全文获取类型
收费全文 | 1573866篇 |
免费 | 29171篇 |
国内免费 | 7788篇 |
专业分类
电工技术 | 35339篇 |
技术理论 | 1篇 |
综合类 | 7323篇 |
化学工业 | 274667篇 |
金属工艺 | 65899篇 |
机械仪表 | 45024篇 |
建筑科学 | 48338篇 |
矿业工程 | 11837篇 |
能源动力 | 50791篇 |
轻工业 | 121397篇 |
水利工程 | 16012篇 |
石油天然气 | 38345篇 |
武器工业 | 256篇 |
无线电 | 199264篇 |
一般工业技术 | 293865篇 |
冶金工业 | 194583篇 |
原子能技术 | 34361篇 |
自动化技术 | 173523篇 |
出版年
2021年 | 16458篇 |
2020年 | 12578篇 |
2019年 | 15141篇 |
2018年 | 17642篇 |
2017年 | 17060篇 |
2016年 | 22556篇 |
2015年 | 18193篇 |
2014年 | 29628篇 |
2013年 | 88992篇 |
2012年 | 38112篇 |
2011年 | 51764篇 |
2010年 | 44867篇 |
2009年 | 52705篇 |
2008年 | 47743篇 |
2007年 | 45009篇 |
2006年 | 46549篇 |
2005年 | 41395篇 |
2004年 | 43118篇 |
2003年 | 42790篇 |
2002年 | 41622篇 |
2001年 | 38759篇 |
2000年 | 36917篇 |
1999年 | 36557篇 |
1998年 | 55126篇 |
1997年 | 45376篇 |
1996年 | 39204篇 |
1995年 | 32975篇 |
1994年 | 30582篇 |
1993年 | 30522篇 |
1992年 | 26718篇 |
1991年 | 23797篇 |
1990年 | 24097篇 |
1989年 | 23176篇 |
1988年 | 21664篇 |
1987年 | 19930篇 |
1986年 | 19241篇 |
1985年 | 22561篇 |
1984年 | 22513篇 |
1983年 | 20461篇 |
1982年 | 19448篇 |
1981年 | 19622篇 |
1980年 | 18210篇 |
1979年 | 18625篇 |
1978年 | 17913篇 |
1977年 | 18091篇 |
1976年 | 19733篇 |
1975年 | 16077篇 |
1974年 | 15543篇 |
1973年 | 15659篇 |
1972年 | 13122篇 |
排序方式: 共有10000条查询结果,搜索用时 44 毫秒
841.
Y.K. Yang F.L. Liu Y.W. Zhang M.F. Li F. Ling H.T. Wu 《Ceramics International》2018,44(11):12238-12244
In this work, ultra-low loss Li2MgTi0.7(Mg1/3Nb2/3)0.3O4 ceramics were successfully prepared via the conventional solid-state method. X-ray photoelectron spectroscopy (XPS), thermally stimulated depolarization current (TSDC) and bond energy were used to determine the distinction between intrinsic and extrinsic dielectric loss in (Mg1/3Nb2/3)4+ ions substituted ceramics. The addition of (Mg1/3Nb2/3)4+ ions enhances the bond energy in unit cell without changing the crystal structure of Li2MgTiO4, which results in high Q·f value as an intrinsic factor. The extrinsic factors such as porosity and grain size influence the dielectric loss at lower sintering temperature, while the oxygen vacancies play dominant role when the ceramics densified at 1400?°C. The Li2MgTi0.7(Mg1/3Nb2/3)0.3O4 ceramics sintered at 1400?°C can achieve an excellent combination of microwave dielectric properties: εr =?16.19, Q·f?=?160,000?GHz and τf =??3.14?ppm/°C. In addition, a certain amount of LiF can effectively lower the sintering temperature of the matrix, and the Li2MgTi0.7(Mg1/3Nb2/3)0.3O4-3?wt% LiF ceramics sintered at 1100?°C possess balanced properties with εr?=?16.32, Q·f?=?145,384?GHz and τf =??16.33?ppm/°C. 相似文献
842.
