共查询到18条相似文献，搜索用时 484 毫秒
1.
This work investigated the microwave dielectric properties of A-site substitution by rare earth La3+in(Pb0.5Ca0.5)(Fe0.5Ta0.5)O3(PCFT) system.A single perovskite phase was obtained only when the doping content was 2%.Suitable La3+ doping improved microwave dielectric performances.Excessive La3+doping caused the formation of secondary phase,which resulted in the decreasing of permittivity εrand quality factor Qfvalues.Especially,when the doping content is 2%-5%,permittivity εrwas above 75 and Qfvalues were 6... 相似文献
2.
Studies on the sintering, microstructure, mechanical properties and low temperature degradation behavior of yttria stabilized-tetragonal zirconia polycrystal （3Y-TZP） were carried out. The results show that the addition of Al2O3 and MgO improve the sintering at low temperatures, and cause exaggerated grain growth and the transformation of tetragonaI-ZrO2 to monoclinic-ZrO2 with corresponding changes in the mechanical properties. The addition of the yttria-free monoclinic zirconia particles change the overall yttria distribution and induce an active transformation toughening mechanism. Furthermore, the dispersed yttria-free ZrO2 can inhibit the tetragonal zirconia transformation, which is beneficial to the improvement of the low temperature degradation behavior of 3Y- TZP ceramics. 相似文献
3.
Aluminum nitride （AlN）/borosilicate glass composites were prepared by the tape casting process and hot-press sintered at 950 ℃ with AIN and SiO2-B203-ZnO-Al2O3-Li2O glass as starting materials. We characterized and analyzed the variation of the microstructure, bulk density, porosity, dielectric constant, thermal conductivity and thermal expansion coefficient （TEC） of the ceramic samples as a function of AIN content. Results show that AIN and SiO2-B2O3-ZnO-Al2O3-Li2O glass can be sintered at 950 ℃, and ZnAI204 and Zn2SiO4 phase precipitated to form glass-ceramic. The performance of the ceramic samples was determined by the composition and bulk density of the composites. Lower AlN content was found redounding to liquid phase sintering, and higher bulk density of composites can be accordingly obtained. With the increase of porosity, corresponding decreases were located in the dielectric constant, thermal conductivity and TEC of the ceramic samples. When the mass fraction of AlN was 40%, the ceramic samples possessed a low dielectric constant （4.5-5.0）, high thermal conductivity （11.6 W/（m.K）） and a proper TEC （3.0× 10^-6 K^-1 which matched that of silicon）. The excellent performance makes this kind of low temperature co-fired ceramic a promising candidate for application in the micro-electronics packaging industry. 相似文献
4.
Different ambient conditions for the synthesis of Ag（Nb0.8Ta0.2）O3 ceramics were investi- gated. The Ag（Nb0.8Ta0.2）O3 powder was synthesized at 950 ℃ under different ambient conditions, and then pressed into disks and sintered between 1060 ℃ and 1100 ℃ respectively. Samples were investigated by X-ray diffraction, scanning electron microscopy and dielectric measurement. The results show that perovskite Ag（Nb0.8Ta0.2）O3 powder was easier to be synthesized in air than in vacuum at 950℃. Grain size of ceramic samples sintered in air was uniform （about 1 μm） and its dielectric loss was small for its high density. However, the samples decomposed greatly and ceramics could hardly be densified when sintered in vacuum, Thus,. higher atmospheric pressure and oxygen atmosphere would benifit the synthesis of Ag （Nb0.8Ta0.2）O3, and suppress its decomposition at high temperature. 相似文献
5.
CuO-doped(K0.5Na0.5)Nb0.9Ta0.1O3 lead-free piezoelectric ceramics have been prepared by conventional sintering technique and the effects of CuO on the phase structure and piezoelectric properties of the ceramics have been studied.The results show that a small amount of CuO can improve the densification of the ceramics,consequently,the electric properties are improved.Good piezoelectric and dielectric properties of d33=125 pC/N,Kp=0.38,εr=700,Qm= 248,and Tc=370°C were obtained for the K0.5Na0.5Nb0.9Ta 0.1O3 ceramics doped with 0.5mol% CuO sintered at 1000℃ for 2h. 相似文献
6.
