloys at low scattering vectors, h, the scattering curve shows an I(h)h⁻² dependence, because of the fractal nature of the inhomogeneities present. Also, a-Ge:H shows these features. Therefore, it is concluded that the inhomogeneities arise principally from clustering of Ge atoms.     

On the engulfment of spherical particles by a moving ice-liquid interface

Second phase particles suspended in a liquid undergoing solidification are either repelled and swept along by the advancing solid-liquid interface or, beyond a critical velocity of the solidification front, engulfed and encapsulated into the solid phase. In this study critical encapsulation velocities were determined for spherical latex particles at an advancing ice-water interface by means of a gradient freezing stage attached to a light microscope. The influence of the particle radius, the temperature gradient at the planar solidification interface, and the viscosity of the melt has been investigated. The experimental data confirm the theoretically predicted 1/R relationship between the particle radius and the critical growth velocity V_c. The effect of the temperature gradient can be described by a power law (V_c ∝ G^1/4), but its influence on V_c may be even weaker. The inverse proportionality between V_c and the dynamic viscosity of the melt may also be weaker than it would result from Stokes' law. The critical distance between particle and solidification interface was calculated to be in the range of 0.76 to 16.3 nm depending on the theoretical model applied. The resulting Hamaker constant of the system ice/polystyrene/water is -1.7 x 10^-21 J.     

Temperature coefficient of electronic polarizability in optical glasses

The temperature coefficient of refractive index dn/dT and that of volume expansion β have been measured by an interferometric procedure designed to determine the coefficients simultaneously, with one sample. The temperature coefficient of electronic polarizability φ have been calculated from dn/dT and β. φ is expressed as the sum of the contributions of each glass component, φ_i·φ_i is expressed as follows: φ_i = ß_iγ_i, where ß_i is the thermal change in cation-oxygen distance and γ_i is the change in electronic polarizability due to the inter-ionic distance change. φ_i decreases as the ionic field strength z/a² of the cation increases, which is the same tendency as that of ß_i. γ_i, calculated from φ_i and ß_i, gradually increases as z/a² increases. This means that φ_i is mainly determined by the elongation of the inter-ionic distance, and it also suggests that the deformation of the electron cloud of oxygen ion due to the inter-ionic distance change, γ_i becomes larger, as the electronic polarizing power of the cation increases.     

A simple method to synthesize single-crystalline β-wollastonite nanowires

The single-crystalline β-wollastonite (β-CaSiO₃) nanowires were prepared via a simple hydrothermal method, in the absence of any template or surfactant using cheap and simple inorganic salts as raw materials. Xonotlite [Ca₆(Si₆O₁₇)(OH)₂] nanowires were first obtained after hydrothermal treatment at a lower temperature of 200 °C for 24 h, and after being calcinated at 800 °C for 2 h, xonotlite nanowires completely transformed into β-wollastonite nanowires and the wire-like structure was preserved. The synthesized β-wollastonite nanowires had a diameter of 10–30 nm, and a length up to tens of micrometers, and the single-crystalline monoclinic parawollastonite structured β-wollastonite was identified by XRD with the space group of P2₁/a and cell constants of a=15.42 Å, b=7.325 Å, c=7.069 Å and β=95.38°. A possible growth mechanism of β-wollastonite nanowires was also proposed. The advantages of this method for the nanowire synthesis lie in the high yield, low temperature and mild reaction conditions, which will allow large-scale production at low cost.     

NMR studies of Na2O---B2O3---SiO2 glasses of mid-alkali content

¹¹B Fourier transform spectra have been used to study the structure of Na₂O---B₂O₃---SiO₂ glasses of mid-alkali content. Based on the measurements of the fraction N₄ of four-coordinated borons, it has been found that for K = mol.% SiO₂/mol.% B₂O₃ 8 and R = mol.% Na₂O/mol.% B₂O₃ = 1, N₄ is obviously smaller than 1 rather than equal to 1 as assumed in the relevant literature. Only when R reaches a value appropriately greater than 1, can the case where N₄ = 1 occur. A structural model suggested in this paper can satisfactorily explain the fact.     

