首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   8101篇
  免费   621篇
  国内免费   554篇
电工技术   140篇
综合类   474篇
化学工业   4513篇
金属工艺   619篇
机械仪表   319篇
建筑科学   78篇
矿业工程   87篇
能源动力   53篇
轻工业   40篇
水利工程   4篇
石油天然气   12篇
武器工业   84篇
无线电   846篇
一般工业技术   1683篇
冶金工业   239篇
原子能技术   19篇
自动化技术   66篇
  2024年   3篇
  2023年   413篇
  2022年   471篇
  2021年   382篇
  2020年   391篇
  2019年   341篇
  2018年   337篇
  2017年   340篇
  2016年   188篇
  2015年   114篇
  2014年   232篇
  2013年   256篇
  2012年   337篇
  2011年   477篇
  2010年   298篇
  2009年   383篇
  2008年   334篇
  2007年   493篇
  2006年   437篇
  2005年   390篇
  2004年   360篇
  2003年   316篇
  2002年   265篇
  2001年   239篇
  2000年   227篇
  1999年   199篇
  1998年   182篇
  1997年   172篇
  1996年   135篇
  1995年   129篇
  1994年   81篇
  1993年   76篇
  1992年   70篇
  1991年   60篇
  1990年   48篇
  1989年   43篇
  1988年   27篇
  1987年   11篇
  1986年   8篇
  1985年   1篇
  1984年   1篇
  1983年   1篇
  1982年   1篇
  1981年   3篇
  1980年   2篇
  1979年   1篇
  1975年   1篇
排序方式: 共有9276条查询结果,搜索用时 15 毫秒
1.
陶瓷的压缩破坏是一个爆炸式破碎的过程,在短时间内急剧释放大量能量,并伴随大量高速飞行碎片的产生,实验上较难获取陶瓷压缩破坏过程中碎片的飞溅速度。本文采用离散元数值模拟氧化铝陶瓷的压缩破碎过程,分析了不同应变率下产生碎片的尺寸分布、碎片的平均飞溅速度,以及试件内部不同区域碎片的速度变化规律。研究表明:(1)陶瓷的表观破坏强度以及破碎后碎片的平均飞溅速度与加载应变率正相关;(2)碎片飞溅速度与其初始位置相关,外侧碎片的飞散速度最大,随着初始位置与试件中心轴距离的减小,碎片飞溅速度逐渐减小;(3)随着加载应变率的提高,碎裂产生的碎片数逐渐增多,对应的碎片平均尺寸变小。进一步讨论了试件压缩破碎过程中的能量守恒和转换模式,并对碎片飞溅的平均速度进行了理论分析。  相似文献   
2.
《Ceramics International》2022,48(21):31738-31745
In this study, novel polyborosilazane-derived SiBCN(O) ceramic was used as self-healing component in self-healing Cf/SiBCN(O) composite, which was prepared by polymer infiltration and pyrolysis (PIP) process. Molecular-level structure design of boron-containing ceramic precursors was utilized to achieve uniform dispersion of boron-containing self-healing components in prepared composites. No elemental diffusion was observed at the interface of ceramic matrix and carbon fibers, which resulted in stable SiBCN(O) structure. In addition, boron was uniformly distributed in Cf/SiBCN(O) composite ceramic matrix, which was beneficial for self-healing of cracks. Cracks and indentations were able to heal at high temperatures in air. The best crack-healing behavior occurred in air atmosphere at 1000 °C, with nearly complete crack healing. This excellent self-healing behavior was achieved because silicon and boron atoms in SiBCN(O) ceramic reacted with available oxygen at high temperatures to form SiO2(l), B2O3(l), and B2O3·xSiO2 liquid phases, which effectively filled cracks. In general, as-prepared Cf/SiBCN(O) composite exhibited excellent self-healing properties and shows great application potential in high-temperature environment applications such as aviation, aerospace, and nuclear power.  相似文献   
3.
