Zn和Dy_2O_3粉末添加对放电等离子烧结纳米晶NdFeB磁体组织形貌和性能的影响

通过对混合Zn或者Dy_2O_3粉末的快淬Nd10.15Pr1.86Fe80.41Al1.67B5.91粉末进行放电等离子烧结(SPS),制备出各向同性Nd Fe B永磁材料,分别研究了两种粉末的添加对磁体组织形貌和性能的影响。结果表明,Zn可以起到细化磁体内部晶粒尺寸的作用,并且会和主相反应生成Nd Zn及Nd Zn5相;Dy_2O_3不利于磁体的致密化,其磁性能的提高被认为是粉末对于磁体内部晶粒的细化作用以及(Nd,Dy)2Fe14B相形成共同作用的结果。对于添加Zn粉末磁体,当Zn添加量为0.6wt%时,磁体获得最佳磁性能;对于添加Dy_2O_3粉末磁体,当Dy_2O_3添加量为2.0wt%时,磁体获得最佳磁性能。     

合金浇铸温度对NdFeB铸片显微组织及磁体性能的影响

采用片铸、氢爆碎、气流磨工艺制备烧结NdFeB磁体。研究了合金浇铸温度对铸片显微组织和烧结磁体性能的影响。在一定范围内,随着浇铸温度降低,铸片的厚度增加;高温浇铸,铸片柱状晶尺寸较大;低温浇铸易产生大量等轴晶,在氢爆过程中不易破碎。激光粒度分析仪和XRD分析表明,等轴晶造成气流磨粉末粒度分布宽,影响磁体的取向度,不利于磁体性能的提高。1460℃浇铸温度可以获得显微组织均匀、无明显等轴晶、柱状晶生长良好的铸片,可以获得粒度分布窄的气流磨粉末,从而制备出50M牌号高性能烧结NdFeB磁体。     

热变形工艺对各向异性NdFeB永磁体显微结构和磁性能的影响

采用热压-热变形法制备各向异性NdFeB磁体,通过正交实验研究了热变形温度、变形量以及变形速率对磁体磁性能及显微结构的影响。结果表明,随变形温度、变形量、变形速率的增高增大,磁体剩磁Br及最大磁能积(BH)max增高,当变形温度升高至700℃、变形量增大到65%、以0.045mm/s的速率变形时,磁体获得最佳磁性能,(BH)max达360kJ/m3(45MGOe),同时主相NdFeB晶粒由最初的球状晶、沿垂直于压力方向长大转变为片状晶,晶粒取向度增高;当变形温度、变形量过高或变形速率过低时,磁体中将会出现异常长大晶粒,使磁性能恶化。     

NdFeB磁体烧结工艺研究

高性能NdFeB磁体的磁性能是结构敏感量,与显微组织密切相关。只有设计合理的烧结温度及保温时间,才能得到较为理想的显微结构,从而保证磁体获得理想的磁性能。本文分析了NdFeB磁体烧结过程中的几个关键步骤,制订出较为合理的工艺路线,为NdFeB磁体的批量生产提供技术保证。     

具有改良表面的粘结NdFeB磁体的磁性能

研究了粉粒表面涂层对粘结NdFeB磁体磁性能的影响.涂层和未涂层的NdFeB粉末分别与聚苯撑硫化物混合,压制成各向同性压坯.用Helmholtz线圈和SQUID测量其磁性能.结果表明:涂层可大大改进粘结磁体的不可逆磁通损失和磁能积.研究还表明:NdFeB粉末中由氧化而引起的软磁材料的存在降低了磁性粒子之间的相互作用,但涂层处理后由于内部粒子的耦合系数增加,从而使粘结磁体的剩磁提高.     

高能球磨法及其在纳米晶磁性材料制备中的应用(二)

4 高能球磨在纳米晶磁性材料制备中的应用 4.1 纳米晶永磁材料的制备 理论预测纳米晶永磁材料具有很高的磁性能,因此近年来备受人们的关注.目前一般制备纳米晶永磁材料的主要方法有快淬法和高能球磨法两种.快淬法是先制备出非晶快淬薄带,然后用等温退火的方法来获得纳米晶材料.而高能球磨法制造纳米晶永磁材料,由于合金成分连续可调,制得粉体颗粒小,尺寸分布均匀,为软、硬磁相在纳米尺度内产生交换耦合提供了较为理想的微结构,比快淬法具有更高的磁性能,从而使其成为开发和研究高性能永磁材料的重要手段.     

氢爆碎(HD)工艺对Nd-Fe-B 磁体的制备、微结构和磁性能的影响

选用40HP和40SHP牌号合金铸锭分别采用氢爆碎和传统制粉工艺制备了烧结磁体,研究了氢爆碎工艺及烧结温度对磁体的微观组织和磁性能的影响.结果表明,HD工艺能明显提高NdFeB磁体的磁性能.     

