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.     

Measurement of flow phenomena using the ultrasonic velocity profile method in a simulated Czochralski crystal puller

An experimental investigation of flow phenomena in a simulated Czochralski (CZ) crystal puller was conducted using an ultrasound velocity profile (UVP) probe to measure fluid velocities. To isolate the various forces that influence the fluid motion, only the crystal was rotated and a water-glycerol mixture was used as the test fluid in this experiment. Measurements of the velocity components, V_z and V_r, under the initial transient from zero rotation to a set rotation rate as well as under steady-state conditions were made. The corresponding Reynolds number based on the crystal radius and rotation varied from Re_Ω,_x ≈ 660–5000 (Ω_x ≈ 110–2000 rpm). The measured velocity profiles, V_z and V_r, were processed using a simple color image scheme based on the magnitude and direction of flow with respect to the probe. These profiles were studied to postulate flow patterns in the simulated CZ device. The measurements revealed three major regions of flow: (1) one beneath the crystal, (2) one near the free surface and crucible wall and (3) one beneath the crystal and occupying the extent of the crucible. In the initial transient, spatially symmetric outward flow beneath the crystal developed until the recirculating flow from below disturbed this pattern. At this point the apparent shear between the flow beneath the crystal and that circulating upwards from the bottom of the crucible induced flow instabilities and a non-axisymmetric flow pattern. The non-axisymmety was verified by the color-coded velocity profiles and a power spectrum analysis. The analysis revealed the critical Reynolds number for the onset of the primary instability to be Re_Ωcrit ≈ 669. The linear variation in the power of the primary peak with the Reynolds number indicates that the onset of the axisymmetry-breaking instability is due to Hopf bifurcation.     

Growth by self-flux method and post-annealing effect of Bi2Sr2Ca1Cu2Ox single crystals

Post-annealing effects on superconducting characteristics have been studied in Bi₂Sr₂Ca₁Cu₂O_x single crystals grown by a conventional flux method. Also, favorable growth conditions and the effect of the pre-sintering process on the starting materials for flux growth have been examined. The best superconducting behavior is obtained in post-annealed crystals grown from pre-sintered powder materials. The critical current density J_c estimated from magnetization hysteresis in annealed crystals grown with pre-sintered materials is roughly 8×10⁵ A/cm² (Hc) and 5×10⁴ A/cm² (Hc) at 4.2 K at zero field.     

The Effect of Solid and Liquid Inclusions on Morphology of Crystallization Front

The interaction between crystallization front and solid particles was studied for substances characterized by the normal growth mechanism (continuous growth). It has been measured the critical velocity below which the particle of radius R is repulsed by moving interface and above it is captured. It has been shown that critical velocity is proportional to R⁻(1.4–1.8) what satisfactorily agrees with Chernov-Temkin's theory. Data have been received about change of interface morphology at capture of solid particles for growth from pure and impurity-contained melt. The dendrites have been found to split at interaction with particles and concentration inhomogeneities. Taking for example Al-Cu alloy, possibility has been shown to disperse the dendritic structure by formation concentration inhomogeneities in the melt.     

Elastic constant anomaly in Fe80−xMoxB20 metallic glasses

The extensional sound velocities V_E of Fe_80−xMo_xB₂₀(x = 6, 7, 8) metallic glasses have been measured between 100 and 600 K in a saturating magnetic field. Young's modulus (E = _E², = density) shows anomalous temperature dependence at and below the magnetic transition temperature of each glass studied. These anomalies are interpreted as arising from the change of long-range spin coupling energy at the Curie temperature in a molecular field approximation. Comparison is made with similar effects in other crystalline and non-crystalline magnetic materials.     

Influence of boron concentration on the oxidation-induced stacking fault ring in Czochralski silicon crystals

The influence of the boron doping level in the range of 1 × 10¹⁵-2 × 10¹⁹ cm⁻³ on the position of the oxidation-induced stacking fault ring (R-OSF) in silicon crystals has been investigated by experiments and numerical simulation. For low boron-doped crystals, the position of the R-OSF is described by a critical value C_crit defined by the ratio of the pull rate and the temperature gradient in the crystal at the solid/liquid interface. Boron concentrations higher than 10¹⁷ cm⁻³ shift the position of the R-OSF towards the wafer center without change of growth parameters. The critical value C_crit converts into a function C_crit(C_B, depending linearly on the boron concentration C_B. Crystal-originated particles (COP) and gate oxide integrity (GOI) yield distributions which are consistent with the R-OSF pattern. A low COP density and a high GOI yield are observed outside the ring; a high COP density and a medium GOI yield in the inner region bordered by the ring. It is assumed that boron atoms modify the thermodynamical properties of vacancies and self-interstitials.     