Low-thermal conductivity ceramics play an indispensable role in maximizing the efficiency and durability of hot end components. Pyrochlore, particularly zirconate pyrochlore, is currently a highly promising and widely studied candidate for its extremely low thermal conductivity. However, there are still few pyrochlores that offer both stiffness, insulation, and good thermal expansion properties. In this work, the solidification method was innovatively introduced into the preparation of titanate pyrochlore, and combined it with the compositional design of high-entropy. Through careful composition design and solidification control, the high-density and uniform elements distributed high-entropy titanate pyrochlore ceramics were successfully prepared. These samples possess high hardness (15.88 GPa) and Young’s modulus (295.5 GPa), low thermal conductivity (0.947 W·m?1·K?1), excellent thermal expansion coefficient (11.6 ×10?6/K) and an exquisite balance between stiffness and insulation (E/κ, 312.1 GPa·W?1·m·K), in which the E/κ exhibits the highest value among the current reported works.  相似文献   
4.
Bismuth layer–structured ferroelectric calcium bismuth niobate (CaBi2Nb2O9, CBN) is considered to be one of the most potential high-temperature piezoelectric materials due to its high Curie temperature Tc of ∼940°C, but the drawbacks of low electrical resistivity at elevated temperature and low piezoelectric performance limit its applications as key electronic components at high temperature (HT). Herein, we report significantly enhanced dc electrical resistivity and piezoelectric properties of CBN ceramics through rare-earth element Tb ions compositional adjustment. The nominal compositions of Ca1−xTbxBi2Nb2O9 (abbreviated as CBN-100xTb) have been fabricated by conventional solid-state reaction method. The composition of CBN-3Tb exhibits a significantly enhanced dc electrical resistivity of 1.97 × 106 Ω cm at 600°C, which is larger by two orders of magnitude compared with unmodified CBN. The donor substitutions of Tb3+ ions for Ca2+ ions reduce the oxygen vacancy concentrations and increase the band-gap energy, which is responsible for the enhancement of dc electric resistivity. The temperature-dependent dc conduction properties reveal that the conduction is dominated by the thermally activated oxygen vacancies in the low-temperature region (200–350°C) and by the intrinsic conduction in the HT region (350–650°C). The CBN-3Tb also exhibits enhanced piezoelectric properties with a high piezoelectric coefficient d33 of ∼13.2 pC/N and a high Tc of ∼966°C. Moreover, the CBN-3Tb exhibits good thermal stabilities of piezoelectric properties, remaining 97% of its room temperature value after annealing at 900°C. These properties demonstrate the great potentials of Tb-modified CBN for high-temperature piezoelectric applications.  相似文献   
5.
以Er2O3-Mg2Si-Yb2O3为三元复合烧结助剂,制备了力学性能优异的高导热氮化硅陶瓷,研究了Er2O3-Mg2Si-Yb2O3体系对氮化硅陶瓷致密化、微观结构、力学性能、热导率的影响。研究表明,当添加5%(质量分数,下同)Er2O3+2%Mg2Si+4%Yb2O3烧结助剂时,烧结助剂对氮化硅陶瓷致密度与晶界相含量的平衡效果最佳,此时氮化硅陶瓷具有最佳性能:抗弯强度为765 MPa,断裂韧性为7.2 MPa·m1/2,热导率为67 W/(m·K)。在烧结过程中,只添加5%Er2O3+2%Mg2Si的烧结助剂产生的液相量少且黏度高,不能使氮化硅陶瓷完成致密化;此外,当添加的Yb2O3含量超过4%时,烧结助剂产生大量的晶界相,降低了氮化硅陶瓷的性能。  相似文献   
6.
《Ceramics International》2022,48(17):24592-24598
Single-phase Al4SiC4 powder with a low neutron absorption cross section was synthesized and mixed with SiC powder to fabricate highly densified SiC ceramics by hot pressing. The densification of SiC ceramics was greatly improved by the decomposition of Al4SiC4 and the formation of aluminosilicate liquid phase during the sintering process. The resulting SiC ceramics were composed of fine equiaxed grains with an average grain size of 2.0 μm and exhibited excellent mechanical properties in terms of a high flexure strength of 593 ± 55 MPa and a fracture toughness of 6.9 ± 0.2 MPa m1/2. Furthermore, the ion-irradiation damage in SiC ceramics was investigated by irradiating with 1.2 MeV Si5+ ions at 650 °C using a fluence of 1.1 × 1016 ions/cm2, which corresponds to 6.3 displacements per atom (dpa). The evolution of the microstructure was investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The breaking of Si–C bonds and the segregation of C elements on the irradiated surface was revealed by XPS, whereas the formation of Si–Si and C–C homonuclear bonds within the Si–C network of SiC grains was detected by Raman spectroscopy.  相似文献   
7.
