X-ray diffraction measurements for expanded liquid alkali halides at very high temperatures

In order to study the change of the atomic arrangement in liquid (1-) alkali halides with volume expansion, the energy-dispersive X-ray diffraction and the density for 1-RbCl were measured at temperatures up to 1600°C, exceeding the boiling point at normal pressure. The structure factor and the pair distribution function were obtained as functions of density. It was found that the average coordination number, N₁, decreases substantially with decreasing density, while the nearest-neighbor distance, r₁, remains almost constant or decreases very slightly with density decrease.     

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.     

Nuclear magnetic resonance studies of alkaline earth phosphosilicate and aluminoborosilicate glasses

The ¹¹B, ²⁷Al, ²⁹Si and ³¹P magic angle spinning (MAS) NMR spectra of MO–P₂O₅, MO–SiO₂–P₂O₅ and MO(M^′₂O)–SiO₂–Al₂O₃–B₂O₃ (M=Mg, Ca, Sr and Ba, M^′=Na) glasses were examined. In binary MO–P₂O₅ (M=Ca and Mg) glasses, the distributions of the phosphate sites, P(Q_n), can be expressed by a theoretical prediction that P₂O₅ reacts quantitatively with MO. In the ternary 0.30MO–0.05SiO₂–0.65P₂O₅ glasses, the 6-coordinated silicon sites were detected, whose population increases in the order of MgOxCaO–0.05SiO₂–(0.95−x)P₂O₅ glasses, its population increases with an increase in f (=([P₂O₅]−[MO]−[B₂O₃]−[Na₂O])/[SiO₂]) and has maximum at f=9. The signal due to the 5-coordinated silicon atoms is also observed when x is smaller than 0.45. When three network-forming oxides such as SiO₂, Al₂O₃ and B₂O₃ coexist, Al₂O₃ reacts preferably with MO. The populations of 4-coordinated boron atoms, N₄, are expressed well with r/(1−r), where r=([Na₂O]−[Al₂O₃])/([Na₂O]−[Al₂O₃]+[B₂O₃]). The correlation of the Raman signal at 1210 and 1350 cm⁻¹ with the NMR signal of Si(Q₆) at −215 ppm is also seen.     

Structural characterization of amorphous silicon nitride by molecular dynamics simulation

Computer simulation, using the molecular dynamics (MD) technique, has been carried out on amorphous silicon nitride (a-Si₃N₄) with simple Busing-type potentials. From the MD simulation, the following points have been deduced. (1) The average Si---N bond length obtained from MD results is r_Si---N=1.74 Å, and its coordination number, N_Si---N, is 3.95. The bond angles around a Si and a N atom, N---Si---N and Si---N---Si, are found to be 109.8° ± 12.36° and 127.08° ± 16.63°, respectively. The N---Si---N value obtained is in very good agreement with the tetrahedral bond angle (= 109.47°). Hence, the short-range structural arrangement of a-Si₃N₄ comprises tetrahedral SiN₄ units. The MD results presented in this study also indicate that there exist only a small number of defects such as dangling bonds. (2) These MD results are in good agreement with the reported X-ray and neutron data. The a-Si₃N₄ structure can be reproduced by the MD simulation given in this study.     

铁离子掺杂对铜铬黑颜料呈色性能的影响

为改善CuCr2O4黑色颜料呈色性能,将Fe3+掺杂进入CuCr2O4晶体中,采用共沉淀法制备CuCr2-xFexO4(x=0,0.04,0.05,0.06,0.07),并对所制备样品进行TG-DTA、XRD、SEM、Raman、XPS、UV-Vis吸收光谱和色度值的测试与表征.结果表明,Fe以三价态固溶进入Cr3+位...     

Sr2MoBO6(B=Os、Re、W)的电子结构与光学性质的第一性原理

双钙钛矿氧化物Sr2 MoBO6(B=Os、Re、W)已被预测为具有高磁转变温度的半金属材料.本论文对Sr2 MoBO6的电子结构和光学性质进行了第一性原理计算,并分析了二者之间的内在关系.从能带结构来看,Sr2 MoOsO6和Sr2 MoWO6为半金属,Sr2 MoReO6为普通金属.B 5d与Mo 4d之间的跃迁过...     

