全文获取类型
收费全文 | 37696篇 |
免费 | 2816篇 |
国内免费 | 2326篇 |
专业分类
化学 | 17804篇 |
晶体学 | 155篇 |
力学 | 1291篇 |
综合类 | 829篇 |
数学 | 12547篇 |
物理学 | 10212篇 |
出版年
2023年 | 292篇 |
2022年 | 436篇 |
2021年 | 1314篇 |
2020年 | 866篇 |
2019年 | 982篇 |
2018年 | 724篇 |
2017年 | 807篇 |
2016年 | 1065篇 |
2015年 | 1009篇 |
2014年 | 1426篇 |
2013年 | 2639篇 |
2012年 | 1575篇 |
2011年 | 1911篇 |
2010年 | 1801篇 |
2009年 | 2254篇 |
2008年 | 2362篇 |
2007年 | 2561篇 |
2006年 | 1977篇 |
2005年 | 1359篇 |
2004年 | 1232篇 |
2003年 | 1261篇 |
2002年 | 1242篇 |
2001年 | 1172篇 |
2000年 | 856篇 |
1999年 | 685篇 |
1998年 | 673篇 |
1997年 | 495篇 |
1996年 | 566篇 |
1995年 | 493篇 |
1994年 | 475篇 |
1993年 | 513篇 |
1992年 | 498篇 |
1991年 | 352篇 |
1990年 | 267篇 |
1989年 | 231篇 |
1988年 | 252篇 |
1987年 | 205篇 |
1986年 | 226篇 |
1985年 | 335篇 |
1984年 | 250篇 |
1983年 | 152篇 |
1982年 | 300篇 |
1981年 | 481篇 |
1980年 | 436篇 |
1979年 | 479篇 |
1978年 | 379篇 |
1977年 | 292篇 |
1976年 | 244篇 |
1974年 | 79篇 |
1973年 | 156篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
22.
弛豫铁电单晶Pb(In1/2Nb1/2)O3-PbTiO3(PIN-PT)相较于常用的Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT)具有更高的居里温度,在高稳定性、高性能的传感器、换能器方面具有应用前景。本工作采用谐振法研究了[001]方向极化的0.66PIN-0.34PT铁电单晶的全矩阵机电性能参数。0.66PIN-0.34PT 单晶的三方-四方相变温度(TRT)约为160 ℃,居里温度(TC)约为260 ℃,室温压电系数d33、d31、d15分别为1 340 pC/N、-780 pC/N、321 pC/N,介电常数εT33、εS33、εT11、εS11分别为2 700、905、2 210、1 927,机电耦合系数 k33、k31、k15、kt分别为 87%、58%、38%、61%。其纵向压电常数(d33)和纵向机电耦合系数(k33)小于 PMN-PT 单晶,但是横向压电性能(d31)和剪切压电性能(d15)都略高于PMN-PT单晶。另外,研究了机电耦合性能随温度的变化趋势,发现0.66PIN-0.34PT单晶在150 ℃以下有较好的温度稳定性。 相似文献
23.
Leandro Augusto Gouvêa De Godoi Carla Eloísa Diniz Dos Santos Eugenio Foresti Marcelo Zaiat Márcia Helena Rissato Zamariolli Damianovic 《International journal of environmental analytical chemistry》2019,99(9):809-823
Recent developments in wastewater treatment have led to a renewed interest to obtain elemental sulphur (S°) as a by-product from bioreactors. However, practical studies are limited by the gap of adequate analytical techniques for its determination. This paper provides a statistical study and matrix effect evaluation of an adapted spectrophotometric method for routine S° analyses in aqueous samples, based on a methodology previously described by Hart (1961). Four complex matrices were tested: domestic sewage and effluent samples from three different bioreactors. Tested performance criteria included linearity, matrix effect, limit of detection and quantification and S° recovery. Results were linear (R2 = 0.99994) in the studied range (5 to 100 mg S° L?1) and no matrix effect was observed. The accuracy was based on recovery values that varied from 100% to 106%. The colloidal S° separation and extraction protocol was also considered suitable for aqueous samples, reaching more than 99.0% of S° recovery. 相似文献
24.
