全文获取类型
收费全文 | 5162篇 |
免费 | 806篇 |
国内免费 | 699篇 |
专业分类
化学 | 1976篇 |
晶体学 | 67篇 |
力学 | 1098篇 |
综合类 | 118篇 |
数学 | 1589篇 |
物理学 | 1819篇 |
出版年
2024年 | 13篇 |
2023年 | 58篇 |
2022年 | 206篇 |
2021年 | 212篇 |
2020年 | 214篇 |
2019年 | 186篇 |
2018年 | 159篇 |
2017年 | 220篇 |
2016年 | 206篇 |
2015年 | 190篇 |
2014年 | 297篇 |
2013年 | 443篇 |
2012年 | 271篇 |
2011年 | 327篇 |
2010年 | 253篇 |
2009年 | 321篇 |
2008年 | 267篇 |
2007年 | 269篇 |
2006年 | 285篇 |
2005年 | 251篇 |
2004年 | 240篇 |
2003年 | 265篇 |
2002年 | 216篇 |
2001年 | 148篇 |
2000年 | 156篇 |
1999年 | 170篇 |
1998年 | 137篇 |
1997年 | 122篇 |
1996年 | 93篇 |
1995年 | 75篇 |
1994年 | 69篇 |
1993年 | 45篇 |
1992年 | 52篇 |
1991年 | 43篇 |
1990年 | 38篇 |
1989年 | 27篇 |
1988年 | 30篇 |
1987年 | 27篇 |
1986年 | 13篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1983年 | 9篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1979年 | 10篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有6667条查询结果,搜索用时 15 毫秒
51.
WADOOD Y. HAMAD 《Cellulose (London, England)》1997,4(1):51-56
Wood-pulp fibres are recognized as concentrically-layered, laminated composite tubes of structural reinforcing material, the
cellulose microfibrils, embedded in a cementing matrix of hemicellulose and lignin. When the single fibres are subjected to
cyclic mechanical action, their morphological behaviour is characterized by the fatigue growth of micro-voids and surface
damage which individually and collectively give rise to stress concentrations, and eventually crack development. The progressive
damage phenomena, evinced by the surface imaging and optical sectioning techniques through utilizing confocal laser scanning
microscopy, are understood to be consequences of the cumulative material micromechanical degradation and subsequent microstructural
breakdown of the cellulose microfibrillar framework. This structural breakdown is believed to effect the fibrillation and
flexibilization of the fibres
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
52.
The surface wave produced plasma belongs to a class of RF and microwave induced plasmas. It results from the propagation of an electromagnetic wave which uses the plasma column it sustains and the plasma tube as its sole propagating media. This type of plasma offers several advantages compared to the positive column plasma of dc discharges or to other RF and microwave produced plasmas. Surface wave plasmas require no internal electrodes, and they can be applied over an extremely broad range of wave frequencies (27 MHz to 10 GHz demonstrated) and gas pressures (about 10–4 Torr to a few times the atmospheric pressures). Using the surface wave plasma technique, a large variety of plasma column diameters have been created (0.5–150 mm demonstrated) and no limitation on plasma column length (column up to 6 m long demonstrated) has been found. The surface wave produced plasma is used in elemental analysis and to sustain emission in lasing media. This article is intended as a guide for potential users of surface wave plasmas in the field of plasma processing and plasma chemistry. 相似文献
53.
Huwe A Mazitschek R Giannis A 《Angewandte Chemie (International ed. in English)》2003,42(19):2122-2138
Cell division (mitosis) is one of the basic requirements for multicellular oranisms. The capability of a cell to replicate enables a complex assembly to be created. Faulty regulation of the control mechanism in the cell cycle leads to an excessive cell proliferation and is the cause of cancer. The key position of the cyclin-dependent kinases (CDKs) and their direct partners, as well as the fact that the majority of malign illnesses show defects in at least one of these key players of the cell cycle, is of great interest for the development of low-molecular-weight CDK inhibitors. In this Review an overview of the different structural classes of ATP-competitive inhibitors of CDKs are given, whose devlopment was aimed at battling cancer. The Review shows how far the development of selective CDK inhibitors has progressed and to what extent the expectations for such drugs have so far been fulfilled. 相似文献
54.
对已经商品化的Perkin-Elmer TMS-2型热机分析仪进行改进,用来测量聚合物材料的热释蠕变行为。按照7SCr分步加载的程序,用弯曲法测量了-83~300℃温度范围内低密度聚乙烯的玻璃化转变松弛过程,并从TSCr谱上计算其相应的分子参数,实验结果与用扭转的TSCr方法所得结果相等,因此这是一种研究聚合物材料松弛行为的方便可行的技术。 相似文献
55.