Ag precipitation and optical behavior in Na2O---B2O3 glasses

Ag particles of different sizes in the nanometer range were produced in Na₂O---B₂O₃ glasses containing Ag₂O by the melt-quenching and heat-treatment method. The quenching rate was 10³ K s⁻¹ and the heat treatment was at 738 K for 2–300 h. The precipitation was dependent on diffusion limited growth. The optical absorption of Ag particles in the glasses was measured and correlated to the distribution of particle radii. The peak energy of the surface plasmon resonance was blue shifted and the width decreased with increasing average particle radius. These results are compared with previous data on similar systems.     

吡啶鎓盐配体构筑的多钼酸基配合物的合成、结构及光催化性能

本文在水热条件下合成了一例同多钼酸阴离子[δ-Mo₈O₂₆]^4-基配合物[Co(bipbc)(δ-Mo₈O₂₆)_0.5(H₂O)₃] (bipbc=4,4-双[(4-羧基吡啶)甲基]联苯),其为一维链状结构,包含环型双核钴配合物[Co₂(bipbc)₂]⁴⁺和[δ-Mo₈O₂₆]^4-簇。该配合物结晶于单斜晶系,P2₁/n空间群,a=1.147 9(8) nm,b=1.440 9(11) nm,c=2.082 9(16) nm,β=93.469(2)°,V=3.438 8(4) nm³,Z=4,M_r=1 129.18,F(000)=2 204,μ=1.979 mm^-1,D_c=2.181 mg·m^-3,S=1.019,R₁=0.056 2,wR₂=0.137 7。光催化性质研究表明,在可见光、近红外光和全光谱的光照条件下,标题配合物对龙胆紫(GV)和亚甲基蓝(MB)的降解,表现出一定的光催化活性。     

原位ATR-IR光谱实时研究DKDP的结晶过程

采用全反模式的红外(ATR-IR)光谱实时研究了不同氘含量KH_2-xD_xPO₄(DKDP)晶体的结晶过程,其中DKDP溶液氘含量范围为0到99%。通过υ₁(PO₄)和υ₃(PO₄)振动表征了DKDP结晶溶液中(H₂PO₄^-)_1-x(D₂PO₄^-)_x离子基团浓度的变化。υ₁(PO₄)振动强度的变化和υ₃(PO₄)振动宽度的变化说明生长溶液中的(H₂PO₄^-)_1-x(D₂PO₄^-)_x离子浓度随着测试时间延长而不断增大。同时,δ(P—O…H/D—O—P)振动峰的形成说明DKDP晶体的生长基元为(H₂PO₄^-)_n-x(D₂PO₄^-)_x离子团簇。波数在1 448 cm^-1到1 653 cm^-1范围内H—O—H和D—O—D振动强度的变化解释了DKDP晶体在结晶过程中氘含量分布不均的现象。     

Formation and dissolution of InAs quantum dots on GaAs

In situ reflection high energy electron diffraction (RHEED) has been used to study the time evolution during self-assembled molecular beam epitaxy (MBE) growth of InAs quantum dots on GaAs. Using a special data acquisition technique, two characteristic time constants are determined very precisely: the time t_c up to the first appearance of InAs dots and the time t_f it takes to complete the 2D–3D transition of all islands. Surprisingly, we find that t_c increases with temperature which disagrees with a thermally activated process. In contrast to this, t_f behaves Arrhenius-like and an activation energy of E_f0.39 eV is determined. Furthermore, the sum t_c+t_f does not depend significantly on temperature and corresponds to an InAs coverage of 2.0 monolayers. A second focus of this paper is the study of dissolution of InAs dots after interruption of the As flux. From the experiments, an activation energy of 3.2 eV for desorption of In located on top of the wetting layer is determined, whereas direct desorption from the wetting layer corresponds to an activation energy of 3.4 eV.     

Selective epitaxial growth by UHV-CVD using Si2H6 and Cl2

The conditions under which selective epitaxial growth (SEG) is achieved in UHV-CVD with Si₂H₆ are determined by the amount of Si₂H₆ molecules being supplied, and there is a critical gas supply amount (F_c) beyond which SEG will break down and lose its selectivity. The value of F_c is itself determined by two factors, growth temperature and the material used for masking, i.e. SiO₂, Si₃N₄. We found that this limiting factor of F_c was increased through the addition of a small amount of Cl₂, and that after such addition, the resulting decrease in growth rate is minimal.     

