全文获取类型
收费全文 | 383316篇 |
免费 | 4587篇 |
国内免费 | 1499篇 |
专业分类
电工技术 | 6244篇 |
综合类 | 2612篇 |
化学工业 | 59215篇 |
金属工艺 | 16689篇 |
机械仪表 | 12463篇 |
建筑科学 | 8995篇 |
矿业工程 | 2375篇 |
能源动力 | 7840篇 |
轻工业 | 33376篇 |
水利工程 | 4431篇 |
石油天然气 | 7220篇 |
武器工业 | 72篇 |
无线电 | 39973篇 |
一般工业技术 | 73684篇 |
冶金工业 | 58402篇 |
原子能技术 | 8315篇 |
自动化技术 | 47496篇 |
出版年
2021年 | 2807篇 |
2019年 | 2607篇 |
2018年 | 18759篇 |
2017年 | 17677篇 |
2016年 | 14728篇 |
2015年 | 3551篇 |
2014年 | 5302篇 |
2013年 | 13248篇 |
2012年 | 11037篇 |
2011年 | 19691篇 |
2010年 | 16471篇 |
2009年 | 14732篇 |
2008年 | 16216篇 |
2007年 | 17235篇 |
2006年 | 8778篇 |
2005年 | 8773篇 |
2004年 | 8423篇 |
2003年 | 8247篇 |
2002年 | 7618篇 |
2001年 | 7058篇 |
2000年 | 6952篇 |
1999年 | 6677篇 |
1998年 | 15734篇 |
1997年 | 11139篇 |
1996年 | 8538篇 |
1995年 | 6640篇 |
1994年 | 5872篇 |
1993年 | 5812篇 |
1992年 | 4516篇 |
1991年 | 4319篇 |
1990年 | 4491篇 |
1989年 | 4351篇 |
1988年 | 4112篇 |
1987年 | 3632篇 |
1986年 | 3679篇 |
1985年 | 4181篇 |
1984年 | 3856篇 |
1983年 | 3540篇 |
1982年 | 3312篇 |
1981年 | 3406篇 |
1980年 | 3274篇 |
1979年 | 3162篇 |
1978年 | 3204篇 |
1977年 | 3610篇 |
1976年 | 4608篇 |
1975年 | 2868篇 |
1974年 | 2723篇 |
1973年 | 2836篇 |
1972年 | 2385篇 |
1971年 | 2225篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Ya. O. Shablovskii 《Inorganic Materials》2007,43(12):1345-1349
The pyroelectric properties of lithium sulfate have been studied theoretically on the hypothesis of a pseudosymmetry of the structure of its polar phase. Analytical expressions are proposed for the temperature dependences of its pyroelectric polarization and pyroelectric coefficient at low temperatures and near the polymorphic transformation. The pyroelectric and piezoelectric coefficients of a polar crystal are shown to be in direct proportion. 相似文献
992.
A new technique for the preparation of CuI nanoparticles from CuSO4 and KI ethanol solutions has been developed. Preparation conditions were optimized through a series of experiments. Under
these conditions, the yield of CuI reached 95.39%. The product was characterized and the reaction kinetics was studied. The
results show that the product takes a roughly spherical shape with an average particle size of less then 50 nm. The activation
energy of the formation of CuI is found to be E
a = 0.58 × 102 kJ/mol, and the preexponential factor in the Arrhenius equation is k
0 = 7.43 × 1016 mol/(l s).
The text was submitted by the authors in English. 相似文献
993.
994.
995.
This paper deals with the time-optimal control problem for a class of control systems which includes controlled mechanical systems with possible dissipation terms. The Lie algebras associated with such mechanical systems have certain special properties. These properties are explored and used in conjunction with the Pontryagin maximum principle to determine the structure of singular extremals and, in particular, time-optimal trajectories. The theory is illustrated by an application to a time-optimal problem for a class of underwater vehicles. 相似文献
996.
This paper discusses another generalization of the direct Routh table truncation method for interval systems. It is shown that the existing generalization of the direct Routh table truncation fails to produce a stable system, in contradiction to the equivalent result for fixed-coefficients systems. The present method guarantees a stable reduced order model for interval systems as well. 相似文献
997.
998.
Solidification time and heat treatment are known to have a large effect on the microstructure of cast aluminum alloys. This
study was conducted to quantify how the fatigue properties of a 319-type aluminum alloy are affected by solidification time
and heat treatment. Both porosity-containing (non-hot isostatically pressed (HIP)) and porosity-free (HIP) samples in the
T6 (“peak aged”) or T7 (“overaged”) heattreated conditions were tested. As the solidification time increased, the average
initiating pore diameter increased and stress-controlled fatigue life decreased. Heat treatment was observed to have a large
effect on fatigue properties of the HIP samples. However, in the non-HIP fatigue samples, heat treatment did not significantly
change the fatigue life or fatigue strength of the cast 319-type alloy. The absence of an influence of heat treatment on fatigue
response is attributed to the predominance of the microporosity in fatigue crack initiation in cast aluminum. 相似文献
999.
1000.
The nave of Santa Maria Novella, a Dominican church in Florence Italy is representative of a Florentine Gothic system of construction. This system, consisting of domical rib vaults on square nave bays, high side aisles, and crypto-buttressing, differs substantially from the French high Gothic system of even-level-crown rib vaults on rectangular bays, with flying buttresses over relatively low aisles. An investigation into the structural aspects of the Florentine Gothic construction system reveals that the domical vaults increase the longitudinal thrust and reduce the transverse thrust, which may require additional precautions to be taken during construction, but entails a less elaborate buttressing system. The domical vaults are found to perform well structurally, with an absence of tensile stresses, and are more suitable for a square bay. In general, the structural system of the nave of Santa Maria Novella is found to be the product of carefully considered structural design, which may be accepted as an alternative to the French Gothic construction system. 相似文献