Effects of SmF3 addition on aluminum nitride ceramics via pressureless sintering

Aluminum nitride (AlN) ceramics with dense structure, high thermal conductivity, and exceptional mechanical properties were fabricated by pressureless sintering with a novel non-oxide sintering additive, samarium fluoride (SmF₃). The results showed that the use of a moderate amount of SmF₃ promoted significant densification of AlN and removed the oxygen impurity. This led to the formation of fine and isolated secondary phase that cleaned the grain boundaries and increased the contact between AlN grains, remarkably enhancing thermal conductivity. Furthermore, SmF₃ also exhibited grain refinement and grain boundary strengthening effects similar to traditional sintering additive, samarium oxide (Sm₂O₃), leading to high mechanical properties in SmF₃-doped AlN samples. The most optimal characteristics (thermal conductivity of 190.67 W·m⁻¹·K⁻¹, flexural strength of 403.86 ± 18.27 MPa, and fracture toughness of 3.71 ± 0.19 MPa·m^1/2) were achieved in the AlN ceramic with 5 wt% SmF₃.     

纳米CaCO_3含量对PP/SBS/CaCO_3复合材料力学性能的影响

通过双螺杆共混方法制备聚丙烯(PP)/苯乙烯-丁二烯-苯乙烯嵌段共聚物(SBS)/Ca-CO3复合材料,并采用扫描电镜(SEM)、力学性能测试和偏光显微镜研究了纳米CaCO3用量对该复合体系微观结构、力学性能以及结晶形貌的影响。结果表明:纳米CaCO3粒子在PP中均匀地分散可细化晶粒并起到了增韧增强的效果。当加入纳米CaCO3的质量分数为2%时,其综合性能最好。     

Low temperature densification of silicon nitride materials

Thin layers of oxides, corresponding to additions of up to 5 w/o Na₂O have been deposited on the surface of grains of a commercial silicon nitride powder using alcoholic solutions containing appropriate amounts of the metal alkoxide. The resulting powders have been densified by hot pressing and pressureless sintering techniques, and their sintering characteristics identified in comparison with equivalent materials produced by adding the oxide in particulate form. In every case, a better sintering performance was observed at all temperatures for the oxide-coated materials, with ∼97% dense sintered materials being obtained at temperatures as low as 1400–1500 °C. Microstructures were observed using a S-2400 Hitachi Scanning Electron Microscope (SEM) and final microstructure was more uniform than that obtained by conventional method. It is concluded that the powder coating technique is an excellent method of homogeneously incorporating minor amounts of sintering additive into a powder.     

磁场对碳酸钙成核结晶过程的影响

利用自行设计的磁化水循环装置,测定了不同温度下,含有Ca2++CO32-+HCO32-的纯水溶液垂直通过均匀可调磁场(最高达4 100 mT)经磁处理后溶液的电导率、浊度和碳酸钙浓度的变化趋势,用XRD测定了不同磁处理条件下得到的碳酸钙晶型,研究了碳酸钙的成核结晶过程,从热力学角度探讨了磁场对碳酸钙成核结晶过程的作用机理.实验结果表明,磁场对碳酸钙晶体生成速率和晶型结构的影响主要是因为磁处理改变了水分子簇的结构,从而改变了碳酸钙晶核先驱水合化碳酸钙的形成和脱水过程.     

CaCO3对增强聚丙烯合金性能影响的研究

本文研究了CaCO3对聚丙烯／三元乙丙橡胶（EPDM）／苯乙烯－丁二烯－苯乙烯（SBS）和聚丙烯／聚烯烃（PO）的力学性能，热性能和流变性能的影响，结果表明,CaCO3的加入，提高了共混体系的冲击强度，获得性能较好的PP合金材料，可用于汽车保险杠。     

表面处理CaCO3对单组分聚氨酯密封剂性能的影响

对轻质CaCO3、重质CaCO3和滑石粉3种填料进行了表面处理,考察了它们对聚氨酯密封剂性能的影响。结果表明：使用表面处理轻质CaCO3作填料的聚氨酯密封剂与使用未处理轻质CaCO3作填料制备的聚氨酯密封剂相比,其邵氏硬度降低,伸长率提高。此外,使用表面处理轻质CaCO3作填料时在增加填料用量的同时,能确保密封剂的物理机械性能。     

Effects of carbothermal reduction on the thermal and electrical conductivities of aluminum nitride ceramics

This study attempts to control the oxygen content by adding various amounts of yttria sintering additives or by introducing an in situ carbothermal reduction using a carbon addition during the Power processing step. While both yttria and the carbon addition increased the thermal conductivity and electrical resistivity, carbon addition was more effective in increasing the DC volume resistivity for specimens that had 3 or 5 wt% Y₂O₃. Among several elements of the electrical conductivities of the grains and grain boundaries, the conductivity of the grains appeared to be more relevant to the thermal conductivities of AlN ceramics. In addition, yttrium enrichment was observed in the grain boundary region. We also found that an in situ carbothermal reduction resulted in a small amount of yttrium-aluminate second phases, which are beneficial to the plasma resistance of the AlN ceramics.     

