全文获取类型
收费全文 | 165757篇 |
免费 | 10615篇 |
国内免费 | 5059篇 |
专业分类
电工技术 | 8212篇 |
技术理论 | 24篇 |
综合类 | 15497篇 |
化学工业 | 10739篇 |
金属工艺 | 5955篇 |
机械仪表 | 15230篇 |
建筑科学 | 48607篇 |
矿业工程 | 5763篇 |
能源动力 | 4054篇 |
轻工业 | 5584篇 |
水利工程 | 4926篇 |
石油天然气 | 4688篇 |
武器工业 | 1591篇 |
无线电 | 9635篇 |
一般工业技术 | 15629篇 |
冶金工业 | 4302篇 |
原子能技术 | 875篇 |
自动化技术 | 20120篇 |
出版年
2024年 | 583篇 |
2023年 | 1580篇 |
2022年 | 2875篇 |
2021年 | 3562篇 |
2020年 | 3980篇 |
2019年 | 2985篇 |
2018年 | 2897篇 |
2017年 | 3744篇 |
2016年 | 4367篇 |
2015年 | 4841篇 |
2014年 | 13031篇 |
2013年 | 9748篇 |
2012年 | 12066篇 |
2011年 | 12754篇 |
2010年 | 10839篇 |
2009年 | 11110篇 |
2008年 | 9915篇 |
2007年 | 11894篇 |
2006年 | 9745篇 |
2005年 | 8516篇 |
2004年 | 6923篇 |
2003年 | 6346篇 |
2002年 | 5103篇 |
2001年 | 4400篇 |
2000年 | 3502篇 |
1999年 | 2817篇 |
1998年 | 2210篇 |
1997年 | 1850篇 |
1996年 | 1530篇 |
1995年 | 1217篇 |
1994年 | 985篇 |
1993年 | 694篇 |
1992年 | 538篇 |
1991年 | 447篇 |
1990年 | 310篇 |
1989年 | 276篇 |
1988年 | 221篇 |
1987年 | 168篇 |
1986年 | 100篇 |
1985年 | 123篇 |
1984年 | 140篇 |
1983年 | 95篇 |
1982年 | 105篇 |
1981年 | 40篇 |
1980年 | 62篇 |
1979年 | 33篇 |
1978年 | 14篇 |
1977年 | 12篇 |
1959年 | 17篇 |
1957年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
23.
Sebastian Roth Peter Stockinger Jakob Steff Simon Steimle Dr. Viktor Sautner Prof. Dr. Kai Tittmann Prof. Dr. Jürgen Pleiss Prof. Dr. Michael Müller 《Chembiochem : a European journal of chemical biology》2020,21(18):2615-2619
The family of NAD(P)H-dependent short-chain dehydrogenases/reductases (SDRs) comprises numerous biocatalysts capable of C=O or C=C reduction. The highly homologous noroxomaritidine reductase (NR) from Narcissus sp. aff. pseudonarcissus and Zt_SDR from Zephyranthes treatiae, however, are SDRs with an extended imine substrate scope. Comparison with a similar SDR from Asparagus officinalis (Ao_SDR) exhibiting keto-reducing activity, yet negligible imine-reducing capability, and mining the Short-Chain Dehydrogenase/Reductase Engineering Database indicated that NR and Zt_SDR possess a unique active-site composition among SDRs. Adapting the active site of Ao_SDR accordingly improved its imine-reducing capability. By applying the same strategy, an unrelated SDR from Methylobacterium sp. 77 (M77_SDR) with distinct keto-reducing activity was engineered into a promiscuous enzyme with imine-reducing activity, thereby confirming that the ability to reduce imines can be rationally introduced into members of the “classical” SDR enzyme family. Thus, members of the SDR family could be a promising starting point for protein approaches to generate new imine-reducing enzymes. 相似文献
24.
For many-objective optimization problems, how to get a set of solutions with good convergence and diversity is a difficult and challenging work. In this paper, a new decomposition based evolutionary algorithm with uniform designs is proposed to achieve the goal. The proposed algorithm adopts the uniform design method to set the weight vectors which are uniformly distributed over the design space, and the size of the weight vectors neither increases nonlinearly with the number of objectives nor considers a formulaic setting. A crossover operator based on the uniform design method is constructed to enhance the search capacity of the proposed algorithm. Moreover, in order to improve the convergence performance of the algorithm, a sub-population strategy is used to optimize each sub-problem. Comparing with some efficient state-of-the-art algorithms, e.g., NSGAII-CE, MOEA/D and HypE, on six benchmark functions, the proposed algorithm is able to find a set of solutions with better diversity and convergence. 相似文献
25.