Radiochemistry - Spent reactor graphite constitutes a significant part of the accumulated solid radioactive wastes, and a search for its isolation from the biosphere is a topical task. Radioactive... 相似文献
843.
844.
Francesco Di Girolamo Andrea Masotti Isabella Lante Margherita Scapaticci Cosima Damiana Calvano Carlo Zambonin Maurizio Muraca Lorenza Putignani 《International journal of molecular sciences》2015,16(9):20896-20912
Extra virgin olive oil (EVOO) with its nutraceutical characteristics substantially contributes as a major nutrient to the health benefit of the Mediterranean diet. Unfortunately, the adulteration of EVOO with less expensive oils (e.g., peanut and corn oils), has become one of the biggest source of agricultural fraud in the European Union, with important health implications for consumers, mainly due to the introduction of seed oil-derived allergens causing, especially in children, severe food allergy phenomena. In this regard, revealing adulterations of EVOO is of fundamental importance for health care and prevention reasons, especially in children. To this aim, effective analytical methods to assess EVOO purity are necessary. Here, we propose a simple, rapid, robust and very sensitive method for non-specialized mass spectrometric laboratory, based on the matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS) coupled to unsupervised hierarchical clustering (UHC), principal component (PCA) and Pearson’s correlation analyses, to reveal corn oil (CO) adulterations in EVOO at very low levels (down to 0.5%). 相似文献
845.
Biological information is stored in DNA, RNA and protein sequences, which can be understood as genotypes that are translated into phenotypes. The properties of genotype–phenotype (GP) maps have been studied in great detail for RNA secondary structure. These include a highly biased distribution of genotypes per phenotype, negative correlation of genotypic robustness and evolvability, positive correlation of phenotypic robustness and evolvability, shape-space covering, and a roughly logarithmic scaling of phenotypic robustness with phenotypic frequency. More recently similar properties have been discovered in other GP maps, suggesting that they may be fundamental to biological GP maps, in general, rather than specific to the RNA secondary structure map. Here we propose that the above properties arise from the fundamental organization of biological information into ‘constrained'' and ‘unconstrained'' sequences, in the broadest possible sense. As ‘constrained'' we describe sequences that affect the phenotype more immediately, and are therefore more sensitive to mutations, such as, e.g. protein-coding DNA or the stems in RNA secondary structure. ‘Unconstrained'' sequences, on the other hand, can mutate more freely without affecting the phenotype, such as, e.g. intronic or intergenic DNA or the loops in RNA secondary structure. To test our hypothesis we consider a highly simplified GP map that has genotypes with ‘coding'' and ‘non-coding'' parts. We term this the Fibonacci GP map, as it is equivalent to the Fibonacci code in information theory. Despite its simplicity the Fibonacci GP map exhibits all the above properties of much more complex and biologically realistic GP maps. These properties are therefore likely to be fundamental to many biological GP maps. 相似文献
846.
847.
848.
849.
The dynamic interaction between the mechanical and electrical drive parameters when damage suddenly appears in the mechanical part of a conveyer belt is investigated. The possibility of using the change in the motor’s electrical parameters as a diagnostic signal is assessed. A model for investigating the dynamic processes in the system consisting of the grid, the induction motor, and the conveyer belt is proposed. The grid and induction motor are described by a model of fourth-order state space, whose output is the torque at the motor shaft. The moments of inertia and pliability of the links between the elements in the mechanical part of the conveyer belt are determined. By gradually identifying the elements with the minimum moment of inertia and distributing their moments of inertia and pliabilities among the adjacent elements, we obtain a three-mass system simulating the mechanical part of the conveyer belt. This dynamic model is solved by means of Matlab Simulink software. The energy characteristics of the drive are determined in dynamic processes following a mechanical accident. 相似文献
850.
通过对粉煤灰砖抗折强度测量结果不确定度的评定,分析出影响试验结果的各种因素,为提高检测水平和精确度提供指导。 相似文献