粘结NdFeB磁体的热稳定性

本文测定了二种粘结NdFeB磁体的80℃，120℃、150℃和180℃温度下的退磁曲线，以及室温至200℃之间不同温度下烘烤2小时后冷却至室温的开路磁通的变化。无论是快淬磁粉的粘结磁体或者HDD磁粉的粘结磁体，热稳定性都比普通的烧结NdFeB磁体略好，其中快淬磁粉的更好一些，这与它们的微细晶粒结构有关。同时对两种磁粉在大气中暴露半年后做的粘结磁体进行了磁性测量，发现磁性无明显变化，说明磁粉在大气环境中有一定的抗氧化能力，这对于运输、存放和使用该磁粉的各过程带来了很大方便。     

烧结剂对Nd-Fe-B永磁合金结构和剩磁的影响

用不同的烧结剂,通过液相烧结一种基本成分的Nd_(140.1)Fe_(79.8)B_(6.3)或者通过烧结同样成分的预熔合金来制造Nd_(15)Fe_(79)B_6磁体。用三种不同Nd含量的富Nd Nd-Fe-B合金作为烧结剂,研究了原材料的各种化合物对发展磁晶结构和提高剩磁的影响。发现对于不同的原材料,在压坯状态,已达到不同程度的磁有序。用较高Nd的烧结剂,产生了较好的磁有序和较高的烧结密度。因此,用烧结剂烧结的磁体,比预熔Nd_(15)Fe_(79)B_6合金磁体,具有更高的剩磁。然而,由于烧结剂的不均匀分布,在一些具有薄晶粒边界的大晶粒中间出现了不规则的晶粒生长,这些小晶粒被厚的富Nd层环绕。在高烧结温度下,由于热骚动使这些小晶粒产生浮动和转动,从而导致结构变坏剩磁降低。     

各向异性纳米晶复合稀土磁体的研制

作者成功地开发了全密度各向异性纳米晶复合稀土永磁体,其制备技术新颖,工艺成本低.采用粉末混合技术,纳米磁体的(BH)max可以达到320～400kJ/m3(40～50MGOe).而采用粉末镀膜技术,(BH)max可达360～440kJ/m3(45～55MGOe).这样,制备各向异性纳米晶复合稀土磁体的主要技术困难就得到了克服.此外,作者观察到纳米晶粒复合磁体中软磁相的尺寸可达数十微米.这一尺寸是目前的界面交换耦合模型所建议的软磁相尺寸上限的一千倍以上.继续减小软磁相的尺寸并改善其分布将会进一步改善纳米晶粒复合磁体的磁性能.     

Nanoblast synthesis and SPS of nanostructured oxides for SOFC

Engineering of Cerium Gadolinium Oxide (CGO) and Yttria Stabilized Zirconia (8Y-SZ) ceramic nanopowder with precise morphology and homogeneity of the compounds by multiple nanoblast technique of preliminary engineered nanoreactors was analysed. Nanoreactors were produced by morphology optimization technique and loaded by colloidal impregnation of the C₃H₆N₆O₆. The subsequent explosive initiation of C₃H₆N₆O₆ followed thermal detonation form gaseous products with local temperatures of ~2500°C. The rapid evolution of a large volume of gaseous products i.e. impacts of the blast waves dissipates the heat of the process and limits temperature increase, thus reducing the possibility of premature local partial sintering among the primary particles; and leads to the fragmentation of the surrounding agglomerates. Uniformly aggregated nanosize CGO and 8Y-SZ consisting of ~50 nm nano-aggregates of ~7 nm crystallites with a remarkably homogeneous composition and uniform morphology were synthesized. The SPS consolidation of nanosized aggregates of 8Y-SZ and CGO was analyzed. The 8Y-SZ ceramic with an average grain size of 90 nm and CGO nanoceramic with average grain sizes of 73, 32 and 12 nm were obtained by SPS at 1100, 1035 and 970°C, respectively under the pressures of 90–150 MPa.     