B. E. Conway 《Journal of solution chemistry》1977,6(1):23-31
A volcano-shaped relation between heats of solution of a series of salts and the difference of hydration energy of the ions of the salt was noted by Fajans. This relation has a maximum when the difference of hydration energy of the cation and anion approaches zero. A rationalization of this type of relation is proposed on the basis of sums of reciprocal radii terms which represent the dependence of electrostatic hydration energy on ion size. The general trend of heats of solution with ionic size in series of salts with either a common cation or a common anion is discussed. 相似文献
56.
J.A. ColladoM.A.G. Aranda A. CabezaP. Olivera-Pastor S. Bruque 《Journal of solid state chemistry》2002,167(1):80-85
The La2W2−xMoxO9 series has been synthesized by the ceramic method. An alternative synthesis using microwave radiation is also reported. La2W2O9 has two polymorphs and the low-temperature phase (α) transforms to the high-temperature form (β) at 1077°C. The influence of the W/Mo substitution in this phase transition has been investigated by DTA. The β structure for x≥0.7 compositions can be prepared as single phase at any cooling rate. The β phase for 0.3≤x≤0.7 compounds can be prepared as single phase by quenching, whereas a mixture of α and β phases is obtained by slow cooling. The W/Mo ratio in both coexisting phases is different with the β-phase having a higher Mo content. The x=0.1 and 0.2 compounds have been prepared as mixtures of phases. The room temperature structure of β-La2W1.7Mo0.3O9 has been analyzed by the Rietveld method in P213 space group. The final R-factors were RWP=9.0% and RF=5.6% with a structure similar to that of β-La2Mo2O9. Finally, the thermal expansion of both types of structures has been determined from a thermodiffractometric study. The thermal expansion coefficients were 2.9×10−6 and 9.7×10−6°C−1 for α-La2W2O9 and β-La2W1.2Mo0.8O9, respectively. 相似文献
57.
This article introduces the relationship between chemistry and fish farming, fish selling, and fish food. The nitrogen cycle process in fish ponds is introduced to explain the fish farming. "Yufuling" is used as the entry point to fish selling to connect food safety with chemistry. Fishy smell is explained from a chemical perspective for fish food. This paper aims at making a scientific explanation of life phenomena, deepening the consciousness of chemical life, helping people establish a correct view of chemical science, and inspiring youth to learn to use the knowledge that they have learned to explain life phenomena. This paper also tries to show the youth that life cannot be separated from chemistry, and chemistry cannot be separated from life. 相似文献
58.
一种低磁矩、大功率复合稀土石榴石铁氧体的工艺和性能研究 总被引:4,自引:0,他引:4
为满足低场区几百兆赫的超大功率器件的性能要求,以钇铁石榴石铁氧体为基础,制备了一种低磁矩复合稀土石榴石铁氧体YGd CaVInIG,研究了少量Mn3+替代Fe3+和预烧温度、烧结温度对铁氧体性能和结构的影响。实验表明,以少量Mn3+替代Fe3+可以提高铁氧体电阻率,降低磁损耗和介电损耗。Mn3+掺入量以x为0.04~0.06比较合适,铁氧体最佳予烧温度为1050℃,最佳烧结温度1350~1380℃,保温5h,氧气中烧结,其性能为:4πMs=500±10%kA·m-1,ΔH=5.25~5.55kA·m-1,TC>180℃,tgδe≤0.5×10-5,这种材料适合微波低频段器件性能要求。X射线衍射分析指出,掺Mn3+的YGdCaVInIG铁氧体相变完全,呈单相复合石榴石铁氧体,空间群为I230,点阵常数1.25057~1.25101nm,单胞分子数n为7.75~7.80。 相似文献
59.
M. Costa-Ferreira A. Dias C. Maximo M. J. Morgado G. Sena-Martins J. Cardoso Duarte 《Applied biochemistry and biotechnology》1994,44(3):231-242
Production of xylanolytic enzymes by anAspergillus niger CCMI 850 isolate was investigated in batch cultures. The effect of the composition of a fermentation medium that did not
include chemical inducers, on β-xylanase, β-xylosidase, α-l-arabinofuranosidase, and total cellulase activity was studied. With 4% xylan as the carbon source, about 65 U/mL of β-xylanase
was obtained, whereas the total cellulase activity was undetectable, under the specified conditions. This β-xylanase activity
represents the highest reported for a wild-type strain ofA. niger. The effect of pH and temperature on the activity of β-xylanase was studied. Partial characterization of the β-xylanase showed
that with insoluble birchwood as substrate theK
m
andV
max were 0.3 mM and 19 μmol/min, respectively. Aspects of using the crude β-xylanase preparation for applications in the pulp and paper industry
were discussed. 相似文献
60.
本文提出了一个硫酸中间盐子循环用以代替分解水制氢的KNO_3-I_2混合循环中的电解反应,从而使原混合循环变为一个纯热化学循环。 相似文献