全文获取类型
收费全文 | 50482篇 |
免费 | 5056篇 |
国内免费 | 2859篇 |
专业分类
电工技术 | 2914篇 |
技术理论 | 4篇 |
综合类 | 3616篇 |
化学工业 | 8241篇 |
金属工艺 | 2850篇 |
机械仪表 | 3169篇 |
建筑科学 | 3855篇 |
矿业工程 | 1663篇 |
能源动力 | 1573篇 |
轻工业 | 4242篇 |
水利工程 | 1167篇 |
石油天然气 | 2639篇 |
武器工业 | 427篇 |
无线电 | 5954篇 |
一般工业技术 | 6127篇 |
冶金工业 | 2335篇 |
原子能技术 | 607篇 |
自动化技术 | 7014篇 |
出版年
2024年 | 249篇 |
2023年 | 851篇 |
2022年 | 1520篇 |
2021年 | 2116篇 |
2020年 | 1650篇 |
2019年 | 1392篇 |
2018年 | 1637篇 |
2017年 | 1747篇 |
2016年 | 1651篇 |
2015年 | 2043篇 |
2014年 | 2587篇 |
2013年 | 3291篇 |
2012年 | 3493篇 |
2011年 | 3582篇 |
2010年 | 3351篇 |
2009年 | 3166篇 |
2008年 | 3087篇 |
2007年 | 2987篇 |
2006年 | 2761篇 |
2005年 | 2315篇 |
2004年 | 1749篇 |
2003年 | 1650篇 |
2002年 | 1763篇 |
2001年 | 1542篇 |
2000年 | 1093篇 |
1999年 | 986篇 |
1998年 | 735篇 |
1997年 | 629篇 |
1996年 | 556篇 |
1995年 | 466篇 |
1994年 | 405篇 |
1993年 | 298篇 |
1992年 | 249篇 |
1991年 | 169篇 |
1990年 | 99篇 |
1989年 | 112篇 |
1988年 | 89篇 |
1987年 | 67篇 |
1986年 | 45篇 |
1985年 | 32篇 |
1984年 | 25篇 |
1983年 | 20篇 |
1982年 | 19篇 |
1981年 | 14篇 |
1980年 | 12篇 |
1979年 | 16篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1973年 | 6篇 |
1959年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
甲基丙烯酸乙酯合成工艺研究 总被引:1,自引:1,他引:0
以大孔强酸性离子交换树脂为催化剂,甲基丙烯酸(MAA)与乙醇直接酯化合成甲基丙烯酸乙酯(EMA)。在MAA与乙醇摩尔比为1:3、反应温度89~93℃、空速0.3h~(-1)的条件下,MAA转化率不低于98%,EMA的选择性不低于99%。 相似文献
102.
In this paper, the influence of viewing distance on subjective assessment of the impairment in video sequences is investigated. Subjective tests using the double-stimulus impairment scale variant II (DSIS II) method have been conducted at viewing distances of 5H (where H stands for the screen height) and 3H, respectively. Several statistical measures have been used to analyze the influence, including correlations and ANOVA (analysis of variance) tests. The results reveal that there is a very high correlation between the subjective scores, the variances are similar under the two viewing distances, the means of subjective data at these two viewing distances are the same, and there is no interaction between the viewing distance and the other two factors, i.e., the codec system and the source sequence. Throughout the tests, there is no evidence that a closer viewing distance such as 3H will vary the subjective test result statistically significantly. 相似文献
103.
本文分析了电信企业面临的社会经济新形势,揭示了在计划经济条件下电信企业内部激励机制存在的弊端,论述了电信企业进行激励机制改革与创新的必要性和必然性,提出了电信企业在生存和发展中改革与创新激励机制的指导思想及原则,提出了如何建立一套科学、合理的激励机制,以便更有效地发挥员工的积极性、主动性和创造性,最大程度地提高激励的效用。 相似文献
104.