Long lasting strontium aluminate phosphors were synthesized by means of coated precipitation in the experiment. SrA12O4 monoclinic phase with long afterglow was synthesized at 1 100 ℃ with the aid of H3BO3,and the synthetic temperature decreased about 100 ℃-150 ℃ than in solid state reaction. The process of heat treatment was studied by TG-DTA. The phase compositions of powders were analyzed by XRD. The micrographs of the samples were investigated by SEM and the luminescent properties were researched by optical spectrum. The effects of H3 BO3 on the phase composition and microstructure of the calcined powders were investigated. The results indicated that H3BO3 accelerated the formation of SrAl2O4 by consuming the intermediate phase. The addition of Dy2O3 stimulated the afterglow property due to the increase in the depth of the trap center in the materials. 相似文献
7.
In this work we study the behaviour of the dielectric constant of BaTiO3 single crystals doped with Cu and Fe for different ion percentages, particularly, the influence of these hetemvalent substitutions on the ferroelectric-paraelectric phase transition whose temperature is found at Tc=120℃ for pure samples. The dielectric constant e in terms of temperature shows that the Curie temperature decreases when the quantity of impurities increases and presents a broadening and flattering of the maximum oft（T） within higher values, with the transition becoming more and more diffuse. It is interesting to have a material with very high permittivity （high-k） because of its capacity to store an important quantity of electric charges. The t anisotropy and the Curie-Weiss law are also verified with a good ratio between the slopes ofε^-l（T） from both sides of the transition, leading to a Curie constant： C= 13 × 10^4 K for BaTiO3：1.6%Fe in the polar phase. BaTiO3 is a displacive ferroelectric going through a first-order phase transition. The substitutions have an effect on the dynamics of the perovskite lattice. They induce charges transfer to Ti and a diminution of elastic forces in BaTiO3. Then we discuss the transition but on the nature of the material. fact that the maximum of permittivity does not depend on the phase 相似文献
8.
The influence of calcination temperature on TiO2 nanotubes＇ catalysis for TiO2/UV/03 was investigated. TiO2 nanotubes （TNTs） were prepared via the sol-gel method and calcined at 300--700 ℃, which were labeled as TNTs-300, TNTs-400, TNTs-500, TNTs-600 and TNTs-700, respectively. TNTs were characterized by transmission electron microscopy （TEM） and X-ray diffraction （XRD）. It is found that TNTs calcined at 400 ℃ showed the best thermal stability. When the calcination temperature increased from 400 ℃ to 700 ℃, the special structure of tubes was destroyed and gradually converted into nanorods and/or particles. The transformation from anatase to rutile occurred at 600 ℃, and the rutile phase was enhanced when the calcination temperature was increased to over 600 ℃. The calcina- tion temperature＇s influence on TNTs＇ adsorption activity for for TiO2/UV/O3 was investigated in landfill leachate solution chemical oxygen demand （COD） and catalytic activity In landfill leachate solution, the adsorption activity of COD decreased in the reduced order of TNTs-300, TNTs-400, TNTs-500, TNTs-600 and TNTs-700. In photocatalytic ozonation, TNTs-400 showed the best catalytic activity while TNTs-700 exhibited the worst. In other three processes, the COD removal of TNTs-300/UV/O3 was higher than those of TNTs-500/UV/O3 and TNTs-600/UV/O3 in the first 20 rain, and then became close to those of the latter two in the following 40 rain. Compared with TNTs-300 and TNTs- 400, TNTs-600 had the best anti-fouling activity, while TNTs-500 and TNTs-700 had lower anti-fouling activity than the former three. In photocatalytic ozonation, the calcination temperature of 400 ℃ was appropriate when TNTs were obtained at the synthesis temperature of 105 ℃. 相似文献
9.