Glass-forming ability versus stability of silicate glasses. II. Theoretical demonstration

Possible relationships between measures of glass stability (GS) against devitrification on heating (evaluated by the Hrubÿ parameter K_H=(T_c^h−T_g)/(T_m−T_c^h), and the parameter K_w=(T_c^h−T_g)/T_m) and a criterion of glass-forming ability (GFA) – the critical cooling rate – were investigated by computing non-isothermal crystallization for typical values of the main quantities that control crystal nucleation and growth in silicate glasses. We limit these quantities to one thermodynamic parameter – the melting entropy (ΔS_m) and two kinetic parameters that control the viscosity (B and T₀ in the Vogel–Fulcher–Tamman equation or T_g and in Avramov’s equation). The effect of heterogeneous nucleation and, in particular, the possible role of the surface as active substrate is tested. The results presented herein demonstrate that GS and GFA are indeed related concepts.     

The effect of mixed alkaline earths on the diffusivity and the incorporation of iron in 5Na2O · xMgO · (15−x)CaO · yAl2O3 · (80−y)SiO2 melts

Diffusion coefficients of iron were measured in glass melts with the basic compositions 5Na₂O · xMgO · (15−x)CaO · yAl₂O₃ · (80−y)SiO₂ with x=5, 10 and y=0, 5, 7.5, 10 and 15. The melts were doped with 0.25 mol% Fe₂O₃ and studied in the temperature range from 1000 to 1600 °C using square-wave voltammetry. The voltammograms exhibited distinct peaks attributed to the reduction of Fe³⁺ to Fe²⁺, from which peak currents mixed diffusion coefficients of iron were calculated. Diffusion coefficients in all melt compositions which did not show crystallization could be fitted to Arrhenius equation. The diffusivities measured in different melt compositions were related to the same viscosity, i.e. not the same temperature. Increasing the alumina concentration from 5 to 10 mol% resulted in an increase of the viscosity corrected diffusivities. At further increasing alumina concentrations, the diffusivities get smaller again. This can be explained by the stabilizing effect of Na⁺ and Ca²⁺ on FeO⁻₄ and AlO⁻₄-tetrahedra, which strengthens the incorporation of Fe³⁺ into the glass structure.     

制备方法对Al2O3-CeO2物化性质及CO2加氢制甲醇催化性能的影响

复合氧化物界面性质与CO₂加氢制甲醇反应的催化性能有着重要的关系。本文对比考察了物理共混法、浸渍法、传统共沉淀法和微流控连续共沉淀法对Al₂O₃-CeO₂复合氧化物界面性质和催化性能的影响。浸渍作用尽管使Al₂O₃/CeO₂界面产生了一定的结构性质调变,但贫瘠的氧空位缺陷导致催化反应效率低。共沉淀样品中固溶结构的存在增强了Al₂O₃/CeO₂界面的相互作用,增大了电子结合能,形成的大量氧空位缺陷有利于CO₂活化转化。而微流控连续共沉淀法合成样品因具有更小的晶粒尺寸、均匀的复合相结构和丰富的氧空位缺陷,表现出更为优异的催化性能。在原料气配比为V(H₂)∶V(CO₂)∶V(N₂)=72∶24∶4,反应温度为320 ℃,反应压力为3 MPa,体积空速为9 000 mL·g^-1·h^-1的条件下,Al₂O₃-CeO₂复合氧化物的CO₂转化率、甲醇选择性及甲醇时空产率分别达到15.3%,86.4%和0.076 g·mL^-1·h^-1。     

Viscosity and IR investigations in the Li2O---B2O3 system

The high viscosity in melts of the Li₂O---B₂O₃ system makes it very difficult to grow large crystals of lithium triborate. The viscosity and IR characteristics of molten li₂O---B₂O₃ system are reported in this paper. When the temperature increases the viscosity of li₂O---B₂O₃ system decreases and follows an Arrhenius-type relationship. With an increasing 13203 ratio in Li₂O---B₂O₃ melts, the viscosity rises gradually to a maximum with a composition Li₂O: 3.513203 then it falls rapidly. In order to find active agents to reduce the viscosity, Na₂O, NaCl, LiF, P205, M003, W03 etc oxides were added to Li₂O---B₂O₃ samples respectively and investigated using the orthogonal method. The experimental results show that the addition of acidic oxides can significantly decrease the viscosity in the Li₂O---B₂O₃ system. For Li₂O: 4.513203, an ideal additive agent is 20wt% Li₂O:: 2MoO₃. Near the composition for crystal growth, the percentage reduction of viscosity is 62.2%. The IR spectra of Li₂O---B₂O₃ system revealed that the BO₄⁻/NO₃⁻ ratio is reduced in the melt using Li₂O: 2MoO₃ as an additive. It is proposed that the M003 reduced the concentration of bridging oxygen atoms of BO₄⁻. The change of structure explains the decline in the viscosity. In the crystal structures of lithium triborate, the matrix spaces are so small that larger other cations than Li+ are very difficult to enter the crystal matrix. So the use of additive agents to reduce the viscosity is a possible method if no new phase appears.     

