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931.
932.
An investigation of pore cracking in titanium welds 总被引:1,自引:0,他引:1
T. Khaled 《Journal of Materials Engineering and Performance》1994,3(1):21-36
Two welded Ti-6A1-4V pressure vessels leaked prematurely in service. The leaks were caused by cracks emanating from weld porosity.
The cracks originated during fabrication, with subsequent growth in service leading to the formation of the leak paths. Pore
cracking is thought to be caused by a mechanism that involves both sustained-load and cyclic contributions, with the former
being the more prominent. It is shown that the tendency for cracking is influenced by pore position and that pore size is
not a deciding factor in that regard. The factors that govern pore cracking are discussed, and the possible role of interstitial
embrittlement is assessed. 相似文献
933.
将Willenborg的超载残余应力场模型与裂尖形变动力学、裂尖表面析氢反应电化学动力学以及裂尖材料内部氢的扩散动力学和热力学相结合,建立了氢脆腐蚀疲劳裂纹扩展(CFCG)超载效应的定量描述模型,给出了超载对裂尖表面析氢电化学反应及裂尖氢扩展影响的一系列判据和表达式.用上述模型进行的预测表明,对于超高强度钢,当两次超载周次之间裂纹扩展长度ad远小于超载塑性区尺寸rpOL与恒载塑性区尺寸rpCLA之差,即ad<<rpOL-rpCLA时,CFCG会发生比空气疲劳还强烈的超载迟滞效应.而当ad>rpOL-rpCLA时,可能会发生超载加速效应.这种预测在后文中由试验的宏观规律与断口特征得到充分验证. 相似文献
934.
应力比和频率对GC-4钢CF裂纹扩展特性的影响 总被引:1,自引:0,他引:1
研究了五种应力比和四种频率对GC-4(40CrMnSiMoVA)钢在35%NaCl中性溶溶液中腐蚀疲劳裂纹扩展(CFCG)特性的影响.结果表明,在高应力比时,CFCG的dα/dt-Kmax曲线存在两个平台区PⅠ和PⅡ,且(da/dt)PⅡ<(da/dt)PⅠ.CFCG的da/dN-△K曲线的平台区起始值面△Kp随频率升高呈幂函数关系而降低.从表面电化学反应动力学、裂尖表面形变动力学以及裂尖材料内部氢的扩散动力学三个方面综合考虑,导出了△KP-f的理论函数关系.该函数关系与CFCG试验数据所显示的函数关系十分一致. 相似文献
935.
The synergistic influence of prior-austenite grain size and silicon content of 9Cr–1Mo steel on the resistance to scale spallation has been studied in air at 773 K (for 500 hr) and 973 K (12 hr). Two steels, irrespective of their grain size and Si content, did not show spallation during oxidation at 773 K. Spallation occurred at 973 K, and fine-grain steels exhibited less spallation resistance than coarse-grain ones (in low-as well as high-Si steels). Among the four possible combinations of grain size ans Si content, the steel with low Si and fine grains showed least resistance to spallation, while the steel with high Si and coarse grains showed the best resistance. Spallation was found to initiate in the areas adjoining the oxide ridges formed at the alloy grain boundaries. Oxide scales at the ridges and within the grains were analyzed by scanning electron microscopy (SEM/EDX) and secondary-ion mass spectrometry (SIMS). These analyses suggest depletion of silicon from the areas adjoining grain boundaries, resulting in thicker scaling that triggers spallation in such areas. For similar grain-size materials, the necessary thickness for spallation was attained earlier with low-Si steel rather than in high-Si steel. 相似文献
936.
