聚合物前驱体法制备纳米Ba0.5Sr0.5TiO3粉体的研究

分析比较了三种工艺对制备纳米Ba0.5Sr0.5TiO3(BST55)粉体的影响,得到了优化的聚合物前驱体法制备工艺.并利用TG-DSC、IR、XRD、SEM分析了BST55干凝胶的热分解过程、化学组成、物相变化以及形貌.实验结果表明,加入适量的乙烯基三乙氧基硅烷偶联剂(TM-12)能改善BST55粒子的粒度及聚集形态;先采用聚合物前驱体法制备BST55/PMMA复合材料干凝胶,然后在700 ℃煅烧可形成粒径介于80～100 nm之间的BST55粒子.     

PMMA／凹凸棒粘土复合材料的制备及其性能研究

以有机改性凹凸棒粘土为填料,采用原位聚合法制备了聚甲基丙烯酸甲酯（PMMA）／凹凸棒粘土（ATP）复合材料,通过红外光谱（FTIR）、扫描电镜（SEM）、热重（TG）、力学性能测试等方法对复合材料进行分析和表征。结果表明,改性凹凸棒粘土与PMMA的相容性良好,在PMMA中分散均匀。而其独特的一维纤维棒状结构也可以对PMMA在一定程度上起到补强、增韧作用,当凹凸棒粘土的加入量在1．5％时,其综合力学性能较佳。     

PU/P（MMA-co-MAH）/OMMT纳米复合材料的制备与性能研究

将插层纳米复合技术与互穿网络共混技术相结合，通过同步插层聚合法制备PU／聚（甲基丙烯酸甲酯-马来酸酐）[P（MMA-co-MAH）]／有机蒙脱土（OMMT）纳米复合材料。试验结果表明，PU／P（MMA-co-MAH）／OMMT纳米复合材料为剥离／插层型纳米复合材料，各项物理性能较优;其优化制备条件是：过氧化苯甲酰，二甲基丙烯酸乙二酯、OMMT和MAH在单体中的质量分数分别为0．008，0．02，0．05和0．05，PU／PMMA共混比为60／40；亚甲基双邻氯苯胺因数为0．8。     

低温烧结制备Ba0.6Sr0.4TiO3陶瓷及厚膜

用改进的溶胶－凝胶工艺制备Ba0.6Sr0.4TiO3（BST）陶瓷及厚膜，研究它们的结晶情况与电学性能。在这种改进的工艺中，将经高能球磨制备的纳米BST陶瓷粉体加入到同组分的BST的溶胶液中，制备成稳定的悬浮性好的浆料，并用此浆料制备BST陶瓷及厚膜。实验结果表明：采用两步烧结法可制备细晶、较致密的BST陶瓷材料，它呈现明显的弥散相变特征，具有较好的介电性能，晶粒尺寸〈1μm。用匀胶法制备的沉积在Pd／Ag电极上的BST厚膜，膜厚约为10μm，也呈现明显的弥散相变特征。在1200℃烧结后的厚膜在0℃，1kHz时，相对介电常数为900，介电损耗为0．03。     

聚甲基丙烯酸甲酯／聚乙二醇／炭黑导电复合材料的制备与导电性能研究

采用原位聚合法制备了聚甲基丙烯酸甲酯／聚乙二醇／炭黑（PMMA/PEG/CB）导电复合材料，研究了其导电性能及其对有机蒸气的电阻响应性能。结果表明，当MMA／PEG质量比为8／1时，原位聚合反应时间为20～30min，单体转化率可达94％以上；复合材料的导电性能显著增强，当CB质量分数为8．5％时，复合材料的电阻率为1．3Ω·cm；该复合材料在较大CB含量范围内对有机蒸气产生显著的电阻响应，对PMMA的良溶剂（如丙酮、乙酸乙酯、氯仿等）蒸气的电阻响应程度可达104，而对非溶剂甲醇蒸气的仅为140。     

改性蒙脱土增强PVC木塑复合材料的研究

采用钛酸酯偶联剂改性木粉（TTWF）制备了PVC／TTWF木塑复合材料，PVC/TTWF体系中进一步加入适量改性插层蒙脱土（OMMT），通过熔融共混挤出制备了PVC／TTWF／OMMT纳米复合材料，并采用傅里叶变换红外光谱仪、X射线光电子能谱仪、X射线衍射仪、透射电子显微镜、场发射扫描电镜等手段进行了表征和分析。结果表明，TTMF改善了PVC木塑复合材料的力学性能，当钛酸酯偶联剂含量为1．5％（质量分数，下同）时，复合材料拉伸强度和弯曲强度分别提高了10．5％和6．0％；在此基础上适量加入OMMT（0．5％）进一步改善了复合材料的力学性能，其中拉伸强度、冲击强度和弯曲强度分别提高了10．2％、2．3％和16．5％。     

