全文获取类型
收费全文 | 3733篇 |
免费 | 57篇 |
国内免费 | 3篇 |
专业分类
电工技术 | 30篇 |
化学工业 | 438篇 |
金属工艺 | 35篇 |
机械仪表 | 77篇 |
建筑科学 | 119篇 |
矿业工程 | 5篇 |
能源动力 | 67篇 |
轻工业 | 183篇 |
水利工程 | 19篇 |
石油天然气 | 9篇 |
无线电 | 240篇 |
一般工业技术 | 408篇 |
冶金工业 | 1828篇 |
原子能技术 | 46篇 |
自动化技术 | 289篇 |
出版年
2021年 | 23篇 |
2020年 | 19篇 |
2019年 | 22篇 |
2018年 | 31篇 |
2017年 | 24篇 |
2016年 | 28篇 |
2015年 | 37篇 |
2014年 | 33篇 |
2013年 | 108篇 |
2012年 | 66篇 |
2011年 | 82篇 |
2010年 | 73篇 |
2009年 | 68篇 |
2008年 | 96篇 |
2007年 | 99篇 |
2006年 | 72篇 |
2005年 | 69篇 |
2004年 | 67篇 |
2003年 | 62篇 |
2002年 | 61篇 |
2001年 | 58篇 |
2000年 | 63篇 |
1999年 | 102篇 |
1998年 | 507篇 |
1997年 | 327篇 |
1996年 | 226篇 |
1995年 | 127篇 |
1994年 | 101篇 |
1993年 | 115篇 |
1992年 | 54篇 |
1991年 | 50篇 |
1990年 | 40篇 |
1989年 | 50篇 |
1988年 | 50篇 |
1987年 | 41篇 |
1986年 | 39篇 |
1985年 | 51篇 |
1984年 | 43篇 |
1983年 | 35篇 |
1982年 | 34篇 |
1981年 | 53篇 |
1980年 | 35篇 |
1979年 | 29篇 |
1978年 | 32篇 |
1977年 | 68篇 |
1976年 | 121篇 |
1975年 | 30篇 |
1974年 | 19篇 |
1973年 | 31篇 |
1972年 | 20篇 |
排序方式: 共有3793条查询结果,搜索用时 15 毫秒
1.
Sensory activity mediates regulation of tyrosine hydroxylase (TH), the first enzyme in the dopamine biosynthetic pathway, in the rodent olfactory bulb. The current studies established for the first time primary cultures of neonatal mouse olfactory bulb expressing TH and tested whether L-type calcium channels mediate the activity-dependent regulation of the dopamine phenotype. After 1 d in vitro (DIV), a small population of TH-immunostained neurons that lacked extensive processes could be demonstrated. After an additional 2 DIV in serum-free medium, the number of TH neurons had doubled, and they exhibited long interdigitating processes. Membrane depolarization for 48 hr with 50 mM KCl produced a further 2.4-fold increase in the number of TH-immunoreactive neurons compared with control cultures. Increased TH neuron number required at least 36 hr of exposure to KCl. Forskolin, which increases intracellular cAMP levels, induced a 1.5- to 1.6-fold increase in the number of TH-immunostained neurons. Combined treatment with KCl and forskolin was not additive. Nifedipine, an L-type calcium channel blocker, completely prevented the depolarization-mediated increase in TH expression but did not block the response to forskolin. Treatment with Bay K8644, an L-type calcium channel agonist, also significantly increased the number of TH-expressing neurons. Depolarization also induced alterations in neuritic outgrowth, resulting in a stellate versus an elongate morphology that, in contrast, was not prevented by nifedipine. These results are the first demonstration that in vitro, as in vivo, depolarization increases TH expression in olfactory bulb and that L-type calcium channels mediate this activity-dependent regulation of the dopamine phenotype. 相似文献
2.
Structural engineers use design codes formulated to consider uncertainty for both reinforced concrete and structural steel
design. For a simple one-bay structural steel frame, we survey typical uncertainties and compute an interval solution for
displacements and forces. The naive solutions have large over-estimations, so we explore the Mullen-Muhanna assembly strategy,
scaling, and constraint propagation to achieve tight enclosures of the true ranges for displacements and forces in a fraction
of the CPU time typically used for simulations. That we compute tight enclosures, even for large parameter uncertainties used
in practice, suggests the promise of interval methods for much larger structures. 相似文献
3.
