影响中国铅精矿进出口的风险因素分析

对我国过去4a铅精矿进出口状况进行了回顾,分析影响中国铅精矿进出口的风险因素,并预测我国在未来一段时期内铅精矿净进口量将呈现持续增加的态势。     

An intrinsic luminescence in binary lead silicate glasses

The low-temperature photoluminescence (PL) of xPbO · (1 − x)SiO₂ glasses (x = 0.20-0.75) was studied at Т = 10 K. The recorded PL-spectra are a superposition of three spectral components with maxima located at 1.8 eV (identified as Pb 6p → metal-bridging O2p radiative electron transition, the “R”-band), 2.0 eV (Pb 6p → non-bridging O2p, the “O”-band) and 2.55 eV (Pb 6p → Pb 6s, the “B”-band), respectively. It was found the essential link for “R”, “O” and “B” PL-bands with chemical composition x of the glasses under study. These concentration dependences are expressed as mutual PL-intensity variations for each recorded luminescence band that allowed to determine their origin. The shape of established dependences well coincides with numerical data on NBO- and MBO-density of chemical bonding, reported previously.The overall PL-manner within the temperature range of 10-295 K is described by an empirical Street’s law. It was shown that experimental photoluminescence quenching curves may be precisely approximated as a superposition of Mott relationships for nonequivalent luminescence centers. The obtained distribution of PL-centers on the activation energy for luminescence quenching reflects the essential donation of the low-energy states into the overall PL-process. The width of this energy distribution affects by the type of PL-emission band and the disordering degree in the arrangement of local PL-centers of a certain kind.     

Large oscillator strength excitons in PbGa2S4 crystals

Exciton states with a large oscillator strength (transverse-longitudinal splitting of 50 meV) and binding energy of 290 meV have been observed in PbGa₂S₄ crystals. The ground exciton states are stable up to the room temperature. The ground (n = 1) exciton state and two excited (n = 2 and n = 3) states of the short-wavelength B-excitons series were observed at low temperature (T = 10 K). The ground (n = 1) and the excited (n = 2) states of the long-wavelength A-exciton series were also revealed. The exciton Bohr radius of the ground exciton state was determined to be 70 Å for the A-exciton and 10 Å for the B-exciton. The band splitting and the main parameters of excitons have been determined from the calculation of the reflectivity spectra contours. The group theory analysis of the band symmetry was performed and the scheme of bands responsible for exciton transitions in the center of the Brillouin zone was proposed.     

Diffuse characteristics and piezoelectric properties of Tb-doped BCZT ceramics with CaCl2 as sintering aid

Lead-free ceramics 47(Ba_0.7Ca_0.3)TiO₃–0.53Ba(Zr_0.2Ti_0.8)O₃ (BCZT) + 0.2 mol% Tb and xCaCl₂ (x = 0.1,0.2 and 0.3 wt%) were prepared at different temperatures (1300, 1350 and 1410 ℃) by solid-state reaction method. The experimental results reveal that dopants can effectively promote the growth of grain at relatively low sintering temperature and CaCl₂ as the well sintering aid can produce a pronounced liquid phase to fill the gap between grains sintered at 1410 ℃ with 0.2 wt%. Relative densities and piezoelectric properties of samples can be improved by the introduction of Tb and CaCl₂. Comparing with pure BCZT, the residual polarization (P_r) and piezoelectric coefficient (d₃₃^*) of ceramics increase from 8.2 to 11.4 μC/cm² and 568–720 pm/V with 0.2 mol% Tb and 0.2 wt% CaCl₂ doped. The enhanced properties of samples may be related to the appearance of liquid phase which can increase relative densities of ceramics to 96% and the form of defect dipole in ceramics. Moreover, introduction of CaCl₂ makes dielectric peaks (T_c) move to the high-temperature zone about 13 ℃ and become more and more diffuse (γ = 1.71–1.86), which may have the potential electro-caloric effect application in ferroelectrics.     

