负热膨胀铁电晶体研究进展

近些年来,负热膨胀材料已成为材料科学的研究热点之一.本文总结了负热膨胀材料发展历程及负热膨胀机理,重点介绍钙钛矿结构和钨青铜结构铁电晶体的生长和热膨胀特性,探讨了晶体结构、铁电相变及负热膨胀机理的关系,对负热膨胀铁电晶体研究和应用提出了建议.     

晶体中负离子配位多面体结晶方位、形变与晶体压电、铁电性

本文研究了压电、铁电晶体中负离子配位多面体的结晶方位与形变,提出了压电晶体中同一种负离子配位多面体的结晶方位是一致的.在铁电晶体中,负离子配位多面体发生形变,伴随着晶体发生顺电-铁电相变,并从这一基本过程出发,对铁电体相变的形成机理进行了讨论.     

铁电晶体KNbO3的研究进展

本文评述了近年来国内外对铁电晶体KNbO3在激光倍频和声表面波应用研究方面取得的显著进展,简述了新近生长该晶体的几种方法.最后对KNbO3晶体产业化前景作出了评估.     

银纳米颗粒表面覆盖度对非晶硅薄膜光吸收特性的影响

本文采用磁控溅射系统在非晶硅薄膜的表面沉积不同覆盖度的银纳米颗粒,并研究纳米颗粒的表面覆盖度对非晶硅薄膜光吸收特性的影响.结果表明随着纳米颗粒表面覆盖度的增加,薄膜的晶格结构并不受影响.但是,随着表面覆盖度的增加,薄膜在近红外区域内对光的吸收范围却发生了明显变化.当纳米颗粒的表面覆盖度达到8;时,薄膜在近红外区域内对光的吸收范围达到了最大.这主要是由于纳米颗粒的局域表面等离子体增强了光吸收.     

AgGaGeS_4晶体介电性能的初步探讨

利用本实验室生长的红外非线性晶体材料AgGaGeS_4(AGGS),常温下进行腐蚀实验并观察畴结构,测试了不同频率、电压下晶体的电滞回线以及同一电压下不同频率介质的电容值.腐蚀图像显示出畴结构,畴尺寸5～10 μm左右,证实AGGS为一热释电晶体.然而,室温下的电滞回线变形为一近似椭圆,介质电容与电场频率关系表现出强色散特性.本文对这一现象进行了系统分析,最后提出了进一步探索AGGS铁电性质的具体建议.     

活性炭负载纳米零价铁去除水溶液中U(Ⅵ)的研究

采用NaBH4还原Fe2+制备活性炭负载纳米零价铁,以去除水溶液中铀酰离子,使用X射线衍射(XRD)对材料进行了表征,考察了活性炭负载纳米零价铁投加量、溶液pH值、反应温度和吸附时间对铀去除效果的影响.分别用动力学和吸附等温模型对吸附数据进行了分析.结果表明:XRD分析活性炭负载纳米零价铁负载的颗粒大部分为纳米零价铁,表面有一层铁氧化物(FeOOH)生成.活性炭负载纳米零价铁对U(Ⅵ)具有很好的去除效果,当投加量为0.5 g/L、U(Ⅵ)初始质量浓度为250 mg/L、pH =5、温度为35℃、时间为60 min时,U(Ⅵ)去除率为98.52;,吸附量为492.6 mg/g.吸附过程符合准二级动力学模型和Freundlich吸附等温模型,所制备的吸附剂有望解决含铀废水难以有效处理等问题.     

反铁电晶体PbZrO3序参量和对称性的研究

在反铁电晶体锆酸铅(PbZrO3)顺电反铁电相变中,伴随有对称性的变化.前人工作中选择反极化参量(P)或子晶格的极化矢量(PA、PB)作为反铁电相的序参量,根据居里原理得出了反铁电晶体锆酸铅反铁电相对称性所属空间群为Pbam(D2h)或Pba2(C2v).但是通过对中子衍射实验结果分析,该晶体反铁电相对称性所属空间群不是Pbam(D2h)或Pba2(C2v),而是P2/m(C2i).本文选取轴矢量R作为反铁电相的序参量,对锆酸铅晶体应用居里原理,恰能得到正确的对称性P2/m(C2i).     