高光泽PC／PMMA共混体系的动态热力学分析

通过熔融共混法制备了高光泽聚碳酸酯（PC）／聚甲基丙烯酸甲酯（PMMA）复合材料,分析了其动态力学性能,探讨不同相容剂、不同PMMA组份等因素对复合材料的储存模量（E＇）、损耗模量（E＂）、损耗因子（tanδ）等动态力学参数的影响。结果表明使用乙烯．丙烯酸甲酯共聚物和苯乙烯马来酸酐无规共聚物组成的复合相容剂能够较好地改善高光PC／PMMA的相容性。在PMMA20份含量以下,当温度超过120℃时合金依然有较高的储存模量,材料的耐高温性能比较好。研究结果可为进一步优化高光PC／PMMA产品配方提供依据。     

热处理温度对Ba0.5Sr0.5TiO3薄膜结构与性能影响的研究

采用HPAgilent4294A阻抗分析仪、X衍射、扫描电镜等测试方法研究了热处理温度对BaxSr1-x，TiO3（BST）薄膜结构与性能的影响。当升／降温速率为0．5～1℃／min时，得到的薄膜表面均匀、无裂纹、孔洞。经过750℃退火处理后，薄膜的介电性能最佳，即具有较高的相对介电常数和较小的介质损耗。     

炭纳米纤维添加剂对炭/炭复合材料力学性能影响

采用等温CVI工艺制备出5种不同炭纳米纤维含量（质量分数分别为0，5％，10％，15％和20％）的炭／炭复合材料。发现添加炭纳米纤维的炭／炭复合材料具有很高的力学性能，在加入炭纳米纤维为5％时，相对于没有添加炭纳米纤维的炭／炭复合材料，弯曲强度增大了76．3％，弹性模量增大了55．5％，但添加量增大到20％时，强度和模量都逐渐降低。     

钙长石／玻璃复合材料的制备和性能研究

采用电子陶瓷工艺制备了一系列钙长石／玻璃复合材料,并对复合材料进行X射线分析、扫描电镜观察和性能测试。结果表明：复合材料的介电常数、热膨胀系数随钙长石含量的增加而增加,而介电损耗和抗折强度随钙长石含量的增加而减小。钙长石含量大于50wt％的复合材料中α-石英和方石英的析出增加了材料的热膨胀系数,但对材料的介电性能影响不大。所制备的复合材料具有低的介电常数（5．4～6．1）、低的介电损耗（0．11％～0．41％）、低的热膨胀系数（4．3×10^-6～6．1×10^-6／℃）和低的烧结温度（≤900℃）,有望用于电子封装领域。     

Ceramic–Polymer Ba0.6Sr0.4TiO3/Poly(Methyl Methacrylate) Composites with Different Type Composite Structures for Electronic Technology

Ba_0.6Sr_0.4TiO₃ (BST)/poly(methyl methacrylate) (PMMA) composites with 0–3, 1–3, and 2–2 type structures were prepared and studied. The effect of composite type on the dielectric properties of BST/PMMA composites was comprehensively investigated by both theoretical and experimental methods. The 1–3 type composite shows the highest permittivity and dielectric tunability, while the 2–2 and 0–3 type composites show lower permittivity as well as lower dielectric tunabilities. The experimentally measured dielectric properties are in agreement with the theoretically calculated values. The results help in understanding and tailoring the dielectric properties of ceramic–polymer composites by choosing a suitable composite structure.     

Electric Field-Dependent Dielectric Properties and High Tunability of Porous Ba0.5Sr0.5TiO3 Ceramics

Porous Ba_0.5Sr_0.5TiO₃ (BST) ceramics were fabricated by the traditional solid-state reaction process, and their structural, microstructural, dielectric, and tunability properties were systemically investigated. Compared with the fully dense BST samples, porous samples exhibit smaller grain sizes, a more uniform microstructure, and much lower dielectric constants, while at the same time, exhibiting little increase in tunability, which is beneficial to the development of microwave-tunable applications. At a frequency of 10 kHz and a temperature of 18°C, as porosity increased from 0% to 28.8%, the dielectric constant of the BST ceramics (under zero bias field) decreased from ɛ_r(0)∼1690 to ɛ_r(0)∼990, while the dielectric losses were still less than 0.2%, and the tunability increased from 17.6% to 19.6% (2.6 kV/mm).     