Two types of transparent Y2O3 ceramics without including large scattering sources such as residual pores, one with very high optical homogeneity (type A) and another one with slightly insufficient optical homogeneity (type B), are purposely prepared, and their optical properties are investigated and compared qualitatively and quantitatively. Type A ceramic exhibits transmittance characteristics with very low internal loss in the visible to infrared wavelength region, while type B ceramic is inferior in various optical performances especially in the short (visible) wavelength region. In type B ceramic, birefringence occurs due to optical inhomogeneity in the visible region, resulting in a decrease in the extinction ratio. Non-uniform refractive index distribution is also observed in the Schlieren image of type B ceramic, hence the laser beam quality through that material is degraded. This study proved the importance of optical homogeneity of transparent ceramics and clarified the problems in actual applications.  相似文献   
8.
In view of the practical application of γ-AlON as a promising transparent structural ceramic, in-depth insight into its mechanical and thermal properties is essential. The solid-state MAS NMR technique was combined with XRD Rietveld refinement to confirm the crystal structure of Al(8+x)/3O4-xNx (x = 0.299–0.575). These structural parameters were further applied to predict hardness and elastic properties based on theoretical exploration, which are in good agreement with the experimental values. A slight enhancement of mechanical properties with increasing nitrogen concentration is attributed to the stronger chemical bond in octahedra. The experimental thermal conductivity of γ-AlON transparent ceramics was improved slightly with the rise of x in the temperature range from 298 K to 1074 K. The intrinsic lattice thermal conductivity was determined by eliminating the extrinsic phonon scattering as well as the thermal radiation. The reason for the discrepancy between experimental and intrinsic thermal conductivity was revealed. The present methods provided powerful and accessible guidelines in optimizing the mechanical and thermal properties of oxynitride materials.  相似文献   
9.
采用快速液相烧结法制备Bi1-xPrxFe1-xTixO3(x=0.00、0.03、0.06、0.12)系列多铁陶瓷样品,研究Pr-Ti共掺杂对BiFe O3结构、缺陷、电学和磁学特性的影响。XRD分析结果表明:所有样品均为菱方钙钛矿结构,Pr-Ti共掺杂可有效抑制杂相生成,当掺杂量高于0.06时杂相基本消失,共掺杂引起结构畸变。正电子湮没寿命谱测试结果表明:所有样品中均存在阳离子空位型缺陷,空位尺寸和浓度均随Pr-Ti掺杂量增加而增大。电学和磁学性能测试结果表明:适量Pr-Ti共掺杂可有效提高Bi Fe O3的介电、铁电和磁学性能。综合上述结果,认为BiFeO3多铁性能的改善可能是由于Pr-Ti共掺杂引起晶格畸变、减少氧空位浓度、改变阳离子空位浓度等多种原因引起。  相似文献   
10.
A type of boron nitride–magnesium aluminum silicate (BN-MAS) composite ceramics was fabricated by hot-press sintering at different sintering temperatures. The relationship between the sintering temperature and microstructure was investigated by analyzing the interaction between hexagonal boron nitride (h-BN) and the MAS phase. The main MAS phase in the composite ceramics is the α-cordierite phase at a sintering temperature of 1300°C. At temperatures above 1400°C, the inhibitory effect of h-BN on the crystallization of the MAS system is significant, and MAS mainly exists in the form of an amorphous phase. The composite sintered at 1700°C exhibited the highest bending strength of 218MPa. h-BN and MAS were co-enhanced. MAS can be used as an effective liquid-phase sintering aid to assist in the sintering of h-BN, whereas h-BN can absorb the fracture energy of the composite ceramics through the pull-out and bridging effect of the particles.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号