高居里温度铁电单晶PIN-PT的机电性能

弛豫铁电单晶Pb(In_1/2Nb_1/2)O₃-PbTiO₃(PIN-PT)相较于常用的Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃(PMN-PT)具有更高的居里温度,在高稳定性、高性能的传感器、换能器方面具有应用前景。本工作采用谐振法研究了[001]方向极化的0.66PIN-0.34PT铁电单晶的全矩阵机电性能参数。0.66PIN-0.34PT 单晶的三方-四方相变温度(T_RT)约为160 ℃,居里温度(T_C)约为260 ℃,室温压电系数d₃₃、d₃₁、d₁₅分别为1 340 pC/N、-780 pC/N、321 pC/N,介电常数ε^T₃₃、ε^S₃₃、ε^T₁₁、ε^S₁₁分别为2 700、905、2 210、1 927,机电耦合系数 k₃₃、k₃₁、k₁₅、k_t分别为 87%、58%、38%、61%。其纵向压电常数(d₃₃)和纵向机电耦合系数(k₃₃)小于 PMN-PT 单晶,但是横向压电性能(d₃₁)和剪切压电性能(d₁₅)都略高于PMN-PT单晶。另外,研究了机电耦合性能随温度的变化趋势,发现0.66PIN-0.34PT单晶在150 ℃以下有较好的温度稳定性。     

Analytical model for chemical vapor deposition of SiO2 films using tetraethoxysilane and ozone

A theoretical model has been developed to analyze chemical vapor deposition (CVD) of SiO₂ from tetraethoxysilane (TEOS) and ozone. The model incorporates both homogeneous gas phase reactions and heterogeneous surface reactions. Two new kinetic mechanisms which include parasitic gas phase reactions are proposed to explain the observed decrease in deposition rate of SiO₂ films as the temperature is increased. The predicted values from the proposed model are found to agree well with all available experimental results over a wide range of experimental conditions. The effect of parasitic gas phase reactions becomes significantly more pronounced with increasing pressure and temperature. Correlations of experimental data on etch rate with surface concentrations predicted by the model at 60 Torr show that one can improve the film quality by increasing the ratio of O₃ to TEOS at the wafer surface, (O₃/TEOS)_wafer, and the wafer temperature.     

Boron-doped a-SiOx:H films prepared by photo-CVD technique

The optoelectronic and structural properties of p-type a-SiO_x:H films have been studied. The deposition parameters e.g. chamber pressure and diborane to silane ratio are optimized to get a film with dark conductivity (σ_d) 7.9×10⁻⁶ S cm⁻¹ and photoconductivity 9.3×10⁻⁶ S cm⁻¹ for an optical gap (E₀₄) of 1.94 eV. The decrease of optical gap accompanied by the increase of conductivity is due to less oxygen incorporation in the film, which is substantiated by the decrease of the intensity of SiO absorption spectra. The properties are very much effected by the chamber pressure and diborane to silane ratio.     

Profiling band structure in GaN devices by electron holography

Electron holography in a field emission gun transmission electron microscope has been used to profile the inner potential V₀ across GaN/x nm In_0.1Ga_0.9N/GaN/(0 0 0 1) sapphire samples (x=10, 40 nm) grown by molecular beam epitaxy and viewed in cross-section. Results are presented which suggest a decrease in V₀ of 3–4 V across the InGaN layer in the [0 0 0 1] direction. It is proposed that the results can be explained by charge accumulation across the InGaN layer and that the opposing contributions due to piezoelectric and polarisation fields are effectively masked by Fermi level pinning.     

Monte Carlo simulations of Meyer-Neldel effect on carrier time-of-flight in a-Si:H

We present Monte Carlo simulations of multiple-trapping transport with Meyer-Neldel effect in a-Si:H assuming exponential band tails. The transit time t_T, and the dispersion parameters, ₁ and ₂, before and after the transit time, are extracted from the simulated currents. The simulations show that including the Meyer-Neldel effect improves the agreement of ₁ and ₂ with experimental data, both at low and high temperatures and fits the time-of-flight drift mobility measurements. Best fits to the data yield T₀ = 263 K, T_{MN = 464 K}, v₀₀ = 5 × 10⁹ s⁻¹ and μ₀ = 4 cm² V⁻¹ s⁻¹ for electrons and T₀ = 409 K, T_{MN = 809 K}, v₀₀ = 6.5 × 10¹⁰ s⁻¹ and μ₀ = 0.5 cm² V⁻¹ for holes.     

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.     

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.     

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.     

