全文获取类型
收费全文 | 294678篇 |
免费 | 5865篇 |
国内免费 | 868篇 |
专业分类
电工技术 | 5382篇 |
综合类 | 365篇 |
化学工业 | 44057篇 |
金属工艺 | 10870篇 |
机械仪表 | 9267篇 |
建筑科学 | 7250篇 |
矿业工程 | 1150篇 |
能源动力 | 7113篇 |
轻工业 | 28346篇 |
水利工程 | 2875篇 |
石油天然气 | 4045篇 |
武器工业 | 73篇 |
无线电 | 36280篇 |
一般工业技术 | 56187篇 |
冶金工业 | 54347篇 |
原子能技术 | 5560篇 |
自动化技术 | 28244篇 |
出版年
2021年 | 2935篇 |
2020年 | 2264篇 |
2019年 | 2918篇 |
2018年 | 4903篇 |
2017年 | 4871篇 |
2016年 | 5133篇 |
2015年 | 3432篇 |
2014年 | 5359篇 |
2013年 | 14068篇 |
2012年 | 8621篇 |
2011年 | 10782篇 |
2010年 | 8544篇 |
2009年 | 9259篇 |
2008年 | 10058篇 |
2007年 | 9937篇 |
2006年 | 8691篇 |
2005年 | 7923篇 |
2004年 | 7607篇 |
2003年 | 7387篇 |
2002年 | 7014篇 |
2001年 | 6911篇 |
2000年 | 6602篇 |
1999年 | 6710篇 |
1998年 | 16526篇 |
1997年 | 11828篇 |
1996年 | 9102篇 |
1995年 | 7051篇 |
1994年 | 6199篇 |
1993年 | 6134篇 |
1992年 | 4789篇 |
1991年 | 4436篇 |
1990年 | 4214篇 |
1989年 | 3946篇 |
1988年 | 3802篇 |
1987年 | 3323篇 |
1986年 | 3187篇 |
1985年 | 3764篇 |
1984年 | 3513篇 |
1983年 | 3137篇 |
1982年 | 2996篇 |
1981年 | 3113篇 |
1980年 | 2920篇 |
1979年 | 2739篇 |
1978年 | 2596篇 |
1977年 | 3225篇 |
1976年 | 4185篇 |
1975年 | 2386篇 |
1974年 | 2370篇 |
1973年 | 2386篇 |
1972年 | 1884篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
972.
Dynamic external fixation for distal radius fractures 总被引:5,自引:0,他引:5
Thirty adult patients with closed comminuted and mostly intraarticular fractures of the distal radius were treated by closed reduction and immobilization with a dynamic external wrist fixator during a 2-year period. In 13 patients with severely comminuted and unstable fractures, additional Kirschner wires were used. After 10 to 14 days of rigid fixation in neutral position, the motion element was unlocked to allow up to 30 degrees flexion. Six weeks later, the fixator was removed. The patients then were observed for an average of 24 weeks. An excellent functional outcome was seen in 6 patients (20%), a good outcome in 20 patients (67%), and a fair outcome in 4 patients (13%). Anatomically, 15 patients (50%) had an excellent result, 14 (47%) a good outcome, and 1 (3%) a fair outcome. Only minimal loss of reduction averaging 1 degree palmar tilt was seen during mobilization. There were 2 major complications: 1 deep Kirschner wire tract infection and 1 index metacarpal fracture. Minor complications such as sensory disturbances and pin tenderness were present but recovered completely after removal of the fixator. This study provides promising data and offers an alternative method in the treatment of distal radius fractures with severe comminution. In cases with postreductive unstable fragments, additional Kirschner wires should be used to allow early mobilization of the wrist. 相似文献
973.
Intracranial tumours, often presenting with progressive hydrocephalus, are rare congenital diseases accounting for 0.5-1.5 per cent of all cases of brain tumours diagnosed during childhood. The differential diagnosis includes vascular malformations, infarctions, and haemorrhages. Sonographic signs suggestive of glioblastoma, teratoma, and astrocytoma do not establish the histological diagnosis, however. We report a case of an undifferentiated fetal glioma detected at 29 weeks' gestation. The diagnosis of an undifferentiated brain tumour was suspected by sonography because of the lack of normal brain structures in conjunction with a diffuse echogenic central lesion and an external hydrocephalus. Because of the very poor prognosis, we induced labour by intravaginal and intravenous administration of prostaglandin E2 and achieved the vaginal delivery of a stillborn child whose head circumference corresponded to 38 weeks of pregnancy. Histological and immunochemical features of this undifferentiated congenital glioma (glioblastoma) are presented. 相似文献
974.
Compression of higher order optical solitons in fibers with anomalous dispersion decreasing along their length is investigated. We demonstrate high-quality compression of pulses with initial soliton order 1相似文献
975.