Influence of microstructure on oxide ionic conductivity in doped CeO2 electrolytes

Doped ceria (CeO₂) compounds are fluorite type oxides, which show oxide ionic conductivity higher than yttria stabilized zirconia, in oxidizing atmospheres. As a consequence of this, considerable interest has been shown in application of these materials for `low (500^∘–650^∘C)’ temperature operation of solid oxide fuel cells (SOFCs). In this study, some rare earth (eg. Gd, Sm, and Dy) doped CeO₂ nano-powders were synthesized via a carbonate co-precipitation method. Fluorite-type solid solution were able to be formed at low temperature, such as 400^∘C and dense sintered bodies were subsequently fabricated in the temperature ranging from 1000^∘ to 1450^∘C by conventional sintering (CS) method. To develop high quality solid electrolytes, the microstructure at the atomic level of these doped CeO₂ solid electrolytes were examined using transmission electron microscopy (TEM). The specimens obtained by CS had continuous and large micro-domains with a distorted pyrochlore structure or related structure, within each grain. We conclude that the conducting properties in these doped CeO₂ systems are strongly influenced by the micro-domain size in the grain. To minimize the micro-domain size, spark plasma sintering (SPS) was examined. SPS has not been used to fabricate dense sintered bodies of doped CeO₂ electrolytes, previously; carbon from the graphite dies penetrates the specimens and inhibits densification. To overcome this challenge, and to be able to produce dense sintered bodies of doped CeO₂ of a grain size that minimizes the microdomain growth, a combination of SPS and CS methods were examined. Using this combined method we report that we were able to produce fully dense specimens with improved conductivity. This is correlated with a reduction in the size of the micro-domains. Consequently we conclude that the control of micro-domain size within the grain structure is a key component in the successful design of electrolyte materials with improved conductivity.     

The dependence of ferroelectric and fatigue behaviors of PZT films on microstructure and orientation

Microstructure of PbZr_0.53Ti_0.47O₃ films about 0.2 μm thick was studied as a function of annealing temperature and time using TEM and SEM. The films were heat treated in the temperature range 550–650°C for times from 10 minutes to 4 hours. The features characterized were crystal structure of individual PZT particles, PZT grain size and shape, porosity, and grain boundaries. The films were prepared by the metallo-organic decomposition (MOD) process which produced films amorphous to x-rays after pyrolysis. A growth mechanism of PZT particles from the amorphous film is discussed. It was found that at mild annealing conditions (i.e., low temperature and short annealing times), agglomerates of microcrystallites of perovskite, and probably pyrochlore, were formed. As the temperature was increased, the microcrystallites grew into porous submicron perovskite single crystals. With excessive heating, intraparticle pores were found to migrate and became pinned at the grain boundaries, causing widening of boundaries which probably was responsible for the degradation in ferroelectric properties observed in such films. The dependence of ferroelectric and fatigue behaviors on grain orientation was also investigated. It was found that PZT films oriented in the [111] direction demonstrated better fatigue behavior but not necessarily higher polarizations.     

Dielectric properties of nanocrystalline TiO2 prepared using spark plasma sintering

Ultra fine rutile powders (below 50 nm) were prepared via the sol-gel process and bulk type TiO₂ specimens were fabricated using spark plasma sintering (SPS). The TiO₂ specimen sintered at a low temperature (720^∘C) exhibited a highly relative density (97%) and a nano-sized grain structure (200 nm). Dielectric properties of spark plasma sintered TiO₂ specimens including dielectric constants (k) and losses (tan δ) were measured. The TiO₂ specimen, obtained by SPS, showed a high dielectric constant (∼780) and a low tan δ (∼0.005), and a relaxation behavior at 1 MHz. After the subsequent annealing process of the TiO₂ specimen in O₂ flow, the dielectric constants remarkably decreased (k = 100s). These dielectric properties of nanocrystalline TiO₂ specimens prepared by SPS were discussed in terms of space charges produced by the reduction of Ti⁴⁺ ions and crystallographic orientations of grains.     

Preparation, microstructure and electrical properties of Li1.4Al0.4Ti1.6(PO4)3 nanoceramics

NASICON-type Li_1.4Al_0.4Ti_1.6(PO₄)₃ solid electrolytes were prepared by various processes, such as crystallization of glasses, spark plasma sintering (SPS) and conventional sintering process from nanosized precursor powders synthesized by a sol–gel route. The experimental results showed that grain size and relative density were the main factors determining the ionic conductivity of the bulk materials. The SPS technique produced ceramics with nearly 100% of the theoretical density. Maximum room temperature conductivities, 1.39?×?10^?3 S cm^?1 and 1.12?×?10^?3 S cm^?1 of grain boundary conductivity and total conductivity, respectively were obtained which were the highest values for Li⁺ inorganic oxide conductors as reported. Crystallization of ceramics from a glass was also certified as a favorable route to fabricate a bulk material with high conductivity.     