An infrared study of crystallization in sodium-disilicate glasses containing iron oxides

The infrared absorption spectra of sodium-disilicate glasses containing various amounts of Fe₂O₃ ([Na₂O · 2 SiO₂]_1−x [Fe₂O₃]_x, where X = 0.05, 0.1 and 0.2) were investigated in the wavenumber range from 200–2000 cm⁻¹. The addition of Fe₂O₃ to the sodium-disilicate glass does not seem to introduce any new absorption band as compared with the spectrum of a pure sodium-disilicate glass; nevertheless, a general shift of the existing absorption bands toward lower wavenumbers is observed. The amount of shift is, in fact, proportional to the content of Fe₂O₃ in the glass. This observation is consistent with the recently proposed structural model for the bonding of Fe³⁺ ions in the iron-sodium-silicate glass system.

Annealing of 20 mol% iron oxide glasses at 550 and 580°C produced an extra sharp infrared absorption peak at about 610 cm⁻¹ wavenumber. This new peak is believed to be related to the crystallized particles of the glass as concluded from both a scanning electron micrograph and an electron diffraction pattern.     

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.     

Raman spectroscopic study on the structure of ZnCl2---ZnX2 and ZnCl2---KX (X = Br, I) glasses

Raman spectra have been measured for ZnCl₂---ZnX₂ and ZnCl₂---KX (X = Br, I) glasses to investigate the structure of the glasses with varying composition. The assignment of each band was made, and the change of the spectra with composition was explained in terms of the bridging and non-bridging states of halide ions and the change of the tetrahedral units, ZnX_nCl_4−n²⁻ (n = 0–4), formed in the glasses. As the content of ZnX₂ in ZnCl₂---ZnX₂ glasses increases (20 → 80 mol%), the peak frequency of the Zn---Cl stretching mode increases (238 → 248 cm⁻¹ in X = I glasses, 238 → 259 cm⁻¹ in X = Br glasses) while the Zn---I and Zn---Br stretching frequencies decrease (173 → 120 cm⁻¹ for Zn---I, 196 → 157 cm⁻¹ for Zn---Br). The decrease of the Zn---I and Zn---Br band frequencies was attributed to the increase of the number n of the ZnX_nCl_4−n²⁻ tetrahedra. The increase of the Zn---Cl frequency suggests the existence of the bonding state of Cl⁻ ions which is intermediate between the bridging and the non-bridging states. In ZnCl₂---KX glasses, the Zn---Cl_non-bridging band at about 300 cm⁻¹ was observed in addition to the bands observed in ZnCl₂---ZnX₂ glasses. The addition of KX produces non-bridging anions while the tetrahedral units, ZnX_nCl_4−n²⁻ are also formed.     

多孔坩埚温度梯度法生长Ho,Y∶CaF2晶体及其光谱性能

采用自主设计改造的温梯炉,成功生长了不同浓度Ho³⁺、Y³⁺掺杂的CaF₂及Sr_xCa_1-xF₂晶体,晶体尺寸约为ϕ15 mm×55 mm,生长周期约为6 d,能够实现7种不同浓度晶体的同步生长,并选取其中的4%(原子数分数)Ho,4%Y∶CaF₂晶体进行分析,吸收测试表明,该晶体448 nm和643 nm处吸收峰的吸收截面分别是1.13×10^-20 cm²和0.84×10^-20 cm², J-O理论分析得到了晶场强度参数Ω_t(t=2、4、6)、辐射跃迁几率、荧光分支比和辐射寿命。在448 nm氙灯激发下,经计算得到该晶体在546 nm、650 nm 和752 nm处的发射截面分别为10.450×10^-21 cm²、8.737×10^-21 cm²和5.965×10^-21 cm²,测得⁵F₄和⁵F₅能级的寿命分别为33.5 μs和17.7 μs。在640 nm LD泵浦激发下,经计算得到该晶体2 031 nm处发射截面为5.375×10^-21 cm²,2 847 nm处发射截面为10.356×10^-21 cm²,测得⁵I₇和⁵I₆ 能级的寿命分别为4.37 ms 和1.85 ms。以上结果表明,多孔坩埚温梯法能够大大提高激光晶体稀土离子掺杂浓度筛选的效率,加快新型激光晶体材料的研发速度。     

Interaction between colloidal particles in SiO2 and TiO2 sols

The effect of the interaction between colloidal particles in SiO₂ and TiO₂ sols on the viscosity of the sols has been examined. It was found that the interaction was significantly influenced by the zeta potential of electrical double layers and terminal bonds associated with particle surfaces. Water/alkoxide mole ratio and pH affect the zeta potential and the terminal bonds and, consequently, change the interaction between the particles and the viscosity of the sols. In addition, the relationship of viscosity with the volume fraction of the colloidal particles in SiO₂ sol has been described by a Dougherty-Kreiger equation when the viscosity reached a stable value with time.