葡聚糖修饰的超顺磁性氧化铁纳米粒子的制备及表征

以乙酰丙酮铁为铁源,聚乙二醇为溶剂,采用高温热分解法制备了超顺磁性氧化铁纳米粒子,得到聚乙二醇修饰的超顺磁性氧化铁纳米粒子,进一步修饰葡聚糖.苯酚-浓硫酸显色法表明葡聚糖修饰在SPIONs表面,获得葡聚糖修饰的氧化铁纳米粒子.采用透射电镜、纳米粒度和Zeta电位分析仪、超导量子干涉仪和热重对产物进行检测.葡聚糖/聚乙二醇修饰的超顺磁性氧化铁纳米粒子的平均粒径为8.7±1.5 nm;水合动力学粒径33 nm,Zeta电位为13mV.葡聚糖/聚乙二醇修饰的超顺磁性氧化铁纳米粒子为超顺磁性,饱和磁化强度为21 emu/g;热重分析表明约有42wt;的葡聚糖和30wt;的PEG修饰在SPIONs表面,计算得到纯氧化铁纳米粒子的饱和磁化强度为75 emu/g.     

铁基结合剂金刚石节块性能的优化

以性能较好的铁基结合剂为基体,加入表面未镀、镀Ni和纳米Al2O3/Ni复合镀层的金刚石,用热压烧结的方法得到铁基结合剂金刚石节块,测量了金刚石铁基结合剂节块的抗弯强度和耐磨性,采用SEM和EDS对复合镀层和金刚石表面的形貌和组分进行了表征.结果表明:在金刚石表面镀覆纳米Al2O3/Ni层后,复合镀层均匀致密,晶粒细小;在热压烧结中,复合镀层能阻止金刚石的石墨化,使金刚石和基体之间有强的化学结合,所以金刚石和铁基结合剂之间的界面结合紧密,结合剂对金刚石的把持力提高,节块的抗弯强度从468.9 MPa增加到563.8 MPa,磨耗比从349升高到700.     

PSN-PMN-PT∶Er3+晶体Judd-Ofelt理论分析

为了探索Er3+在铁电晶体场中的发光特性,采用高温溶液法生长了Er3+掺杂Pb(Sc1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3弛豫铁电单晶.该晶体在紫外-可见-近红外吸收光谱380～1800 nm波段出现了7个吸收峰;在980 nm泵浦光源照射下,该晶体的发射光谱出现了强绿光和强红光上转换发射带,分别对应于Er3+的4S3/2→4I15/2和4F9/2→4I15/2跃迁;利用Judd-Ofelt理论计算了晶体的振子强度三参数(Ω2＝2.54×10-20 cm2,Ω4＝1.36×10-20 cm2,Ω6 ＝2.38× 10-20 cm2)以及Er3+在PSN-PMN-PT晶体场中的跃迁几率、能级寿命和荧光分支比.研究结果表明 PSN-PMN-PT∶Er3+弛豫铁电晶体是一种新型发光晶体.     

The influence of a ferroelectric domain wall on the fabrication of a pyroelectric laser energymeter

A region across a ferroelectric domain wall has been scanned, using an Nd:YAG laser beam (beam size 440 μm), in a thin crystal of TGS (triglycine sulphate), cut normal to the polar axis and the pyroelectric response of the crystal has been recorded. If a laser pulse of fixed beam size falls on the domain completely, then the output pyroelectric signal is maximum. But when the laser beam (diameter of beam greater than width of domain wall) falls on the domain wall, the pyroelectric signals from two opposite domains tries to cancel each other and net signal depends on the position of laser beam across the domain wall. When the domain wall lies in the middle of the laser beam the output pyroelctric signal will be zero. In our experiment when beam of size 440 μm falls on positive domain the pyroelectric voltage of ‐2.76 mV recorded and when beam starts crossing the domain wall (i.e from positive domain to negative domain), the pyoelectric signal changes its sign from negative to positive through zero. After 220 μm the signal decreased to minimum and again increases to maximum (+2.46 mV) in opposite direction after 440 μm i.e when beam falls on the negative domain completely. This shows that the polarization in the domain wall region (separating two domains) is zero i.e the paraelectric nature of domain wall. So before detector fabrication the sample must be poled sufficiently to avoid the non uniformity of pyroelectric signal due to the domain wall. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Balanced and unbalanced domain walls in Pb3(PO4)2

The equations of domain walls in the vicinity of a phase transition are obtained from the condition of the deformation balance at phase boundaries and domain walls for a ferroelastic lead orthophosphate crystal. All domain walls can be divided into three groups, in accordance with the conditions of their formation-balanced, conventionally balanced, and unbalanced domain walls. The structure of staggered domain walls is suggested.     

Study of the domain structure of GASH single crystals and its correlation with their dielectric and switching behaviour

The domain structure topography of GASH single crystals is investigated by electron microscope decoration technique. A large variety of domain shapes was found in “young” crystals immediately after growth, whereas after one or more years of ageing the domain structure became coarse. The domain structure of the samples is related with their dielectric and switching behaviour.     