Effect of tacticity on the dielectric properties of polystyrene

Dielectric properties of polystyrene (PS) were investigated by measuring electrical conduction current and frequency spectra of complex permittivity at temperatures from 20 to 199 °C. To clarify the effects of tacticity, the properties were compared among two kinds of syndiotactic PS (SPS) with different crystallinity values and one atactic PS (APS). Experimental results indicate that the two SPS samples show a high melting point of about 250 °C. The conductivity is lowest in SPS with the low crystallinity in the whole temperature range from 20 to 199 °C, while APS had a breakdown at 140 °C. Moreover, the low‐crystallinity SPS shows the smallest permittivity values and the smallest increase in dielectric loss factor toward low frequencies among the three samples. The high‐crystallinity SPS has α crystals. Since the density of α crystals is lower than the amorphous region, the charge transport and polarization are easier in the high‐crystallinity SPS than in the low‐crystallinity SPS. © 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Optimization of the thermoelectric properties of Bi<Subscript>2</Subscript>O<Subscript>2</Subscript>Se ceramics by altering the temperature of spark plasma sintering

The n-type polycrystalline Bi₂O₂Se ceramics were fabricated by solid state reaction and SPSed at different temperatures (873–973 K). The grains of the sample grow up gradually and the grain size enlarges from about 200 nm to micrometer level with the increase of sintering temperature. The highest electrical conductivity of 6.23 S/cm is obtained for the sample sintered at 898 K which benefits from grain orientation along (00l) plane and the highest measured density. The electrical transport properties tend to decline with further increase of the sintering temperature due to the decrease of density and the orientation degree. The maximum power factor of 78.39 μWK^?2m^?1 is obtained at 773 K. The thermal conductivity is intrinsically suppressed owing to the layered crystal structure of Bi₂O₂Se and fine grains within the nanometer size. The ZT value reaches 0.09 at 773 K for the sample SPSed at 898 K and the optimal temperature during the SPS process has been determined.     

固溶强化型镍基合金的时效析出行为

为了给700 ℃超超临界机组选材提供技术参考,研究了2种固溶强化型镍基合金（617和625）经时效处理（时效）后的析出行为。结果表明：2种镍基合金760 ℃时效后,均析出M₂₃C₆、M₆C碳化物和γ′相;M₂₃C₆、M₆C碳化物分布于晶界和晶内,γ′相分布于晶内;γ″相在625合金晶界、层错处析出并向晶内生长。617合金时效3 000 h,晶界析出相不连续分布;时效5 000 h后晶界析出相聚集且连续分布,晶内析出相的尺寸基本稳定。625合金时效3 000 h的晶内析出相尺寸无明显变化,晶界M₂₃C₆颗粒聚集长大;时效5 000 h后M₂₃C₆转变为M₆C碳化物。617合金和625合金时效后的冲击吸收能量均明显下降,时效后的625合金脆化程度较时效后的617合金严重。     

同轴介质阻挡放电初步处理粮菌的研究

为了研究同轴介质阻挡放电装置等离子体对粮菌的杀灭效果,运用同轴介质阻挡放电装置产生的大气压下氦等离子体对根癌菌和水稻恶苗菌进行了灭菌实验研究。定量分析发现等离子体处理延缓乃至杀灭根癌菌和恶苗菌,对前者的杀灭更为容易。采用扫描电镜观察发现经处理的菌均有不同程度的受损情况,且可以看出有明显的高速粒子打击的痕迹和一定程度的脱水现象。等离子体灭菌的机理目前尚无定论,但从扫描电镜的照片中初步推测是由高速粒子的穿透作用造成的,并且温度也起了一定的作用。     

Grain growth kinetics of textured 0.92Na0.5Bi0.5TiO3—0.08BaTiO3 ceramics by tape casting with Bi2.5Na3.5Nb5O18 templates

Plate-like Bi_2.5Na_3.5Nb₅O₁₈ particles were used as templates to fabricate grain-oriented Na_0.5Bi_0.5TiO₃—BaTiO₃ (NBTBT) ceramics by reactive-templated grain growth. The effects of sintering conditions on the grain orientation and microstructure of the textured NBTBT ceramics were investigated, and the kinetic mechanism of grain growth is discussed. The results show that textured ceramics were successfully obtained with orientation factor more than 0.6. NBTBT specimens are composed of strip-like grains and equiaxed shaped grains. The textured ceramics have a microstructure with strip-like grains aligning in the direction parallel to the casting plane and exhibit an {h00} preferred orientation. The degree of grain orientation increases initially, then decreases with increasing sintering temperature and soaking time. The maximum texture fraction is 0.69 when sintered at 1185 °C for 6 h. The kinetic exponent n and activation energy Q of the two types of grain in textured NBTBT ceramics were calculated. The results show that the grain growth mechanism of oriented grains is controlled not only by grain lattice diffusion, but also by grain boundary diffusion. The grain growth mechanism of matrix grains is mainly controlled by the grain boundary curvature.