全文获取类型
收费全文 | 41624篇 |
免费 | 3590篇 |
国内免费 | 5089篇 |
出版年
2023年 | 925篇 |
2022年 | 1008篇 |
2021年 | 1417篇 |
2020年 | 1515篇 |
2019年 | 2084篇 |
2018年 | 1678篇 |
2017年 | 1512篇 |
2016年 | 1434篇 |
2015年 | 1374篇 |
2014年 | 2105篇 |
2013年 | 2937篇 |
2012年 | 1751篇 |
2011年 | 1883篇 |
2010年 | 1535篇 |
2009年 | 2053篇 |
2008年 | 2036篇 |
2007年 | 2181篇 |
2006年 | 1903篇 |
2005年 | 1745篇 |
2004年 | 1508篇 |
2003年 | 1393篇 |
2002年 | 1255篇 |
2001年 | 1027篇 |
2000年 | 918篇 |
1999年 | 822篇 |
1998年 | 768篇 |
1997年 | 641篇 |
1996年 | 535篇 |
1995年 | 554篇 |
1994年 | 557篇 |
1993年 | 466篇 |
1992年 | 433篇 |
1991年 | 456篇 |
1990年 | 365篇 |
1989年 | 312篇 |
1988年 | 325篇 |
1987年 | 266篇 |
1986年 | 301篇 |
1985年 | 480篇 |
1984年 | 588篇 |
1983年 | 327篇 |
1982年 | 459篇 |
1981年 | 444篇 |
1980年 | 396篇 |
1979年 | 291篇 |
1978年 | 214篇 |
1977年 | 234篇 |
1976年 | 208篇 |
1975年 | 162篇 |
1973年 | 176篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
Troy MacAvelia Ahmad Ghasempoor 《Computer methods in biomechanics and biomedical engineering》2014,17(12):1285-1294
The advent of haptic simulation systems for orthopaedic surgery procedures has provided surgeons with an excellent tool for training and preoperative planning purposes. This is especially true for procedures involving the drilling of bone, which require a great amount of adroitness and experience due to difficulties arising from vibration and drill bit breakage. One of the potential difficulties with the drilling of bone is the lack of consistent material evacuation from the drill's flutes as the material tends to clog. This clogging leads to significant increases in force and torque experienced by the surgeon. Clogging was observed for feed rates greater than 0.5 mm/s and spindle speeds less than 2500 rpm. The drilling simulation systems that have been created to date do not address the issue of drill flute clogging. This paper presents force and torque prediction models that account for this phenomenon. The two coefficients of friction required by these models were determined via a set of calibration experiments. The accuracy of both models was evaluated by an additional set of validation experiments resulting in average R2 regression correlation values of 0.9546 and 0.9209 for the force and torque prediction models, respectively. The resulting models can be adopted by haptic simulation systems to provide a more realistic tactile output. 相似文献
23.
24.
25.
26.
Risks and benefit evaluation for controlled human infection studies, where healthy volunteers are deliberately exposed to infectious agents to evaluate vaccine efficacy, should be explicit, systematic, thorough, and non-arbitrary. Decision analysis promotes these qualities using four steps: (1) determining explicit criteria and measures for evaluation, (2) identifying alternatives to the study, (3) defining the models used to estimate the measures for each alternative, and (4) running the models to produce the estimates and compare the alternatives. In this paper, we describe how decision analysis might be applied by funders and regulators, as well as by others contemplating the use of novel controlled human infection studies for vaccine development and evaluation. 相似文献
27.
28.
29.
Makoto Misono 《Molecular Engineering》1993,3(1-3):193-203
First, fundamental properties (structure, acid and redox properties) and advantages of solid polyoxometalate catalysts (catalyst design by acid and redox control, molecularity, unusual reaction field and unique basicity) are explained. Then, the mechanism of alcohol dehydration elucidated by direct observation of reaction intermediates by solid-state NMR and the very high activity of Cs2.5H0.5PW12O40 are described. Finally several industrial applications of polyoxometalate catalysts are briefly introduced placing stress on the role of unique chemical properties of polyoxometalates. 相似文献
30.
B. Singh 《Biometrical journal. Biometrische Zeitschrift》1993,35(7):823-831
The distribution and moments, of ANOVA estimator of heritability are given under unbalanced random model. These expressions are used to investigate the effect of unbalancedness on the bias and variance/MSE of the estimator and also the validity of certain approximations for its variance, numerically. The computed results reveal that the unbalancedness increases both the bias and variance/MSE of the estimator and the Smith-approximation for the variance of the estimator provides better accuracy. 相似文献