The radius of spatial analyticity for solutions of the KdV equation is studied. It is shown that the analyticity radius does not decay faster than as time t goes to infinity. This improves the works of Selberg and da Silva (2017) [30] and Tesfahun (2017) [34]. Our strategy mainly relies on a higher order almost conservation law in Gevrey spaces, which is inspired by the I-method. 相似文献
25.
Maurice Dörr Sebastian Lips Prof. Dr. Carlos Alberto Martínez-Huitle Dr. Dieter Schollmeyer Prof. Dr. Robert Franke Prof. Dr. Siegfried R. Waldvogel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(33):7835-7838
We report an innovative, sustainable and straightforward protocol for the synthesis of N,N-diarylamides equipped with nonprotected hydroxyl groups by using electrosynthesis. The concept allows the application of various substrates furnishing diarylamides with yields up to 57 % within a single and direct electrolytic protocol. The method is thereby easy to conduct in an undivided cell with constant current conditions offering a versatile and short-cut alternative to conventional pathways. 相似文献
26.
Kinetics and mechanism of nitration of aromatic compounds using trichloroisocyanuric acid (TCCA)/NaNO2, TCCA-N,N-dimethyl formamide (TCCA-DMF)/NaNO2, and TCCA-N,N-dimethyl acetamide (TCCA-DMA)/NaNO2 under acid-free and Vilsmeier-Haack conditions. Reactions followed second-order kinetics with a first-order dependence on [Phenol] and [Nitrating agent] ([TCCA], [(TCCA-DMF)], or [(TCCA-DMA)] >> [NaNO2]). Reaction rates accelerated with the introduction of electron-donating groups and retarded with electron-withdrawing groups, but did not fit well into the Hammett's theory of linear free energy relationship or its modified forms like Brown-Okamoto or Yukawa-Tsuno equations. Rate data were analyzed by Charton's multiple linear regression analysis. Isokinetic temperature (β) values, obtained from Exner's theory for different protocols, are 403.7 K (TCCA-NaNO2), 365.8 K (TCCA-DMF)/NaNO2, and 358 K (TCCA-DMA)/NaNO2. These values are far above the experimental temperature range (303-323 K), indicating that the enthalpy factors are probably more important in controlling the reaction. 相似文献
27.
28.
This paper experimentally studies the natural convection heat transfer characteristics on horizontal and vertical plates at pressures lower than atmospheric value. Eight different pressures ranging from 10Pa to 101.325kPa, and three heating loads were used in the experiments. It is found that natural convection heat transfer characteristics at reduced pressures are independent of the plate arrangements and present different behaviors in two pressure ranges, 10Pa < p < 1000Pa and p > 1000Pa. The experimental heat transfer results are correlated into a non-dimensional equation to help the thermal design of stratosphere aircrafts and airborne equipment. 相似文献
29.
Rasmus Havelund Martin P. Seah Ian S. Gilmore 《Surface and interface analysis : SIA》2019,51(13):1332-1341
Four simple methods are evaluated to determine their accuracies for establishing the interface location in secondary ion mass spectrometry intensity depth profiles of organic layers where matrix effects have not been measured. Accurate location requires the separate measurement of each ion's matrix factor. This is often not possible, and so estimates using matrix-less methods are required. Six pure organic material interfaces are measured using many secondary ions to compare their locations from the four methods with those from full evaluation with matrix terms. For different secondary ions, matrix effects cause the apparent interface positions to vary over 20 nm. The shifts in the intensity profiles on going from a layer of P into a layer of Q are in the opposite direction to that for going from Q into P, so doubling layer thickness errors. The four methods are as follows: M1, use of the median interface position in the intensity profiles for the five lightest ions for 15 ≤ m/z ≤ 150; M2, extrapolation of the position for each ion to m/z = 0 for ions with m/z ≤ 150; M3, as M2 but for m/z ≤ 300; and M4, the extreme positions for all m/z ≤ 100. Comparison with the location using matrix terms shows their ranking, from best to worst, to be M4, M3, M1, and M2 with average errors of 10%, 12%, 14%, and 17%, respectively, of the profile interface full widths at half maximum. Use of pseudo-molecular ions is very much poorer, exceeding 50%, and should be avoided. 相似文献
30.
We describe matrices with extremal generalized centralizers over algebraically closed fields. 相似文献