全文获取类型
收费全文 | 8242篇 |
免费 | 875篇 |
国内免费 | 2075篇 |
专业分类
化学 | 5957篇 |
晶体学 | 266篇 |
力学 | 2081篇 |
综合类 | 48篇 |
数学 | 332篇 |
物理学 | 2508篇 |
出版年
2024年 | 36篇 |
2023年 | 152篇 |
2022年 | 335篇 |
2021年 | 339篇 |
2020年 | 353篇 |
2019年 | 238篇 |
2018年 | 188篇 |
2017年 | 323篇 |
2016年 | 435篇 |
2015年 | 368篇 |
2014年 | 453篇 |
2013年 | 551篇 |
2012年 | 521篇 |
2011年 | 589篇 |
2010年 | 556篇 |
2009年 | 591篇 |
2008年 | 600篇 |
2007年 | 646篇 |
2006年 | 531篇 |
2005年 | 499篇 |
2004年 | 474篇 |
2003年 | 406篇 |
2002年 | 308篇 |
2001年 | 262篇 |
2000年 | 214篇 |
1999年 | 197篇 |
1998年 | 150篇 |
1997年 | 154篇 |
1996年 | 132篇 |
1995年 | 87篇 |
1994年 | 90篇 |
1993年 | 57篇 |
1992年 | 67篇 |
1991年 | 50篇 |
1990年 | 37篇 |
1989年 | 25篇 |
1988年 | 34篇 |
1987年 | 27篇 |
1986年 | 31篇 |
1985年 | 11篇 |
1984年 | 10篇 |
1983年 | 6篇 |
1982年 | 14篇 |
1981年 | 14篇 |
1980年 | 5篇 |
1979年 | 12篇 |
1978年 | 3篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1957年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
聚丙烯微孔膜的等离子体接枝聚合改性 总被引:2,自引:1,他引:1
利用拉伸时的晶型转变致孔特性,从β晶相聚丙烯经单向或双向拉伸制得了新型聚丙烯微孔膜、用等离子体接枝聚合技术接上了亲水性聚合物如聚丙烯酸、聚丙烯腈等。研究了等离子体处理时间、单体性质对得到的亲水膜的亲水性、离子/溶质渗透性的影响。发现接枝聚丙烯酸的双向拉伸膜对Na~+、Mg~(++),尿素有较大的渗透性,并具有优良的形状稳定性。 相似文献
52.
NongYueHE JianXinTANG SongLI HongCHEN AnCunZHOU 《中国化学快报》2005,16(3):397-400
Polypropylene microporous membranes were treated with plasma in a mixture of N2 and H2 (1:2 in volume). Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and ultra-violet (UV) spectra demonstrated the success of grafting amino groups. The density of the polar amino groups on the membrane surface is about 0.59 μmol/cm^2. The as-treated membranes were successively applied to the in situ synthesis of oligonucleotides and an average coupling yield was more than 98%. The surface feature of the treated membrane is suggested to be responsible for its advantage over a glass slide. 相似文献
53.
54.
The nanogranular nature of C-S-H 总被引:6,自引:0,他引:6
Despite its ubiquitous presence as binding phase in all cementitious materials, the mechanical behavior of calcium-silicate-hydrates (C-S-H) is still an enigma that has deceived many decoding attempts from experimental and theoretical sides. In this paper, we propose and validate a new technique and experimental protocol to rationally assess the nanomechanical behavior of C-S-H based on a statistical analysis of hundreds of nanoindentation tests. By means of this grid indentation technique we identify in situ two structurally distinct but compositionally similar C-S-H phases heretofore hypothesized to exist as low density (LD) C-S-H and high density (HD) C-S-H, or outer and inner products. The main finding of this paper is that both phases exhibit a unique nanogranular behavior which is driven by particle-to-particle contact forces rather than by mineral properties. We argue that this nanomechanical blueprint of material invariant behavior of C-S-H is a consequence of the hydration reactions during which precipitating C-S-H nanoparticles percolate generating contact surfaces. As hydration proceeds, these nanoparticles pack closer to center on-average around two characteristic limit packing densities, the random packing limit (η=64%) and the ordered face-centered cubic (fcc) or hexagonal close-packed (hcp) packing limit (η=74%), forming a characteristic LD C-S-H and HD C-S-H phase. 相似文献
55.
Kwangwon Park 《Talanta》2007,73(4):791-794
It is very challenging to decompose a plastic product for the purpose of analysis of hazardous elements contained. To circumvent such technical problem, it is imperative that an analyst employ a nondestructive analytical method free of any pretreatments. The analytical results of the concentrations of toxic metals such as Cd and Cr in polypropylene for seven samples at two different levels were obtained using the instrumental neutron activation analysis. This work was intended ultimately to establish certified reference materials (CRMs) of these metals in the polypropylene, traceable to the SI. The uncertainties associated with the analytical procedures were estimated in accordance with the ISO guideline. The results were subsequently validated by a comparison with those for CRM-680 and −681 of the Bureau Communautaire de Reference (BCR), which demonstrated acceptable agreement within their uncertainty ranges. 相似文献
56.
57.
A routine method for the determination of chloride, nitrate and sulfate anions in rainwater by capillary electrophoresis was developed. The system uses an end-column non-suppressed conductivity detector. Linear calibration plots were generated from 0.050 to 20 mg/l, which is the range generally found in wet depositions. Accuracy and precision were evaluated by analyzing certified standards of simulated rainwater and environmental samples, or by comparing CE results with those obtained by IC, the reference technique for anion analysis in wet deposition. The reproducibility of the method was satisfactory except at the lower and upper limits of the analytical range. Sensitivity lay in the range of few μg/l. 相似文献
58.
59.
炭黑填充聚乙烯材料电阻—温度特性研究 总被引:8,自引:1,他引:8
研究了炭黑/聚乙烯导电复合材料的PTC特性及在不同条件下的电阻变化。发现PTC特性与体积膨胀及聚乙烯晶相的熔融有许多一致性。认为材料的体积膨胀及聚乙烯晶相熔融时炭黑颗粒均匀化扩散导致了电阻随温度上升。在较高温度下,炭黑颗粒在分子链段热运动的推动下会发生相对聚集使电阻不断减小,这是材料出现NTC现象的原因。材料总的电阻温度特性是体积膨胀、炭黑向聚乙烯熔融区扩散及相互聚集三个因素共同作用的结果。 相似文献
60.
一水草酸铵(简称AOM)是一种优良的电光晶体 ̄[1]。文献 ̄[1,2]报道了该晶体的晶体结构,化学式为(NH_4)_2C_2O_2·H_2O,属正交晶系,P222空间群,晶胞参数a=0.8035nm,b=1。031nm,c=0.3801nm,z=2。本文拟报道AOM单晶生长,热化学分析,透过波段以及非线性光学性质的初步研究。 相似文献