Dielectric Properties and Leakage Current Characteristics of Sol-Gel Derived (Ba0.5Sr0.5)TiO3:MgTiO3 Thin Film Composites

(Ba_1-xSr_x)TiO₃ (BST) is a suitable material for microelectronic device applications due to its high response of the dielectric permittivity to an applied electric field. However, the large dielectric constants found in this system limit its usefulness at microwave frequencies. In our recent studies sol-gel prepared Ba_0.5Sr_0.5TiO₃:MgO heterostructured thin films have shown an increase in the figure of merit (tunability/loss) compared to pure Ba_0.5Sr_0.5TiO₃ films, but the leakage characteristics of these films did not improve significantly. Moreover, due to the hygroscopic nature of MgO, the BST:MgO heterostructured films may suffer with longer period of time instability. In this paper we used the low loss and non-hygroscopic MgTiO₃ for the fabrication of the Ba_0.5Sr_0.5TiO₃:MgTiO₃ (BST50:MT) heterostructured thin films by sol-gel technique and studied the effects of MgTiO₃ layers on the structural, physical, dielectric properties, and leakage current behavior of the BST:MT films. The X-ray studies indicated that both MT and Ba_0.5Sr_0.5TiO₃ are highly oriented and remain as two distinct individual entities in the composite films and a considerable reduction in the dielectric loss and leakage currents has been observed. High frequency phase shifter studies (at low voltage) suggest that these films are suitable for the tunable microwave device applications.     

Giant dielectric phenomenon of Ba0.5Sr0.5TiO3/CaCu3Ti4O12 multilayers due to interfacial polarization for capacitor applications

Ba_0.5Sr_0.5TiO₃/CaCu₃Ti₄O₁₂ (BST/CCTO) multilayers with different stack sequences were deposited on LaNiO₃(LNO)/SiO₂/Si substrates by a sol-gel process. The dielectric properties of BST/CCTO multilayers are significantly affected by the deposition sequence, the layer thickness and the impurities, effects that are interpreted using the Maxwell-Wagner interfacial polarization model. Impurities are generated by elemental interdiffusion at the interfaces of BST/CCTO, and less at the interfaces of BST/LNO and CCTO/LNO. The dielectric permittivity of the CCTO/BST/LNO/SiO₂/Si sample reaches 352,200 at 10 kHz, and stabilizes above 20,000 in the range of 100 kHz to 1 MHz. This work demonstrates an effective approach to enhance dielectric properties for film capacitor applications by constructing multilayers with specific deposition sequences and layer thicknesses.     

Ferroelectric–Dielectric Composites Model of Ba0.5Sr0.5TiO3/Mg2AO4(A = Ti,Si) for Tunable Application

The correspondence between the theoretical model and the experimental results of the dielectric response in two‐phase composites of Ba_0.5Sr_0.5TiO₃ and Mg₂AO₄ (A = Ti, Si)was studied. The Ba_0.5Sr_0.5TiO₃ (BST50)/Mg₂AO₄ composites in 2‐2 model structure consisting of BST50 layers and Mg₂AO₄ layers were fabricated by tape casting and multilayer technique. The 3‐0 model of the two‐phase composites is fabricated by conventional ball mill mixing and solid‐state reaction process. The ceramics samples with dense structure were obtained because the coefficient of thermal expansion (CTE) of Mg₂SiO₄ (12.84 ppm/°C) and Mg₂TiO₄ (12.11 ppm/°C) ceramic specimens are close to the pure BST50(13.15 ppm/°C) ceramic. The microstructure, dielectric, and tunable properties of 2‐2 and 3‐0 model composites were investigated. The experimental results agree well with the theoretical prediction in 2‐2 model. An important feature of 2‐2 model composites is that the DC field is efficiently applied to the high‐permittivity ferroelectric phase. With the increase in Mg₂AO₄ volume fraction q, the tunability of the composite remains almost unchanged whereas the permittivity greatly reduced in the 2‐2‐//model. These results show that the 2‐2‐//model sample is good candidates for the tunable devices.     

