全文获取类型
收费全文 | 293717篇 |
免费 | 3965篇 |
国内免费 | 1093篇 |
专业分类
电工技术 | 5057篇 |
综合类 | 2509篇 |
化学工业 | 41309篇 |
金属工艺 | 12476篇 |
机械仪表 | 8983篇 |
建筑科学 | 6645篇 |
矿业工程 | 1339篇 |
能源动力 | 6358篇 |
轻工业 | 20294篇 |
水利工程 | 3078篇 |
石油天然气 | 3269篇 |
武器工业 | 9篇 |
无线电 | 37323篇 |
一般工业技术 | 55355篇 |
冶金工业 | 49244篇 |
原子能技术 | 3933篇 |
自动化技术 | 41594篇 |
出版年
2022年 | 1215篇 |
2021年 | 1927篇 |
2020年 | 1413篇 |
2019年 | 1817篇 |
2018年 | 16814篇 |
2017年 | 15755篇 |
2016年 | 12544篇 |
2015年 | 2530篇 |
2014年 | 3475篇 |
2013年 | 9224篇 |
2012年 | 8427篇 |
2011年 | 16337篇 |
2010年 | 13726篇 |
2009年 | 11930篇 |
2008年 | 13433篇 |
2007年 | 14282篇 |
2006年 | 6212篇 |
2005年 | 6807篇 |
2004年 | 6637篇 |
2003年 | 6557篇 |
2002年 | 5767篇 |
2001年 | 5664篇 |
2000年 | 5299篇 |
1999年 | 5659篇 |
1998年 | 16085篇 |
1997年 | 10637篇 |
1996年 | 8190篇 |
1995年 | 5932篇 |
1994年 | 5129篇 |
1993年 | 5129篇 |
1992年 | 3460篇 |
1991年 | 3334篇 |
1990年 | 3266篇 |
1989年 | 3082篇 |
1988年 | 2835篇 |
1987年 | 2263篇 |
1986年 | 2330篇 |
1985年 | 2646篇 |
1984年 | 2342篇 |
1983年 | 2094篇 |
1982年 | 1919篇 |
1981年 | 2062篇 |
1980年 | 1811篇 |
1979年 | 1677篇 |
1978年 | 1665篇 |
1977年 | 2047篇 |
1976年 | 2738篇 |
1975年 | 1431篇 |
1974年 | 1376篇 |
1973年 | 1325篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
In this paper, the space-vector transformation used in machine vector control is applied to power system analysis. The proposed method is used to model electric machines, power electronic converters, transformers, and transmission lines and to analyze power sources and loads with different connections (delta and wye). This method can also be applied to analyze steady-state (or transient phenomena) and unbalanced sources, including harmonics. Models obtained with this method are as simple as those of the per-phase approach. With the space-vector transformation, instantaneous active and reactive power concepts can be generalized, and new power system control strategies can be developed when power electronic converters are used. Steady-state, transient behavior, and harmonic analyses examples and applications are presented to illustrate the performance and advantages of the proposed method. This method can be extended to unbalanced systems (e.g., unsymmetric faults) using instantaneous symmetrical components in polyphase balanced circuits. 相似文献
43.
R. Sankarasubramanian C. S. Jog T. A. Abinandanan 《Metallurgical and Materials Transactions A》2002,33(4):1083-1090
We examine the symmetry-breaking transitions in equilibrium shapes of coherent precipitates in two-dimensional (2-D) systems
under a plane-strain condition with the principal misfit strain components ε*
xx
and ε*
yy
. For systems with cubic elastic moduli, we first show all the shape transitions associated with different values of t=ε*
yy
/ε*
xx
. We also characterize each of these transitions, by studying its dependence on elastic anisotropy and inhomogeneity. For
systems with dilatational misfit (t=1) and those with pure shear misfit (t=−1), the transition is from an equiaxed shape to an elongated shape, resulting in a break in rotational symmetry. For systems
with nondilatational misfit (−1<t<1; t ≠ 0), the transition involves a break in mirror symmetries normal to the x- and y-axes. The transition is continuous in all cases, except when 0<t<1. For systems which allow an invariant line (−1≤t<0), the critical size increases with an increase in the particle stiffness. However, for systems which do not allow an invariant
line (0<t≤1), the critical size first decreases, reaches a minimum, and then starts increasing with increasing particle stiffness;
moreover, the transition is also forbidden when the particle stiffness is greater than a critical value. 相似文献
44.
