六角片状氢氧化铝的制备

介绍了一种制备六角片状氢氧化铝的方法。对影响氢氧化铝晶体形状的因素如添加剂、铝酸钠溶液的分解温度、分解时间、分解率以及产物氢氧化铝的性质等进行了深入的研究。结果表明，控制适当的分解条件可以产出适用于造纸涂层材料的六角片状氢氧化铝，其平均粒度为2μm左右，白度≥95％，比表面积6.5-8m^2/g。     

Current Understanding of Structure–Processing–Property Relationships in BaTiO3–Bi(M)O3 Dielectrics

As part of a continued push for high permittivity dielectrics suitable for use at elevated operating temperatures and/or large electric fields, modifications of BaTiO₃ with Bi(M)O₃, where M represents a net‐trivalent B‐site occupied by one or more species, have received a great deal of recent attention. Materials in this composition family exhibit weakly coupled relaxor behavior that is not only remarkably stable at high temperatures and under large electric fields, but is also quite similar across various identities of M. Moderate levels of Bi content (as much as 50 mol%) appear to be crucial to the stability of the dielectric response. In addition, the presence of significant Bi reduces the processing temperatures required for densification and increases the required oxygen content in processing atmospheres relative to traditional X7R‐type BaTiO₃‐based dielectrics. Although detailed understanding of the structure–processing–property relationships in this class of materials is still in its infancy, this article reviews the current state of understanding of the mechanisms underlying the high and stable values of both relative permittivity and resistivity that are characteristic of BaTiO₃‐Bi(M)O₃ dielectrics as well as the processing challenges and opportunities associated with these materials.     

Mechanosynthesis and Thermal Stability of Piezoelectric Perovskite 0.92Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 Powders

The mechanosynthesis of piezoelectric perovskite 0.92Pb(Zn_1/3Nb_2/3)O₃–0.08PbTiO₃ (PZN–PT) by direct mechanochemical activation of the constituent oxides has been studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). This and the PbO flux method are the only two procedures that have succeeded in synthesizing this phase, which has recently been shown to present very high electromechanical response. The thermal stability of the single perovskite phase powders has been studied by differential thermal analysis/thermogravimetry and by high-temperature XRD as a function of mechanical activation time and pressure. The phase was found to transform into a pyrochlore type structure at temperatures above 400°C. The transformation presented a significant time dependence, and it was slowed down by increasing mechanical activation time and by the application of pressures by hot pressing. Sintering experiments were accomplished and 85% density, 77% perovskite-phase ceramics were obtained after heating at 1000°C for 1 h. Hot pressing at this temperature failed to increase the percentage of perovskite phase. Results are discussed, and procedures for obtaining dense single-phase PZN–PT-based ceramics with ultrahigh piezoelectricity are proposed.     

A-Site and B-Site Order in (Na1/2La1/2)(Mg1/3Nb2/3)O3 Perovskite

(Na_1/2La_1/2)(Mg_1/3Nb_2/3)O₃ undergoes a series of phase transitions that involve cation order on the A- and B-sites of the parent perovskite structure. At high temperatures both sites contain a random distribution of cations; below 1275°C a 〈111〉 layering of Mg and Nb leads to the formation of a 1:2 ordered structure with a monoclinic supercell. A second transition was observed at 925°C, where the Na and La cations order onto alternate A-site positions along the 〈001〉 direction of the parent subcell. By quenching samples from above 1275°C to preserve the disorder on the B-site, a fourth variant of this compound was obtained by inducing A-site order through a subsequent anneal at 900°C. Although the changes in structure do not produce significant alterations in the relative permittivity (ɛ_r∼ 35), they do have a significant effect on the value of the temperature coefficient of the capacitance.     

Dielectric Behavior of Na0.5Bi0.5TiO3-Based Composites Incorporating Silver Particles

The dielectric properties of Na_0.5Bi_0.5TiO₃ (NBT) -based composites incorporating silver particles prepared by sintering at a low temperature of ∼900°C are reported. The dielectric constant increases with the amount of metal silver particles in the measured frequency range (150 Hz to 1 MHz), and could be enhanced up to ∼20 times higher than that of pure NBT ceramics, which was ascribed to the effective electric fields developed between the dispersed particles in the matrix and the percolation effect. Further investigation revealed that the dielectric constant of the composites has weak frequency and temperature dependence (−50°C to +50°C).     