Low dielectric loss BST/PTFE composites for microwave applications

Ba_0.3Sr_0.475Ce_0.03La_0.12Ti_0.997Mn_0.003O₃/Polytetrafluoroethylene (PTFE) composites were prepared using powder processing technique. The effects of the ceramic filler volume fraction and the coupling agent on the phase composition, microstructure, dielectric and thermal properties of the composites were investigated in this paper. The ceramic filler dispersion in the PTFE matrix, thus the dielectric loss, permittivity, and dimensional thermal stability of the composite was considerably improved by the modification of BST filler surface using phenyl trimethoxy silane (PTMS) coupling agent. Variation of the dielectric permittivity of the composite with composition was well fitted by the effective medium theory (EMT) model in the experimental compositional range. The obtained silane-treated composite with 0.5 V_f BST exhibits extremely low dielectric loss: ε_r = 16, tan δ = 5.4 × 10⁻⁴ @1 MHz and 5.16 ± 0.6 × 10⁻³ @ 10 GHz. The CTE of the composites was reduced to 43 ppm/°C.     

Effects of vacancy defects caused by non-stoichiometric ratio on dielectric properties of ABO3 perovskite (Ba0.5Sr0.5)xTiO3 ceramics

Non-stoichiometric Ba_0.5Sr_0.5TiO₃ (BST50) ceramics with varying A/B ratios, namely (Ba + Sr)/Ti, were prepared by a conventional solid-state reaction approach. The effects of vacancy defects caused by varying the A/B ratio on the structure and dielectric properties of BST50 ceramics were systematically investigated. A remarkable change in grain size was found when the A/B ratio was increased, which led to apparent variations in the dielectric properties of the BST50 ceramics. The Curie temperature (T_c) and dielectric permittivity peak (ε_max) increased first and then decreased with increasing A/B ratio, and reached the maximum at A/B = 1. Simultaneously, the dielectric diffusion parameter of BST50 ceramics was studied by the Lorenz-type formula. All samples exhibited diffusion phase transition behavior, and T_c was frequency independence. When A/B < 1, the Q value remained at a high level; in contrast, when A/B > 1, the Q value was significantly reduced. For this BST50 system, high tunability of 24.95% (at 30 kV/cm), low dielectric loss of 0.0017 (at 10 kHz), and high figure of merit (FOM) of 147 were achieved simultaneously at A/B = 1.01.     

In situ synthesis of Ba0.5Sr0.5TiO3–Mg3B2O6 composites and their dielectric response

Ba_0.5Sr_0.5TiO₃–Mg₃B₂O₆ (BST–MB) composites have been prepared in situ by a citrate gel process, and their structure and effective dielectric response have been investigated. The precursor with pH≥7 is suitable for in situ formation of the diphase structure consisting of BST and MB. Accordingly, MB particles homogeneously disperse in BST particles, accompanied by the formation of boron-rich grain boundary resulting from liquid phases sintering of B₂O₃. Related with the existence of boron-rich grain boundary and the incorporation of Mg²⁺ into BST lattice, permittivity decreases rapidly with increasing volume fraction of MB from 0.0 to 0.2 and then decreases slowly with further increasing, which coincides with theoretical prediction of the layered model.     

Enhanced Dielectric Properties of Ba0.5Sr0.5TiO3/MgO Composites Prepared by Coprecipitation Derived Core–Shell Powders

Ba_0.5Sr_0.5TiO₃/MgO (BST/MgO) composite ceramics have been prepared in situ by using an oxalate precipitation process. SEM and TEM images show that oleic acid modified BST/MgO nanocomposite exhibits helminth‐like particles consisting of BST core and MgO shell. Results reveal that oleic acid modification is chemisorbed on MgO particles as a carboxylate to form MgO shell, which connects to interparticles. Accordingly, the BST particles are well surrounded naturally by a thin MgO layer in the composite ceramics. As a result, the oleic acid modified BST/MgO composite ceramics shows better dielectric properties, with permittivity of 203, loss tangent of 0.0025 (at 2.4 GHz), and tunability of 10.2%.     

Flexible Composite Transducers

Flexible piezoelectric composites made from PZT spheres and polymers with 1-3 connectivity were fabricated using several types of polymers. The dielectric constants of the composites were 300 to 400 and the piezoelectric voltage coefficients g33 were 45 to 55×lO⁻³ V-m/N. The high-frequency properties of the composites were measured in both the thickness and radial mode of resonance. The frequency constants and the coupling coefficients of the composites for the thickness mode of resonance are comparable with the corresponding values for PZT. Possible applications of the composites are indicated.