Norris Fran H.; Murphy Arthur D.; Baker Charlene K.; Perilla Julia L.; Rodriguez Francisco Gutiérrez; Rodriguez José de Jesús Gutiérrez 《Canadian Metallurgical Quarterly》2003,112(4):646
Prevalence rates of trauma and posttraumatic stress disorder (PTSD) were estimated from a probability sample of 2,509 adults from 4 cities in Mexico. PTSD was assessed according to Diagnostic and Statistical Manual of Mental Disorders (American Psychiatric Association, 1994) criteria using the Composite International Diagnostic Interview (CIDI; WHO, 1997). Lifetime prevalence of exposure and PTSD were 76% and 11.2%, respectively. Risk for PTSD was highest in Oaxaca (the poorest city), persons of lower socioeconomic status, and women. Conditional risk for PTSD was highest following sexual violence, but nonsexual violence and traumatic bereavement had greater overall impact because of their frequency. Of lifetime cases, 62% became chronic; only 42% received medical or professional care. The research demonstrates the importance of expanding the epidemiologic research base on trauma to include developing countries around the world. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
4.
M. A. Amer D. B. Kupranycz B. E. Baker 《Journal of the American Oil Chemists' Society》1985,62(11):1551-1557
Both summer and winter butterfats were fractionated using a laboratory procedure which was designed to simulate a commercial
fractionation process. The process is based on a slow, controlled cooling of the melted fat, a short stabilization time at
the fractionation temperature, and separation of the crystals from the liquid oil by vacuum filtration using a stainless steel
perforated disc. Fractionation temperatures of 29, 26, 23 and 19 C for winter butterfat and 29 and 19 C for summer butterfat
were used to obtain solid and liquid fractions at each temperature. Three replications at each temperature showed good reproducibility
of results. The fractions were characterized by their fatty acid and triglyceride compositions, melting and crystallization
behavior, iodine value, peroxide value and melting point.
Presented at the AOCS meeting in Philadelphia in May 1985. 相似文献
5.
The performance of netted radar is evaluated in terms of a netted version of the radar equation and the ambiguity function. A software simulation has been developed to allow assessment of netted radar sensitivity and ambiguity properties in both two-and three-dimensional space. Simulation results demonstrate that coherent netted radar sensitivity depends not only on the radar parameters, but also on the numbers, locations and specification of transmit and receive nodes. The netted radar ambiguity function is affected by a combination of many factors. In addition to those that determine the form of the monostatic ambiguity function, the locations of transmit and receive nodes as well as the position and velocity of the target all are shown to play an important role in evaluating system performance. As a result of these dependencies, it is shown that there is severe degradation of range and Doppler resolution when a target crosses sufficiently close to a transmit-receive baseline. 相似文献
6.
Ten groups of 14 immunosuppressed NMRI-mice (nu/nu) were raised and kept under germ-reduced conditions. The control animals were fed a germ-reduced diet, nine other groups received the same diet with selegiline (CAS 14611-51-9, Deprenyl) or lipoic acid (thioctic acid, CAS 62-46-4) admixed at various amounts. The 50% survival rate, the total life span of each group and the areas under the curves were determined to evaluate life expectancy as compared to the controls. The racemate of lipoic acid at high dosage (350 mg/kg body weight) reduced the life span significantly. The S(-)-enantiomer of lipoic acid (75 mg/kg body weight) increased the 50% survival rate, whereas the physiologic R(+)-enantiomer (9 mg/kg body weight) expanded the total life span of its group. Alteration of only one out of three parameters was not considered significant. All other groups except for one did not differ from controls: only animals which obtained 75 micrograms selegiline per kg of body weight and per day exerted increased life expectancies by all three parameters. This group exhibited also in statistical evaluation a significantly (p < 0.05) prolongated survival time up to about 200% as compared to the control animals. 相似文献
7.
DI Timokhin AV Istomin TS Shushkova TG Voronel' IG Mikha?lov 《Canadian Metallurgical Quarterly》1997,(2):19-21
Based on generalization and analysis of instructions and guidelines for therapeutical and prophylactic diets of workers, ways of its optimization were proposed, by using bifide-containing acid dairy products as a preventive agent against possible occupational diseases caused by occupational factors. 相似文献
8.
9.
Kiang S.-Z. Baker R.L. Sullivan G.J. Chiu C.-Y. 《IEEE transactions on image processing》1992,1(2):162-169
A pruning algorithm of P.A. Chou et al. (1989) for designing optimal tree structures identifies only those codebooks which lie on the convex hull of the original codebook's operational distortion rate function. The authors introduce a modified version of the original algorithm, which identifies a large number of codebooks having minimum average distortion, under the constraint that, in each step, only modes having no descendents are removed from the tree. All codebooks generated by the original algorithm are also generated by this algorithm. The new algorithm generates a much larger number of codebooks in the middle- and low-rate regions. The additional codebooks permit operation near the codebook's operational distortion rate function without time sharing by choosing from the increased number of available bit rates. Despite the statistical mismatch which occurs when coding data outside the training sequence, these pruned codebooks retain their performance advantage over full search vector quantizers (VQs) for a large range of rates. 相似文献
10.
Cancer statistics, 1993 总被引:1,自引:0,他引:1