Effects of titanium and aluminum incorporations on the structure of boron nitride thin films

Boron nitride (BN)-based composite thin films have been prepared by ion-beam-assisted deposition (IBAD) employing two electron-beam evaporators. Approximately 3–5 at.% of either Ti or Al was incorporated into the BN composite films. Fourier-transform infra-red (FTIR) spectroscopy was used for phase identification of the BN composite films. The influences of the Ti and Al additions on the cubic phase formation in the BN films are reported. It has been found that Al incorporation has a strong negative effect on cubic BN (cBN) formation. No cubic phase can be obtained under the presently chosen ion-bombardment parameters. However, the disturbance of 3∼5 at.% Ti addition, depending on the preparation conditions for the BN thin films, only shifts the threshold of the ion/atom ratio of the IBAD process, which is required for cBN formation to a higher value. In order to understand the different behaviors of the Ti and Al incorporations, the chemical states of the Ti and Al additions in the BN composite films were examined by X-ray photoelectron spectroscopy (XPS), indicating preferential formation of TiB₂ and AlN, respectively.     

Effect of the filler size and content on the thermomechanical properties of particulate aluminum nitride filled epoxy composites

Polymer matrix composites based on brominated epoxy as the matrix and aluminum nitride (AlN) particles as the filler were prepared. The influences of the size, content, and size distribution of AlN on the thermomechanical properties, including the glass‐transition temperature (T_g), coefficient of thermal expansion (CTE), dynamic storage modulus (E′), dynamic loss modulus (E″), and loss factor (tan δ), of the composites were investigated by thermomechanical analysis and dynamic mechanical analysis. There was a total change trend for T_g; that is, T_g of the composites containing nano‐aluminum nitride (nano‐AlN; 50 nm) was lower than that of the micro‐aluminum nitride (micro‐AlN; 2.3 μm) filled composites, especially at high nano‐AlN contents. The T_g depression of the composites containing nano‐AlN was related to the aggregation of nano‐AlN and voids in the composites. On the other hand, the crosslink density of the epoxy matrix decreased for nano‐AlN‐filled composites, which also resulted in a T_g depression. The results also show that E′ and E″ increased, whereas tan δ and CTE of the composites decreased, with increasing the AlN content or increasing nano‐AlN fraction at the same AlN content. These results indicate that increasing the interfacial areas between AlN and the epoxy matrix effectively enhanced the dynamic modulus and decreased CTE. In addition, at a fixed AlN content of 10 wt %, a low E′ of pre‐T_g (before T_g temperature) and high T_g were observed at the smaller weight ratio of nano‐AlN when combinations of nano‐AlN plus micro‐AlN were used as the filler. This may have been related to the best packing efficiency at that weight ratio when the bimodal filler was used. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effects of sintering atmosphere on the densification and microstructure of yttrium aluminum garnet fibers prepared by sol-gel process

Yttrium aluminum garnet (YAG) fibers were prepared by a sol-gel method, and then sintered in air or nitrogen atmosphere, respectively. The effects of sintering atmosphere on the densification process and microstructure of YAG fibers were investigated. No obvious difference can be found in the fibers sintered below 800 °C. At 1100 °C, the grain size of YAG fibers sintered in nitrogen is much smaller than in air. This difference is much clearer at the higher temperature of 1200 °C. The fine grains are explained by the existence of residual carbon in YAG fibers, which can be trapped at the grain boundaries to hinder the movement of grain boundary. Meanwhile, the densification degree of fibers sintered in nitrogen is higher than in air at 1200 °C, due to the smaller grain size and higher oxygen vacancy concentration generated in the nitrogen atmosphere, which leads to a higher fiber densification rate.     

Heat dissipation in 3D printed cellular aluminum nitride structures

The improvement of heat dissipation in electronic and energy devices is a challenge that can be addressed through the use of highly porous materials. Presently, the additive manufacturing of 3D aluminum nitride is described, and different lattice patterns with porosities in the range 45–64 % are achieved by direct ink writing. All the structures are robust and the effective thermal conductivity (k_eff) for cuboid structures decreases by 50–75 % with the filament separation and shows anisotropic characteristics, since k_eff along the longitudinal axis of the scaffold is up to six times greater than for the transversal one. Heat transfer during free cooling experiments for cuboid and cylinder scaffolds, after rapid heating at temperatures above 1000 °C, takes place by radiation for temperatures >500 °C and by convection through the complete cooling process. The heat dissipation time constants of both processes decrease almost linearly with the designed scaffold parameters of porosity and rod separation.     