DeTienne D.H. Gray G.R. Agrawal G.P. Lenstra D. 《Quantum Electronics, IEEE Journal of》1997,33(5):838-844
Most of the previous treatments of semiconductor lasers subject to optical feedback from a phase-conjugate mirror (PCM) have assumed that the PCM responds instantaneously. Furthermore, the mechanism responsible for phase conjugation does not usually enter into the analysis. In this paper, we derive the time-dependent reflectivity of a PCM created through nondegenerate four-wave mixing in a Kerr-type nonlinear medium. The resulting laser dynamics are compared with the case of the ideal PCM, as a function of the external-cavity length, the PCM reflectivity, and the PCM interaction depth. The PCM with a significant interaction depth tends to suppress otherwise chaotic output and produces pulses whose repetition rate is tunable by varying PCM reflectivity. At high feedback levels, it stabilizes the laser output. We use the circle-map formalism to explain our numerical results 相似文献
976.
Reid D.T. McGowan C. Ebrahimzadeh M. Sibbett W. 《Quantum Electronics, IEEE Journal of》1997,33(1):1-9
We describe the design and operation of a critically phase-matched femtosecond optical parametric oscillator based on KTA. By employing a small pump-signal noncollinear angle, tuning of the idler to beyond 4 μm is achieved using a Ti:sapphire pump laser. A Gaussian-beam model is described which can be used to identify the optimal noncollinear phase-matching geometry. Idler and signal pulses are characterized fully both temporally and spectrally and interferometric autocorrelation data at 3.5 μm showing idler pulses of only eight optical cycles duration are presented 相似文献
977.
Knippels G.M.H. van der Meer A.F.G. Oepts D. van Amersfoort P.W. 《Quantum Electronics, IEEE Journal of》1997,33(1):10-17
We have investigated the optical output of the free-electron laser for infrared experiments (FELIX) when it is driven by an electron beam with a ramped energy. We show that the applied slow ramp on the electron beam energy leads to a frequency chirp on each picosecond optical pulse. Typical values for the chirp are 0.2% frequency sweep across a 1.5-ps-long optical pulse. The optical pulses were analyzed with a double-grating pair and with a second-order autocorrelator. The pulse duration was reduced in the double-grating pair by 20%. A linear dependence of the chirp on the cavity desynchronization was measured 相似文献
978.
Rogers D.V. Ippolito L.J. Jr. Davarian F. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1997,85(6):810-820
Accurate estimates of the propagation impairments that affect link quality and availability and determine signal interference fields are essential for the reliable design of telecommunication systems and the efficient use of the electromagnetic spectrum. Recent announcements by commercial entities of their intent to use Ka-band spectrum to supply satellite services have heightened interest in propagation data and models for these frequencies. This paper provides a brief overview of Ka-band Earth-satellite systems and requirements in relation to the need for specific types of propagation data 相似文献
979.
Kinema is a fermented soya bean food of Nepal and the hilly regions of North-eastern States of India. Generally, the fermentation is dominated by Bacillus spp. that often cause alkalinity and desirable stickiness in the product. The present study was undertaken in a limited number of commercial (market) kinema samples to test for the presence of foodborne pathogens and their properties. Bacillus cereus was present in numbers exceeding 104 cfu/g product in five of the tested 15 market samples. Enterobacteriaceae and coliform bacteria exceeded 105 cfu/g in 10 of the 15 samples. Escherichia coli exceeding 105 cfu/g was found in two samples. Staphylococcus aureus was not detected in any of the tested samples. Of 31 isolated typical and atypical strains of B. cereus, 18 representative strains were tested qualitatively for the ability to produce diarrhoeal type enterotoxin (BCET) using an Oxoid BCET-RPLA test kit. Overall, BCET was formed by 12 strains in BHIG (brain heart infusion broth +1% glucose), by seven strains on sterilized cooked rice, and by five strains on sterilized cooked soya beans. Semi-quantitative tests on BCET revealed that levels exceeding 256 ng/g soya beans, produced by single pure culture inoculation with the isolated B. cereus strains, were reduced to ≤ 8 ng/g by frying kinema in oil, a common procedure when making kinema curry. It was also shown in a mixed pure culture experiment that a kinema strain B. Subtilis DK-W1, is able to suppress growth and BCET formation by a selected toxin producing strain (BC7-5) of B. cereus. It is concluded that the traditional way of making kinema and its culinary use in curries is safe. However, for novel applications of kinema, safety precautions are advisable. 相似文献
980.
An exact and simple method to evaluate the frequency reuse efficiency for the reverse link of cellular CDMA systems over a non-uniform traffic distribution is presented. This method is based on a traffic redistribution and decomposition process and on standard curves with which partial reuse efficiencies are obtained and conveniently added to give the final parameter 相似文献