带关键字搜索的公钥加密(PEKS)是一种有用的加密原语,它允许用户将在加密数据上搜索的功能委托给不可信的第三方服务器,而不影响原始数据的安全性和隐私性。但是,由于缺乏对于数据的加密以及解密能力,PEKS方案不能单独进行使用,必须与标准的公钥加密方案(PKE)相结合。因此,Baek等人在2006年引入了一种新的加密原语,称为结合PKE和PEKS的加密方案(PKE+PEKS),它同时提供了PKE和PEKS的功能。目前,已有文献提出了几种PKE+PEKS方案。然而,他们都没有考虑关键字猜测攻击的问题。本文提出一个新的高效且能够抵抗关键字猜测攻击的PKE+PEKS方案,与已有方案相比,该方案在性能上有很大的提升,并且在生成关键字和数据密文时,不需要使用双线性对,极大地降低了计算和存储成本。安全性分析表明,本文中所提出的方案能够满足密文隐私安全性、陷门不可区分性和抗关键字猜测攻击的安全性。效率分析表明,本分提出的方案更加高效。 相似文献
26.
A conceptualization of intended learning outcomes supporting self‐regulated learners in indicating learning paths 下载免费PDF全文
P. Tangworakitthaworn L. Gilbert G.B. Wills 《Journal of Computer Assisted Learning》2015,31(5):393-404
Intended learning outcomes (ILOs) indicate what learners will be able to achieve after they are taught. Traditionally, ILOs are expressed as plain text or unstructured documents. What if all ILOs of a specific course of study can be conceptualized through a structured diagrammatic technique? It was hypothesized that learners can benefit from this conceptualization in learning, especially in self‐regulated learning. The aim of this study was to investigate whether the ILOs represented in unstructured or structured formats can facilitate learners to identify learning paths. The results revealed that the mean ratings of all learning paths were statistically significantly higher with structured ILOs. 相似文献
27.
Soheil Khosrogorji Sepehr Soori Hossein Torkaman 《International Journal of Circuit Theory and Applications》2019,47(10):1645-1663
In this paper, a new design procedure for LLC converter has been introduced. In fact, this method is a computer-based design algorithm based on a numerical technique. In the process of designing, the value of the resonant element is obtained by solving the LLC converter fundamental equation. This converter will be controlled by using state feedback, such as output voltage variable. As a matter of fact, in a control system, the change of output voltage (because of load variation) will affect the switching frequency, so the output voltage will be tuned. In the designing process, the fundamental equations of LLC converter are obtained, and the value of the resonant elements is calculated. Also, a comparison analysis is carried out between the proposed and typical methods. The simulation is done to investigate the validity of the proposed method. Moreover, a prototype is manufactured, and the experimental test is done to evaluate its applicability. 相似文献
28.
29.
针对高可靠度机载多余度EWIS各组成部分寿命服从指数分布但参数未知的情况,提出采用无失效数据可靠度分析方法评估EWIS的可靠度水平。通过Monte-Carlo仿真方法对连接形式为“先并联、后串联”EWIS各组成部分寿命进行抽样,利用“最小最大值”方法获得系统寿命的抽样值,用概率纸检验法初步判断EWIS寿命是否服从威布尔分布,再用Pearson拟合优度检验法判断EWIS寿命是否服从威布尔分布。结合无故障飞行时间的样本值与EWIS寿命服从威布尔分布的假设,采用无失效数据分析方法评估EWIS的可靠度水平。研究方法对机载多余度EWIS无失效数据可靠度分析有一定的贡献。 相似文献
30.
《Ceramics International》2022,48(5):6322-6337
To optimize the corrosion, bioactivity, and biocompatibility behaviors of plasma electrolytic oxidation (PEO) coatings on titanium substrates, the effects of five process variables including frequency, current density, duty cycle, treatment time, and electrolyte Ca/P ratio were evaluated. In our systematic study, a Taguchi design of experimental based on an L16 orthogonal array was used. For this, the coatings characteristics such as the surface roughness, wettability, rutile to anatase and Ca/P ratios, and corrosion polarization resistance were investigated. After determining the optimum process variables for each response, the apatite forming ability in SBF (bioactivity behavior) and MG63 cell attachment and flattening (biocompatibility behavior) for two groups of coatings were examined. The first group was optimized based on the maximum corrosion polarization resistance and the variables were set as the frequency of 2000 Hz, the current density of 5 A/dm2, the duty cycle of 30%, the treatment time of 5 min, and the Ca/P ratio of 0.65 at. % in the electrolyte. For the second group, the maximum surface roughness, greatest Ca/P ratio, and highest wettability as well as the minimum rutile to anatase ratio in coatings, could be obtained when the variables were set as the frequency of 10 Hz, the current density of 12.5 A/dm2, the duty cycle of 50%, the treatment time of 12.5 min, and the Ca/P ratio of 1.70 at. % in the electrolyte. The results showed that while both groups of coatings indicated a significant apatite forming ability and can serve as bioactive coatings, a proper attachment and flattening of cells and consequently, the favorable biocompatibility properties were seen only in the first group. 相似文献