Y^3＋-doped （Bi 1/2 Na 1/2） TiO 3-CaTiO 3-BaTiO 3 （BNCBT） positive temperature coefficient of resistivity （PTCR） ceramics sintered in air atmosphere were investigated in this study. （Bi 1/2 Na 1/2） TiO 3 （BNT） component can remarkably increase the onset temperature T c of PTCR ceramics with the expense of the resistivity R 25 increase. CaTiO 3 （9–27 mol%） component can decrease the resistivity, and adjust the effects of BNT phase on the T c point. For the sample containing 3 mol% CaTiO 3 , T c raises from 122 ℃ to 153 ℃ when only 0.6 mol% BNT added, while for the ones with higher CaTiO 3 content （9–27 mol%）, T c is only increased by a rate of 8–9℃/1.0 mol% BNT. The effects of BNT and CaTiO 3 components on R25/Rmin （negative temperature coefficient effect） are also discussed. 相似文献
10.
Appropriate addition of CuO/V2O5 and the reduction of the granularity of the raw materials particle decrease the sintering temperature ofNiZn ferrite from 1200℃ to 930 ℃. Furthermore, the magnetic properties of the NiZn ferrite prepared at low temperature of 930℃ is superior to that of the NiZn ferrite prepared by sintering at high temperature of 1200 ℃ because the microstructure of the NiZn ferrite sintered at 930℃ is more uniform and compact than that of the NiZn ferrite sintered at 1200℃. The high permeability of 1700 and relative loss coefficient tanδ/μi of 9.0-10^-6 at 100 kHz was achieved in the (Ni0.17Zn0.63Cu0.20)Fe 1.915O4 ferrite. 相似文献
11.
Abstract The compliance of an integrated approach, principal component analysis （PCA）, coupled with Tagu chi＇s robust theory for simultaneous optimization of cor related multiple responses of wire electrical discharge machining （WEDM） process for machining SiCp rein forced ZC63 metal matrix composites （MMCs） is investi gated in this work. The WEDM is proven better for its efficiency to machine MMCs among others, while the particulate size and volume percentage of SiCp with the composite are the utmost important factors. These improve the mechanical properties enormously, however reduce the machining performance. Hence the WEDM experiments are conducted by varying the particulate size, volume fraction, pulseon time, pulseoff time and wire tension. In the view of quality cut, the most important performance indicators of WEDM as surface roughness （Ra）, metal removal rate （MRR）, wire wear ratio （WWR）, kerf （Kw） and white layer thickness （WLT） are measured as respon ses. PCA is used as multiresponse optimization technique to derive the composite principal component （CPC） which acts as the overall quality index in the process. Consequently, Taguchi＇s S/N ratio analysis is applied to optimize the CPC. The derived optimal process responses are confirmed by the experimental validation tests results. The analysis of vari ance is conducted to find the effects of choosing process variables on the overall quality of the machined component.The practical possibility of the derived optimal process conditions is also presented using SEM. 相似文献
12.
L2 （second language） writing teaching and research is greatly influenced by the advent of computer technology. This paper reviews papers, in the field of DDL （data-driven learning） in L2 writing, published in international journals and in Chinese journals, and differences are found in research methodology, research perspective, and research realm. Based on the analysis of the differences, the study aims at finding out the effects of the application of DDL in L2 teaching classroom. The present study, employing semi-structured writing and learning journals, explores how English learners perceive the use of corpora in their writing process. Based on the statistics, it is concluded that DDL in L2 writing is beneficial for vocabulary acquisition. However, corpus consultation does not work well for all learners. Learners＇ attitudes toward the use of corpus are various. The major findings may find application in teaching practice, and implications for teaching are discussed. 相似文献
13.
采用静电纺丝技术制备了PVP/[Co(NO3)3+Ni(CH3COO)2+Fe(NO3)3]复合纤维,通过X射线衍射(XRD)、扫描电镜(SEM)和振动样品磁强计(VSM)对复合纤维及其焙烧产物进行了表征,较为系统地考察了制备参数,如PVP含量、醇/水比例、电场强度以及无机盐含量对复合纤维形貌和直径的影响,获得了最佳制备条件。将所得复合纤维在550℃焙烧2h可以制得单相尖晶石结构的晶态Co0.5Ni0.5Fe2O4纳米纤维,其平均直径约为80nm,在室温下具有良好的磁性,比饱和磁化强度和矫顽力分别为52.3A.m2/kg和81.0kA/m。 相似文献
14.