Serum total cholesterol, high density lipoprotein (HDL) cholesterol, and triglyceride levels were studied in healthy male
and female subjects consuming for one-week periods a diet of conventional food (CF) providing 42% of energy as fat, principally
butter fat, and then in random order nutritionally complete, defined formula diets of moderate (32%) to very low (1%) fat
content. Compared to CF, the formula with 32% of energy as corn oil lowered serum cholesterol by 25% and the ratio of total
to HDL-cholesterol by 13%. Low (9%) and very low (1–3%) fat formulas reduced HDL-cholesterol by as much as 40%, raised the
total: HDL-cholesterol ratio by about 20% and raised serum triglyceride levels by as much as 100%. When low and very low fat
formulas were ingested for three weeks, these effects persisted although maximal responses occurred during the first week.
These results demonstrated that a moderate fat formula diet with a high P/S ratio had a more favorable effect on serum lipid
levels than various low fat formulas. Low fat conventional food diets should be studied in long-term controlled metabolic
experiments before such diets are recommended to the general population for coronary heart disease or cancer prevention. 相似文献
45.
Summary The equilibrium water contents of linear poly(acrylic acid) sodium salts with different degrees of neutralisation were found to be dependent on temperature and relative humidity. An octahedral model for the primary hydration of poly(acrylic acid) sodium salts (HIRAOKA and YOKOYAMA 1980) was critically evaluated in the light of these findings and an anomaly in the water uptake versus neutralisation curve at approximately 33% neutralisation was explained by the counterion condensation theory. (MANNING 1979). 相似文献
46.
47.
V. L. Ozol' L. F. Kandyba N. T. Bychenkov L. A. Zbarskii B. E. Koropov 《Metallurgist》1989,33(8):156-156
Lenin Dnepropetrovsk Pipe Plant. Translated from Metallurg, No. 8, p. 39, August, 1989. 相似文献
48.
Kuh S.S. Strozier T.A. Ryan C.R. 《Selected Areas in Communications, IEEE Journal on》1989,7(9):1462-1469
A continuous phase quadrature phase shift keyed (CPQPSK) modulation technique is presented. This method utilizes a conventional QPSK modulator and a phase trajectory converter to approximate M =4, h =1/4 continuous phase signal and allows low cost, low complexity, and high rate (>1 Gbit/s) CPM modem implementation for bandwidth efficient transmission through nonlinear satellite channels. Using a communications analysis computer program it has been found that CPQPSK has 99 percent out-of-band power of 0.8R (MSK has 99 percent out-of-band power of 1.2 R where R is defined as bit rate), continuous phase trajectories, and nearly constant envelope amplitude. Simulation of realistic hardware designs indicate that the CPQPSK will require an Eb /No of 14 dB to achieve a bit error rate (BER) of 10-6. Forward error correcting techniques using block codes with an overhead of 10 percent indicate that the Eb /No requirements can be reduced to 11.2 dB for 10-6 BER 相似文献
49.
It is shown that direct compression grade tricalcium phosphate yields linear Heckel plots only if the particle density used is that obtained by liquid pycnometry or mercury porosimetry. If the true density is used (i.e. that obtained by gas displacement), then linearization is not obtained in a pressure range where bonding evidently occurs. It is shown that the pore volume in the range of diameters from 0.5 to 2 μm decreases as tableting pressure increases, and that bonding is associated with the loss of pores of this size. 相似文献
50.
Tipton C.W. Bayne S.B. Griffin T.E. Scozzie C.J. Geil B. Agarwal A.K. Richmond J. 《Electron Device Letters, IEEE》2002,23(4):194-196
This paper reports on the first demonstration of a half-bridge power inverter constructed from silicon carbide gate turn-off thyristors (GTOs) operated in the conventional GTO mode. This circuit was characterized with input bus voltages of up to 600 VDC and 2 A (peak current density of 540 A/cm2) with resistive loads using a pulse-width modulated switching frequency of 2 kHz. We discuss the implications of the thyristor's electrical characteristics and the circuit topology on the overall operation of the half-bridge circuit. This work has determined the conservative critical rate of rise value of the off-state voltage to be 200 V/μs in these devices 相似文献