Studies of the facetting of the polished (100) face of CaF2

The present paper deals with studies of the facetting of the polished (1 0 0) surface of CaF₂ during annealing and growth in UHV using low energy electron diffraction (LEED), atomic force microscopy (AFM), and transmission electron microscopy (TEM). First morphological modifications of the polished surfaces become visible at temperatures of T=874 K. Surfaces annealed at T=974 K exhibit a micro-roughening with pyramidal protrusions and corresponding depressions. LEED studies indicate the evolution of {1 1 1} facets. Reflexes from the (1 0 0) surface are not seen. After growth of about 660 monolayers of CaF₂ at T=1093 K and a saturation ratio S=33 from the vapor phase, larger pyramid-like or hip roof-like crystallites are developed. The results of AFM height profiles as well as of the LEED investigations indicate again the formation of {1 1 1} facets as proved by their angles of 54.7 ^o with the base (1 0 0) surface. This shows that the crystallites are homoepitaxially grown on the underlying CaF₂ substrate.     

Crystal growth of KTiOPO4 isomorphs from tungstate and molybdate fluxes

Large inclusion-free crystal of KTiOPO₄, RbTiOPO₄, KTiOAsO₄ and RbTiOAsO₄ have been grown from the tungstate and molybdate fluxes. In this paper, we report the typical growth parameters, the solubilities, and the heats of solution (δH_sol) for these crystals, and briefly comment on their optical quality. In addition to being nonvolatile and of low viscosity, the tungstate and the molybdate fluxes also have significantly faster dissolution kinetics than the pure phosphate or arsenate self-flux. This makes them especially versatile for the exploratory crystal growth of the KTP isomorphs and their solid solutions. Using these fluxes, single crystal boules of up to 2.5×3×5 cm³ have been grown from a 250 ml melt. We have also observed a modification in the crystal growth habit, which in the case of KTiOAsO₄, results in the formation of stable, singular {111} faces.     

基于神经网络和遗传算法的铸锭晶体硅质量控制及工艺优化

铸锭晶体硅是太阳能级晶硅材料的重要来源之一,为了进一步降低硅片成本,需要在保证晶体质量的同时发展大尺寸铸锭晶硅。影响铸造晶体硅质量的热场控制核心参数包括晶体生长速度与生长界面温度梯度之比V/G、壁面热流q、生长界面高度差Δh和硅熔体内部温差ΔT等。针对铸锭晶体硅生长过程中的质量控制问题,本研究基于人工神经网络(ANN)模型对晶体生长过程建立了工艺控制优化方法,利用实验测量数据和数值仿真模拟结果构建铸锭晶体硅生长过程的工艺控制数据集,以底部隔热笼开口和侧、顶加热器功率比作为主要工艺控制参数,V/G、|q|、|Δh|和ΔT为优化目标,建立用于研究晶体生长工艺控制参数和热场参数之间映射关系的神经网络模型。使用训练完成的模型分析底部隔热笼开口及侧、顶加热器功率比对晶体生长过程热场的影响规律,并采用遗传算法(GA)对铸锭晶体硅生长过程的工艺控制参数以提高晶体质量为目标进行优化,最后结合实际生产中的检测图像讨论了V/G对晶体质量的影响。研究表明晶体生长中期的V/G沿横向变化较平缓,对应缺陷较少且分布均匀,因此增大V/G在横向上的均匀度也是提高晶体质量的一个重要因素。     

High-resolution XRD study of stress-modulated YBCO films with various thicknesses

High-quality epitaxial YBa₂Cu₃O_7−δ (YBCO) superconducting films with thicknesses between 0.2 and 2 μm were fabricated on (0 0 l) LaAlO₃ with direct-current sputtering method. The influence of film thickness on the structure and texture was investigated by X-ray diffraction conventional θ–2θ scan and high-resolution reciprocal space mapping (HR-RSM). The films grew with strictly c-axis epitaxial, and no a-axis-oriented growth was observed up to a thickness of 2 μm. Lattice parameters of the YBCO films with different thicknesses were extracted from symmetry and asymmetry HR-RSMs. The X-ray lattice parameter method was used to determine the residual stress in YBCO films by measuring the a-, b-, c-axis strains, respectively. The results showed that YBCO films within thinner than 1 μm were under compressive stress, which was relieved increasing of film thickness. However, beyond 1 μm in thickness, YBCO films exhibited a tensile stress. Based on the experimental analysis, the variety of residual stresses in the films is mainly attributed to oxygen vacancies with thickness of YBCO film increasing.