Coarsening Nature of Liquid-Ordered Domain in Model Membrane

We present the coarsening dynamics of a liquid-ordered (Lo) domain in a supported lipid membrane under an electric field. In a relatively small nanosmooth region surrounded by geometrical walls of nanocorrugated regions, full coarsening of the Lo domain was produced while in a large nanosmooth region, an intermediate, less-ordered Lo domain was developed so that the diffusion of charged phospholipids in the liquid-disordered (Ld) domain was allowed across the geometrical walls in the presence of an electric field. No appreciable diffusion of the charged lipids into a fully coarsened Lo domain by the electric field implies that the structural ordering of the membrane components plays a significant role on the formation of lipid rafts for biological processes.     

PERFORMANCE OF SHORTER PERIODICAL DOMAIN OF NEMATIC LIQUID CRYSTAL INDUCED BY ELASTIC WAVE PROPAGATION

Abstract

The light diffraction properties of a shorter periodical domain, which is caused by an elastic wave propagating along one of two glass plates composing a liquid crystal cell, are described. The shorter periodical domain is controllable for the application of an electric field to the liquid crystal cell. The response time of the disappearance of the shorter periodical domain decreases with increasing the applied voltage. On the other hand, the response time of the domain formation is independent of the applied voltage. These response times agree quantitatively with those of the birefringence electrooptic effect. The domain disappearance by applying the voltage to the liquid crystal indicates the Freedericksz transition.     

Ferroelastic domain walls in BiVO<Subscript>4</Subscript>

Analysis of the domain structure is performed using the method of matching strains at the interface and the model of thin domain walls. Equations describing the matching interlayers for crystal lattices are derived and the interlayer parameters are obtained for all possible types of domain walls in BiVO₄. It is shown that the matching interlayers have monoclinic symmetry. A difference between the elastic energy of a domain wall and the parameters of adjacent orientation states is revealed.     

Polarization mechanism of LiNbO3 crystal due to thermoelectric power and current-induced electric fields during growth via micro-pulling-down method

The mechanism of polarization due to thermoelectric power and current-induced electric fields during the growth of LiNbO₃ crystals was studied using a micro-pulling-down method. With no applied electric current, a +c single-domain crystal was grown regardless of the domain orientation of the seed crystal. This +c domain growth was consistent with the direction of the electric field caused by the thermoelectric power in the liquid, despite an opposing electric field in the solid due to the opposite sign of the Seebeck coefficient. Thus, it was the electric field in the liquid that determined the domain structure of the growing crystal. On the other hand, when a current was applied from the melt to the crystal, a −c domain crystal was grown. The electric current required for this domain inversion to occur became larger as the temperature gradient in the solid phase decreased. This shows that the electric field in the solid phase became large enough to induce domain inversion from +c to −c through a combination of the thermoelectric power in the solid phase and current-induced electric field.     

Single domain wall dynamics in thin magnetic wires

Here we present the low-field domain wall dynamics in thin magnetic wires. It is shown that the domain wall dynamics at low applied fields is described by the power law. The role of all parameters in the domain wall dynamics was studied in a wide temperature range. The power exponent β, which should reflect the changes of the domain wall shape due to its pinning on the local defects, is almost temperature independent. This points to the fact the mechanism of the domain wall pinning does not change with the temperature in the temperature range 77–350 K. This fact is also consistent with the temperature dependence of the switching field H_sw.     

A study of ferroelectric domain patterns in orthorhobic KNbO3 with optical microscopy

Ferroelectiric domain patterns in natural surface layers of Czochralski as-grown crystals are very complicated. The inner domain patterns are comparatively simple and consist of a regular arrays of so-called thin lens-like 90° domains. Only few inner 60° and 120° domains crossing the 90° domain arrays are observed. The inversion or 180° domains are shape-like separated islands. The 60° microdomains are easily introduced into the surface layers during the specimen processing. There are some elastic interactions between the different kinds of ferroelectric domain walls, the orientations of which are in good coincidence with the theory proposed by Janovec.     

Investigation for ferroelectric domains by scanning electron microscopy

Examples for the direct observation of domain structures in ferroelectric crystals by means of scanning electron microscopy are given. The results for TGS and AFB crystals with direction of the polarization vector perpendicular to the observed surface are presented with respect to temperature changes, to domain enlargement under the influence of an external electric field and to the influence of X-ray- and γ-radiation on the domain formation.