烟花爆竹用氧化铜原材料杂质分析及其影响

针对烟花爆竹常用的原材料氧化铜来源各异,可能含有铅化物、铁、钠盐等杂质的问题,采用AAS法对35家烟花爆竹企业所用的氧化铜原材料进行了铅含量分析,结果表明约70%厂家的铅含量超出了欧盟标准,3%厂家的铅含量超出了美国标准。此外,通过XRF对氧化铜样品扫描,发现部分氧化铜还含铁、氯化物、钠盐、砷化物等杂质,并分析其对烟花爆竹产品的影响。本研究可为烟花爆竹原材料厂家和生产企业提供一定的指导。     

Static structure and dynamic properties in liquid Sn96.2 Ag3.8 lead free solder: Structure factor,diffusion coefficients and viscosity

Starting from the pseudopotential formalism, we developed a new pseudopotential able to describe accurately both heavy metals like tin and noble metals like copper and silver which are the basic elements of the new lead free solders. The pseudopotential describes the interaction between electrons and ions. To describe the atomic structure it is necessary to use an effective pair potential describing the interaction between ions taking into account the electron screening cloud. This pair potential is deduced from our new pseudopotential. It enters into a simulation program using molecular dynamics. First we calculate the atomic structure factor that has been compared to that measured experimentally by neutron or X ray scattering. In the case of a binary alloy, we determined three partial structure factors describing the system. These can be calculated but in general cannot be obtained experimentally. To do a comparison we must combine adequately the calculated partial structure factors and compare it to the experimental total structure factor. By molecular dynamics we can follow the individual movement of particles to obtain the different diffusion coefficients which, as partial structure factors, are very difficult to measure accurately. We followed the collective movement of all particles and got the shear viscosity by Green–Kubo formalism. We compared our results to the existing experimental viscosity which is a little easier to measure. However, in the bibliography, we observe high discrepancies between the different authors. It is thus important to be able to calculate accurately these properties as functions of temperature and composition.     

Domain switching characteristics of lead zirconate titanate piezoelectric ceramics on a nanoscopic scale

Domain switching characteristics of lead zirconate titanate ceramics with and without poling under compressive loading are investigated using electron backscatter diffraction. For loading in the poling direction, the switching strain is stronger than that for loading perpendicular to the poling direction. There is strong domain switching when the domain (c-axis of the tetragonal structure) is orientated close to the loading direction. A large number of domains are switched between 85.4° and 90.0°, with many crossing the loading axis. Each grain consists of domains with three different patterns; i.e., with c-axis orientated in three directions in each grain. The patterns remain unchanged even with domain switching and strong deformation. However, the ratios among the patterns depend on compressive stress. Under stress, one or two specific domain modes are switched to about 90°, although others are not switched as much. These domain switching characteristics are related to the poling and loading directions. 90° domain switching model is proposed on the basis of twin deformation model. Due to the aspect ratio of c/a = 1.014 (tetragonal structure), the angle of the switching is less than 90° (89.2°). This angle is corresponding to the switching angle obtained by an electron backscatter diffraction analysis (Ave. 88.9°).     

Free carrier absorption in self-activated PbWO4 and Ce-doped Y3(Al0.25Ga0.75)3O12 and Gd3Al2Ga3O12 garnet scintillators

Nonequilibrium carrier dynamics in the scintillators prospective for fast timing in high energy physics and medical imaging applications was studied. The time-resolved free carrier absorption investigation was carried out to study the dynamics of nonequilibrium carriers in wide-band-gap scintillation materials: self-activated led tungstate (PbWO₄, PWO) ant two garnet crystals, GAGG:Ce and YAGG:Ce. It was shown that free electrons appear in the conduction band of PWO and YAGG:Ce crystals within a sub-picosecond time scale, while the free holes in GAGG:Ce appear due to delocalization from Gd³⁺ ground states to the valence band within a few picoseconds after short-pulse excitation. The influence of Gd ions on the nonequilibrium carrier dynamics is discussed on the base of comparison the results of the free carrier absorption in GAGG:Ce containing gadolinium and in YAGG without Gd in the host lattice.     