Microporous polypropylene sheets containing CaCO3 filler: Effects of stretching ratio and removing CaCO3 filler

Microporous polypropylene sheets are prepared by biaxially stretching polypropylene sheets containing CaCO₃ filler. Here, the stretching ratio is one of the most important factors in the preparative process, and removing the CaCO₃ filler also affects the sheet properties. Their effects were studied in relation to the properties and the structure of the microporous polypropylene sheets. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1543–1553, 1998     

Effect of Y2O3 and ZnO co-doping on the densification and properties of magnesium aluminum spinel

The effects of Y₂O₃ and ZnO co-doping on the densification and properties of magnesium aluminum spinel were investigated. The physical phase composition, microstructure, elemental distribution, densification, apparent porosity, particle size distribution and average corrosion depth of the specimens were investigated by XRD, SEM-EDS, Archimedes drainage method, Nano Measurer 1.2 software, and molten salt corrosion tests. The results showed that after co-doping with Y₂O₃ and ZnO, the cations in the sintering aids could dissolve into the spinel structure, forming solid solution ZnAl₂O₄, second phase Y₃Al₅O₁₂ and Al₂Y₄O₉, which inhibited the abnormal grain growth and made the grain distribution more uniform, thus promoting the densification of the samples. The best co-doping amount of Y₂O₃ and ZnO was 1 wt% Y₂O₃-3 wt.% ZnO, the relative density of the sample was 99.3%, the apparent porosity was 0.021%, and the grain size was the most uniform (6.52 μm). After the sample was placed into the aluminum electrolytic molten salt electrolyte for 1 h, and it was found that the sample doped with 1 wt% Y₂O₃-3 wt.% ZnO had the minimum average corrosion depth (131.9 μm) and the best corrosion resistance.     

表面活性剂对纳米碳酸钙结晶过程的影响

在Ca(OH)2的碳化反应过程中加入阴离子、阳离子和非离子表面活性剂。采用pH、电导率在线跟踪和透射电子显微镜等手段考察了表面活性剂对碳化反应的影响和对碳酸钙结晶过程的影响。实验发现：①无表面活性剂存在下在碳化反应开始后约4min～7min时，反应液呈凝胶态，体系黏度发牛突变，而在表面性剂存在下，这一凝胶化现象被消除，体系的黏度仅小幅升高；②表面活性剂的存在可加速碳化反应的进行，有效地缩短反应时间；③表面活性剂的存在可影响碳酸钙的结晶行为，在适宜的表面活性剂存在下，碳酸钙的晶形发生改变，可得到直径20nm左右、长数十到数百纳米的链球形碳酸钙晶体。     

Spark plasma sintering of Sm2O3-doped aluminum nitride

The high sintering temperature required for aluminum nitride (AlN) at typically 1800 °C, is an impediment to its development as an engineering material. Spark plasma sintering (SPS) of AlN is carried out with samarium oxide (Sm₂O₃) as sintering additive at a sintering temperature as low as 1500–1600 °C. The effect of sintering temperature and SPS cycle on the microstructure and performance of AlN is studied. There appears to be a direct correlation between SPS temperature and number of repeated SPS sintering cycle per sample with the density of the final sintered sample. The addition of Sm₂O₃ as a sintering aid (1 and 3 wt.%) improves the properties and density of AlN noticeably. Thermal conductivity of AlN samples improves with increase in number of SPS cycle (maximum of 2) and sintering temperature (up to 1600 °C). Thermal conductivity is found to be greatly improved with the presence of Sm₂O₃ as sintering additive, with a thermal conductivity value about 118 W m⁻¹ K⁻¹) for the 3 wt.% Sm₂O₃-doped AlN sample SPS at 1500 °C for 3 min. Dielectric constant of the sintered AlN samples is dependent on the relative density of the samples. The number of repeated SPS cycle and sintering aid do not, however, cause significant elevation of the dielectric constant of the final sintered samples. Microstructures of the AlN samples show that, densification of AlN sample is effectively enhanced through increase in the operating SPS temperature and the employment of multiple SPS cycles. Addition of Sm₂O₃ greatly improves the densification of AlN sample while maintaining a fine grain structure. The Sm₂O₃ dopant modifies the microstructures to decidedly faceted AlN grains, resulting in the flattening of AlN–AlN grain contacts.     

碳酸钙与光稳定剂对聚丙烯编织袋防老化性能的影响

研究了重质碳酸钙与受阻铵光稳定剂ＨＡＬＳ相互作用对聚丙烯编织袋防老化性能的影响。结果表明,含有重质碳酸钙≤１．２％且受阻胺光稳定剂Ｔｉｕｎｖｉｎ７７０≥０．１６％的聚丙烯编织袋,可以符合欧洲ＥＮ２７７－９５中抗紫外线试验的规定要求。