This work focuses on distributed control of film thickness, surface roughness and porosity in a porous thin film deposition process using the deposition rate as the manipulated input. The deposition process includes adsorption and migration processes and it is modeled via kinetic Monte Carlo simulation on a triangular lattice with vacancies and overhangs allowed to develop inside the film. A distributed parameter (partial differential equation) dynamic model is derived to describe the evolution of the surface height profile of the thin film accounting for the effect of deposition rate. The dynamics of film porosity, evaluated as film site occupancy ratio, are described by an ordinary differential equation. The developed dynamic models are then used as the basis for the design of a model predictive control algorithm that includes penalty on the deviation of film thickness, surface roughness and film porosity from their respective set-point values. Simulation results demonstrate the applicability and effectiveness of the proposed modeling and control approach in the context of the deposition process under consideration. 相似文献
937.
This work is devoted to the kinetic study of densification and grain growth of LaPO4 ceramics. By sintering at a temperature close to 1500 °C, densification rate can reach up to 98% of the theoretical density and grain growth can be controlled in the range 0.6–4 μm. Isothermal shrinkage measurements carried out by dilatometry revealed that densification occurs by lattice diffusion from the grain boundary to the neck. The activation energy for densification (ED) is evaluated as 480 ± 4 kJ mol−1. Grain growth is governed by lattice diffusion controlled pore drag and the activation energy (EG) is found to be 603 ± 2 kJ mol−1. The pore mobility is so low that grain growth only occurs for almost fully dense materials. 相似文献
938.
目的制备重组碱性成纤维细胞生长因子(bFGF)凝胶剂。方法以卡波姆940、甘油、1,2丙二醇为基质,加入复方保护剂(肝素钠、人血白蛋白、甘露醇和HP-β-CD),制成bFGF凝胶剂,观察25和4℃保存的稳定性,并进行家兔局部刺激试验、急性毒性试验、促烧伤创面愈合试验。结果bFGF凝胶剂在25℃保存18个月、4℃保存24个月,效价维持在原效价的80%以上;局部用药未见皮肤刺激反应和急性毒性反应;促家兔烧伤创面愈合的效果明显优于水溶液剂。结论制备的bFGF凝胶剂安全有效,具有缓释作用,稳定性良好,具有广阔的临床应用前景。 相似文献
939.
Crystallization of Al(OH)3 that occurs during the decomposition of caustic soda solutions is an important part of Bayer process for alumina production. Several phenomena, which influence the physicochemical characteristics of precipitated Al(OH)3, occur simultaneously during this process. They are nucleation, agglomeration, and crystal growth of Al(OH)3. In this article, we have investigated the mechanism of Al(OH)3 crystal growth from pure caustic soda solutions and in the presence of oxalic acid. The results have shown that the growth of Al(OH)3 crystals from caustic soda solutions follow the B + S model (birth and spread). New Al(OH)3 particles, formed during the decomposition process of pure caustic soda solutions, are characterized by regular hexagonal shape. The nuclei have the same geometry as the contact face. However, microstructural investigations of Al(OH)3 samples, obtained by crystallization from caustic soda solutions in the presence of oxalic acid, have shown the presence of nuclei of irregular shape in addition to regular ones. So, the presence of oxalic acid in the caustic soda solutions leads to a change in crystal habit. Besides, the results obtained by kinetic investigation confirmed the mentioned mechanism of Al(OH)3 crystal growth. 相似文献
940.
FGF21生物学特性的研究进展 总被引:1,自引:0,他引:1
FGF21是成纤维细胞生长因子家族的一名新成员,其氨基酸序列与其他亚家族成员之间有不同程度的同源性。FGF21在肝脏中特异性表达,能降低血糖、甘油三酯、胰高血糖素和果糖胺的浓度,改善胰岛β细胞的功能,因此有望成为治疗糖尿病、肥胖症及其他一些代谢综合症的药物。FGF21还有很多不同于其他FGFs家族的特点,如FGF21的信号传导需要βKlotho协同才能稳定结合FGFR;FGF21能通过血脑屏障等。另外,在斑马鱼体内还发现FGF21能促进造血功能。本文就FGF21的分子结构及其生物学特性的研究进展作一综述。 相似文献