本文采用粉末冶金法制备了不同成分的PSZ/Mo复合材料。并对其密度、弹性模量和热膨胀系数进行了测量和分析 ,用XRD进行了物相分析。实验结果表明 :纳米氧化锆的烧结性能比金属钼好 ;弹性模量估算选用简单混合法则进行计算时应进行修正 ;钼和氧化锆在烧结时不发生化学反应。通过热压PSZ/Mo功能梯度材料的断口扫描分析发现 :虽然金属钼和纳米氧化锆具有一定增韧作用 ,但材料断裂方式仍以脆性断裂为主 相似文献
105.
A hybrid finite element method/method of moments (FEM/MoM) technique is used to analyze a printed circuit board power bus structure where the source and observation points are in the near field. The FEM is used to model the lossy region between the planes of the board including the source. The MoM is used to model the region outside the planes and provide a radiation boundary condition to terminate the FEM mesh. Numerical results for a bridged power bus structure are compared with measurements. A nonphysical interior resonance of the electric field integral equation is observed. The problem can be avoided by using a hybrid technique based on a combined field integral equation 相似文献
106.
卞韵 《常州信息职业技术学院学报》2005,4(4):95-96
以票据为主体的结算制度,是我国结算改革的重大突破,也是结算改革取得显著成绩的表现。根据票据的特征,对票据使用在我国的可能性和必要性进行分析,并结合我国结算改革中存在的一些问题,提出几点设想。 相似文献
107.
In the literature the concept of representative volume element (RVE) was introduced to correlate the effective or macroscopic properties of materials with the properties of the microscopic constituents and microscopic structures of the materials. However, to date little quantitative knowledge is available about minimum RVE sizes of various engineering materials. In our recent paper [J. Mech. Phys. Solids 50 (2002) 881], a new definition of minimum RVE size was introduced based on the concept of nominal modulus. Numerical experiments using the finite element method (FEM) were then carried out for determining the minimum RVE sizes of more than 500 cubic polycrystals in the plane stress problem, under the assumption that all grains in a polycrystal have the same square shape––called the simple polycrystal model. The major finding is that the minimum RVE sizes for effective elastic moduli have a roughly linear dependence on crystal anisotropy degrees. The present paper takes into account the effect of grain sizes, shapes, and distribution on the minimum RVE sizes for real cubic polycrystals that are formed by crystallization processes. Similar roughly linear dependence is found again, with the slope about 19% lower than that in the simple polycrystal model. This finding is interesting and useful because numerical experiments on minimum RVE sizes for a large number of crystals are quite time-consuming and the simple polycrystal model reduces significantly the FEM pre- and post-processing works. This should be particularly true in numerically testing minimum RVE sizes for three-dimensional polycrystals and for nonelastic properties in future works. With a maximum relative error 5%, all the polycrystals tested have a minimum RVE size of 16 or less times the grain size. 相似文献
108.
109.
A practical resource management method that can significantly reduce cochannel interference (CCI) and improve spectrum utilisation in FH-OFDMA packet-based cellular networks is presented. The proposed method seeks an effective combination of dynamic resource allocation with fractional coding and bit loading to respectively minimise CCI and maximise system throughput for a desired performance. 相似文献
110.
Ho Nam Kwon Man Geun Kim Il-Han Hwang Young Kab Yun Seon-Ju Kim Jong-Hyun Lee 《Photonics Technology Letters, IEEE》2005,17(4):774-776
We developed a micromachined X-type 2/spl times/4 optical add-drop module (OADM) featuring no difference in propagation length. Four pairs of lensed fibers are aligned in "X" position, and four micromirrors are located between the pairs of optical fibers. The OADM was fabricated utilizing a silicon-on-insulator process. Electrostatic comb actuators can be driven up to 90 /spl mu/m to change the light path within 1 ms. The insertion loss and the on-off ratio were less than 3 and 70 dB, respectively. The loss uniformity in every channel was 1.5 dB. 相似文献