Relationship between the Bond Valence and the Temperature Coefficient of the Resonant Frequency in the Complex Perovskite (Pb1−xCax)[Fe0.5(Nb1−yTay)0.5]O3

The temperature coefficient of the resonant frequency (TCF) of complex perovskite (Pb_1−xCa_x)[Fe_0.5(Nb_1−yTa_y)_0.5]O₃ ceramics (x= 0.5, 0.55; 0.0 ≤y≤ 1.0) was investigated, relative to the bond valence of the A- and B-site ions in the ABO₃ perovskite structure (such as the barium-, strontium-, and calcium-based complex perovskites). The TCF of these complex perovskite compounds varied with the bond valence of the A- and B-sites and the tolerance factor (t) in the perovskite structure. In the tilted region (t < 1.0), the tilting of the oxygen octahedra increased and the TCF decreased, because of the increased bond valence of the B-site. Also, the dependence of TCF on the bond valence of the A-site was similar to its dependence on t.     

以三乙醇胺硼酸酯为前驱体制备大颗粒六方氮化硼

以三乙醇胺和硼酸为原料,甲苯为带水剂,合成了三乙醇胺硼酸酯,在最佳工艺条件下以三乙醇胺硼酸酯为前驱体在氨气气氛中烧结制得六方氮化硼颗粒。采用IR、XRD、SEM、NMR等测试方法对中间物和产物进行了表征,确定了中间物及产物的组成、物相、粒度及形貌。研究结果表明：以三乙醇胺硼酸酯为前驱体在1200℃烧结4h,可制得高纯度、晶型良好,粒径在20μm左右的六方氮化硼颗粒。经过高温精制后的六方氮化硼颗粒具有较高的结晶度和致密度,其粒径可以达到30μm以上,纯度为98.5%。     

Nanocrystalline buildup,relaxor behavior,and polarization characteristic in PZT–PNZN quaternary ferroelectrics

The Pb(Zr_0.47Ti_0.53)O₃–Pb[(Ni_0.6Zn_0.4)_1/3Nb_2/3]O₃ (PZT–PNZN) quaternary ferroelectrics presenting the high piezoelectric coefficients are the active element in a range of piezoelectric devices. However, the buildup of nanoceramics with stable perovskite structure is still a big challenge, which prohibits the miniaturization of piezoelectric devices. In this work, the high dense PZT–PNZN nanoceramics with average grain size of 130 nm were prepared by a combination of high‐energy ball milling (HEBM) and spark plasma sintering (SPS). The domain configuration and relaxor behavior in this nanoceramic are reported for the first time. Although the noncubic crystal structure and dielectric anomaly suggest a ferroelectric behavior, only very slim polarization–electric (P–E) hysteresis loops are observed. The suppression of macroscopic ferroelectric polarization mainly originates from the decrease in crystal tetragonality and the enhanced clamping effect on domain wall motion. The piezoelectric properties, d₃₃=65 pC/N and k_p=17%, raise the possibility of PZT–PNZN nanoceramics for application in multilayer piezoelectric devices.     

Microstructure and electric properties of (Na0.85K0.15)0.5Bi0.5TiO3 composited films with alternative TiO2 layers

In this work, in order to investigate the effect of TiO₂ layer on the microstructure and piezoelectric properties of (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (NKBT) thin films, TiO₂ layer was inserted at the interface between the NKBT thin film and substrate and on both sides of the NKBT, i.e., at the interface and on the top of the NKBT thin film. NKBT composited films with alternative TiO₂ layer were deposited on Pt/Ti/SiO₂/Si substrate by aqueous sol‐gel method. X‐ray diffraction observation found that the degree of (100) preferred orientation strengthened with TiO₂ layers added, especially on both sides of NKBT thin film. The TiO₂/NKBT/TiO₂ composited film with both TiO₂ layer of 40 nm thickness exhibited a remnant polarization value P_r of 22.6 μc/cm³ and effective piezoelectric coefficient of approximate 77 pm/V, which are much larger than that of the single‐layered NKBT thin film with P_r value of 13.7 μc/cm³ and of 56 pm/V, respectively. According to the investigation of the temperature‐dependent ferroelectric property, it was found that the P_r gradually increased, and in the meantime the coercive voltage gradually moved to higher voltage with testing temperature varied from 20 to ?150°C. Besides, applied voltage dependence of leakage current density measurement indicated that the TiO₂ layer would effectively lower the leakage current of the films, and the TiO₂/NKBT/TiO₂ composited film both TiO₂ layer of 40 nm exhibited the lowest leakage current.     

二维氮化铝材料传热性能的模拟研究

二维氮化铝材料是一种新型Ⅲ-Ⅴ族二维材料,具有与石墨烯相似的分子结构和材料性能,受到了广泛的关注,然而其导热性能尚未被充分探讨。应用分子动力学模拟的方法研究了单层二维氮化铝在不同温度的热稳定性和导热性能,并分析了其声子频谱。结果表明,单层二维氮化铝材料可以在极高温度（3500 K）下保持结构稳定性,同时在常温情况热导率可达264.2 W·m^-1·K^-1;在500 K以上温度时,声子色散现象使得该材料热导率明显降低。为二维氮化铝材料导热过程的调控和高温导热材料的应用提供了理论指导。