Effect of wall wettability condition on drift-flux parameters in lead–bismuth two-phase flow in circular and annular bubble columns

Lead–bismuth two-phase flow in a cylindrical vessel and annulus was experimentally investigated by varying the surface wettability of the vessel wall. The test section used in this study was a cylindrical stainless vessel with/without inner sleeve to change the hydraulic diameter. Volume-averaged void fraction was measured by varying the surface wettability of the test section, which was enhanced by using a soldering flux. Measured void fraction was compared with existing two-phase flow correlations and with one-dimensional theoretical simulations assuming one-dimensional drift-flux model. From experimental results, measured distribution parameters of the lead–bismuth two-phase flow are much larger than that of ordinary two-phase flow regardless of the surface wettability. In the present work, the one-dimensional analysis was carried out for the cylindrical vessel to reproduce the distribution parameter. From the simulation results, predicted value for the cylindrical vessel showed good agreement with experimental results. However, in annulus, the distribution parameters in annulus were underestimated by the present model. It was suggested that, in case of annulus, steeper void fraction profile might be formed near the inner surface for poor wettability condition.     

A comprehensive theoretical study of halide perovskites ABX3

Solar cells based on halide perovskites have recently been attractive due to their excellent power conversion efficiency (PCE), lower cost and simple manufacture. Here, a series of halide perovskites (ABX₃: A = CH₃NH₃, CH(NH₂)₂, Cs, Rb; B = Pb, Sn, Ge; X = I, Br, Cl, F) were investigated by Density Functional Theory (DFT) calculations, together with Shockley-Queisser Maximum Solar Cell Efficiency (S-Q) and Spectroscopic Limited Maximum Efficiency (SLME) mathematical models. The results indicate that: the electronic structure of germanium perovskites bears a close similarity to that of lead perovskites with a small energy difference between the nonbonding orbital and antibonding orbitals, but with a large energy difference comparing with that of tin perovskites (0.6–1.7 eV for CsGeI₃ at Z point of the Brillouin zone, 0.7–1.4 eV for CH₃NH₃PbI₃ and 1.4–2.2 eV for CH₃NH₃SnI₃ at R point of the Brillouin zone), which is attributable to the atomic level, where the 4s orbital energy of Ge (−11.5 eV) is close to the 6s orbital energy of Pb (−11.6 eV), but the 5s orbital energy of Sn (−10.1 eV) is significantly high. Therefore, germanium perovskites possess as high absorption coefficient around solar spectrum as lead perovskites, while tin perovskites only have low absorption coefficient, which makes the short-circuit current of CsGeI₃ and CH₃NH₃PbI₃ (0.017 Acm⁻² and 0.016 Acm⁻², simulated by SLME with a 200 nm absorber under AM1.5G) are higher than that of CH₃NH₃SnI₃ (0.015 Acm⁻²) even if the bandgap of CsGeI₃ and CH₃NH₃PbI₃ (1.51 eV and 1.55 eV) are larger than that of CH₃NH₃SnI₃ (1.21 eV). Meanwhile, the effective mass of electrons and holes are approximate for germanium perovskites and lead perovskites (0.14:0.19 for CsGeI₃ and 0.12:0.12 for CH₃NH₃PbI₃), indicating a balanced electrons and holes transport, whereas the electrons transport is much slower than the holes transport for tin perovskites due to the effective mass of electron is much larger than that of hole (0.17:0.04 for CH₃NH₃SnI₃). As a result, the PCE of CsGeI₃ (27.9%) and CH₃NH₃PbI₃ (26.7%) is higher than that of CH₃NH₃SnI₃ (19.9%).