This paper presented an automatic gain control(AGC) circuit suitable for FM/cw ladar. The proposed architecture was based on two-stage variable gain amplifier(VGA) chain with a novel DC offset canceller circuit, which contained an improved Gilbert cell and a Gm-C feedback loop. To keep the VGA with a linearity in dB characteristic, an improved exponential gain control circuit was introduced. The AGC was implemented in 0.18 μm standard CMOS process. Simulation and measurement results verified that its gain ranged from-20 dB to 30 dB, and bandwidth ranged from 100 kHz to 10 MHz. Its power consumption was 19.8 mW under a voltage supply of 3.3 V. 相似文献
15.
We designed and used a series of activities （named activities to teaching electric field （ATEF）） based on the Physics by Inquiry （PBI） strategy to teaching the electric field concept with university students. We used a concentration factor tool to assess the effectiveness of those activities used during three weeks of instruction. Results indicate that students who had traditional teaching obtained a very tiny gain （less than 1%）; concentration factors indicate that most of their responses fall into a random area, which is not favorable. On the other hand, the experimental group using our activities had greater gains （〉 10%） than the control group. Concentration factors showed that after instruction, students＇ mental models changed toward a favorable pattern about comprehension of electric field concept and Coulomb＇s law. A hypothesis test of two samples indicated that there is a significant difference between both traditional teaching and ATEF based on PBI. 相似文献
16.
The aims of the study were to census and compare the technological pedagogical and content knowledge （TPACK） of secondary science teachers and their most used information and communication technology （ICT） in two contexts, Taiwan and Shaanxi Province in China. A questionnaire was employed to examine secondary school science teachers＇ most used ICT and their TPACK in science teaching. Eight hundred and six secondary science teachers from Taiwan and 164 teachers from Shaanxi participated in the study. The analytical results showed that multimedia was the most used ICT in Taiwan, followed by PowerPoint （PPT）, Internet platforms, and interactive whiteboards （IWBs）. In Shaanxi, PPT was the most used ICT reported by science teachers, followed by multimedia, IWBs, and Intemet platforms. The findings indicated that in Taiwan, science teachers＇ TPACK was statistically significant in relation to different types of ICT, whereas in Shaanxi, science teachers＇ TPACK did not demonstrate significant difference. In Taiwan, science teachers who reported their most used ICT to be multimedia were found to show significant differences in TPACK according to gender and teaching experience. In Shaanxi, science teachers who reported their most used ICT to be PPT did not show any significant difference on TPACK by gender; however, they showed significant differences on TPACK in regard to teaching experience. The research implications of this study are provided below along with suggestions for future research. 相似文献
17.
There are some certain factors affecting L2(English)vocabulary acquisition such as L2(English)proficiency,readers’purpose and the habit of reading,and the difficulty of target words and context,among which the first plays a key role.The paper,through a review of the relevant experimental researches,reaches a conclusion that L2(English)proficiency does influence much upon L2vocabulary acquisition. 相似文献
18.
Serkan Arikan 《美中教育评论》2014,(10):716-727
There are many studies that focus on factors affecting achievement. However, there is limited research that used student characteristics indices reported by the Programme for International Student Assessment （PISA）. Therefore, this study investigated the predictive effects of student characteristics on mathematics performance of Turkish students. In PISA studies, sampling design, sampling weights, and plausible values have to be taken into consideration in order not to have biased multiple regression results. In order to conduct multiple regression analyses in PISA, software called the International Association for the Evaluation of Educational Achievement （lEA） International Database （IDB） Analyzer is required to be used because the dependent variable consists of several plausible values. This study aims to identify student characteristics that are significant in predicting mathematics performance in Turkey. Results showed that being successful in mathematics is a combination of several factors in which students＇ beliefs, motivation, and other factors must be organized to achieve mathematics. Among these beliefs and motivation, strong self-efficacy, positive self-concept, and minimum level of anxiety seem to be the key for success. 相似文献