Inhomogeneous nature of UV absorption bands of bulk and surface oxygen-deficient centers in silica glasses

Photochemical and photoluminescence studies of oxygen-deficient centers stabilized in the bulk and on the surface of silica glasses clearly demonstrate the inhomogeneous nature of the absorption and luminescence spectra of oxygen-deficient centers. The conclusion is drawn that the inhomogeneity of the absorption spectra is due to the dispersion of the energy of the S₀-S₁ transition, while the inhomogeneity of the luminescence spectra is due to the dispersion of the energy barrier of intersystem crossing. The inhomogeneous nature of the oxygen-deficient centers in silica glasses is assumed to be caused by a small dispersion in the geometrical parameters of different groups of centers with similar chemical properties.     

Radical-assisted metalorganic chemical vapor deposition of ZnSe

Radical-assisted metalorganic chemical vapor deposition (MOCVD) of ZnSe has been performed by using diethylzinc (DEZn) and diethylselenide (DESe) as a source and azo-t-butane ((t-C₄H₉)₂N₂)and nitrogen trifluoride (NF₃) as co-reactants. The growth rate was significantly increased in the measured temperature range of 623 to 723 K.     

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.     

Structural study of magnesium polyphosphate glasses

Magnesium polyphosphate glasses with molar ratios, y=n(MgO)/n(P₂O₅), ranging from 1.0 to 1.9 have been examined by X-ray and neutron scattering and ³¹P magic angle spinning nuclear magnetic resonance spectroscopy to extract information on their short-range, intermediate-range and submicroscopic structure. The depolymerization of the PO₄ chains with rising MgO content is quantitatively described by the increasing concentration of Q¹ and Q⁰ groups determined by NMR. In the pyrophosphate region the Q¹ sites disproportionate to Q⁰ and Q² groups, whereas there is no disproportionation of the Q² sites at metaphosphate composition. The shortening of the real-space distances, r_m, indicates that the structure of the glasses becomes more compact with progressive depolymerization which is due to the increasing connection of the MgO_n polyhedra by sharing the non-bridging oxygen atoms. The Mg–O co-ordination sphere was found to change not significantly in dependence on composition. Heterogeneities about 1 nm in diameter exist in the glasses with MgO content exceeding 46.6 mol% indicated by a weak small angle X-ray scattering.     

Hydrolysis and condensation reactions of Si(OC2H5)4 related to silica fiber drawing

The hydrolysis and condensation reactions of Si(OC₂H₅)₄ (TEOS) at 80°C in the TEOS---H₂O---C₂H₅OH---HCl solutions with H₂O/TEOS molar ratios (r) from 1.0 to 2.0 were followed by gas chromatography (GC) and measurement of molecular weight ( ) of the hydrolyzates, in order to explain the viscosity change of the solutions. It has been found that the siloxane oligomers with average polymerization degree (n) from 2 to 7 are formed in the early stage of reaction and undergo condensation to form higher polymers. The n and the number of silanol groups of the oligomers are increased with increasing r. The fact that the solution viscosity increased once rapidly around the so-called gelation point followed by a sluggish increase with the increase of reaction time for an r of 1.7, while it increased rapidly without a break till gelation for an r of 2.0, was attributed to fewer silanol groups and higher steric hindrance as a result of more −OC₂H₅ groups remaining in the oligomers for an r of 1.7.     

Dendrite spacing in unidirectionally solidified Al-Cu alloy

Directional solidification experiments have been carried out to study the variation in primary and secondary arm spacings with solidification parameters in the Al-Cu system. It is found that the primary arm spacing Z₁ obeys the common correlation Z₁ = KG^-aV^-b in the high velocity regime or low temperature gradient regime, where a and b are constants, the value of K included the composition dependence, G is the temperature gradient in the liquid and V the growth rate; however, it does not obey this correlation for the low velocity regime or high temperature gradient regime but goes through a maximum or a catastrophe as a function of V or G at V = V_cs/k or G = kG_cs, where k is the equilibrium distribution coefficient, and V_cs and G_cs are the critical velocity and temperature gradient at the limit of constitutional undercooling respectively. The initial secondary arm spacings Z₂₀ are nearly independent of G and mainly depended on V, Z₂₀ = 0.016 V^-0.54 (mm). The secondary arm spacing Z₂ tends to c oarsen with time and thus is a function of coarsening time t_f, Z₂ = 0.016t^0.34_f (mm). Theoretical analyses of the primary arm spac ing and the initial secondary arm spacing have been proposed, and the derived relationships agree reasonably well with the above experimental results.