全文获取类型
收费全文 | 5902篇 |
免费 | 96篇 |
国内免费 | 1篇 |
专业分类
电工技术 | 32篇 |
综合类 | 2篇 |
化学工业 | 449篇 |
金属工艺 | 103篇 |
机械仪表 | 50篇 |
建筑科学 | 137篇 |
矿业工程 | 12篇 |
能源动力 | 62篇 |
轻工业 | 381篇 |
水利工程 | 41篇 |
石油天然气 | 11篇 |
无线电 | 201篇 |
一般工业技术 | 417篇 |
冶金工业 | 3792篇 |
原子能技术 | 41篇 |
自动化技术 | 268篇 |
出版年
2022年 | 45篇 |
2021年 | 49篇 |
2020年 | 39篇 |
2019年 | 50篇 |
2018年 | 51篇 |
2017年 | 46篇 |
2016年 | 50篇 |
2015年 | 51篇 |
2014年 | 55篇 |
2013年 | 137篇 |
2012年 | 103篇 |
2011年 | 156篇 |
2010年 | 127篇 |
2009年 | 134篇 |
2008年 | 159篇 |
2007年 | 117篇 |
2006年 | 120篇 |
2005年 | 109篇 |
2004年 | 90篇 |
2003年 | 77篇 |
2002年 | 94篇 |
2001年 | 59篇 |
2000年 | 70篇 |
1999年 | 136篇 |
1998年 | 1052篇 |
1997年 | 618篇 |
1996年 | 379篇 |
1995年 | 219篇 |
1994年 | 209篇 |
1993年 | 240篇 |
1992年 | 58篇 |
1991年 | 65篇 |
1990年 | 89篇 |
1989年 | 70篇 |
1988年 | 67篇 |
1987年 | 65篇 |
1986年 | 50篇 |
1985年 | 47篇 |
1984年 | 26篇 |
1983年 | 26篇 |
1982年 | 17篇 |
1981年 | 37篇 |
1980年 | 58篇 |
1979年 | 14篇 |
1978年 | 17篇 |
1977年 | 112篇 |
1976年 | 236篇 |
1975年 | 12篇 |
1973年 | 9篇 |
1972年 | 11篇 |
排序方式: 共有5999条查询结果,搜索用时 15 毫秒
991.
Purification and characterization of a troponin C-like phosphodiesterase activator from bovine thyroid 总被引:1,自引:0,他引:1
A troponin C-like phosphodiesterase activator from bovine thyroid has been purified to homogeneity. The overall purification was about 9,800-fold with a yield of 8%. Bovine thyroid activator protein is identical in biologic properties to that isolated from bovine brain. They have the same specific activity regarding stimulation of bovine brain cyclic nucleotide phosphodiesterase. Both proteins form a Ca2+-dependent complex with heart muscle troponin I which is stable in 6M urea-polyacrylamide gel and which is similar, but not identical, to the troponin C-troponin I complex. The physiochemical properties of bovine thyroid activator protein are identical with those of bovine brain and other phosphodiesterase activator proteins and are similar to heart muscle and skeletal muscle troponin C as follows: (A) they bind 3-4 exchangeable calcium ions/mol with dissociation constants between 10(-5) and 10(-6) M, (B) they are highly acidic with a high content of aspartic and glutamic acids and isoelectric points of approximately 4.1, (C) these proteins have an unusual ultraviolet absorption spectrum with six discrete maxima between 250 and 284 nm which are characteristic of phenylalanine and tyrosine, and (D) these proteins have a low content of cysteine, histidine, tyrosine and proline. The tryptic peptide maps of bovine thyroid and brain activator protein are very similar. However, despite a very similar amino acid composition, the peptide map of bovine heart muscle troponin C is significantly different from that of the other two proteins. The molecular weight of thyroid and brain activator protein is 16,500, while that of heart troponin C is 18,500. Thyroid and brain activator protein, as well as heart troponin C, appear to undergo significant Ca2+-dependent conformational changes, as measured by the difference in the circular dichroism spectrum and electrophoretic mobility observed in the presence and absence of calcium ion. 相似文献
992.
993.
994.
The level of PGE is increased 10-fold in intestinal tissues during primary infection of the rat with Nippostrongylus brasiliensis. Peak levels (ca. 7,000 pg/cm) were assayed in the jejunal site of infection on day 7 of infection and similar levels were recorded in 'post-infection' (ileal) segments at this time. The level of PGE in 'post-infection' segments showed further increase to 12,000 pg/cm on day 10. The level of PGE also increased in 'pre-infection' (duodenal) but this was delayed by 4-5 days. The level of PGF also increased during primary infection (from about 100 to 950 pg/cm) but this occurred after expulsion. Increase in the level of PGE occurred earlier (at 3-4 days) during secondary challenge given 19 days after primary infection, but the PGE levels followed the primary response when challenge was given 10 weeks after primary infection. It is suggested that PGE plays a dual role in parasite immunity. (1) PGE may directly affect metabolism of the parasite. In this event it is also suggested that protective antibodies cause the release of PG. (2) Elevated levels of PGE act indirectly by affecting gastrointestinal function which alters the microenvironment at the site of infection. The duodenal migration of parasites may be due to this effect of PGE. 相似文献
995.
LA Sharzer CE Horton JE Adamson JH Carraway JB McCraw 《Canadian Metallurgical Quarterly》1976,3(3):495-509
The effect of prostaglandins (PGs) on the hepatic biotransformation of hexobarbital (Hechi) and p-chloro-N-methylaniline (PCMA) was determined in male rats. PCMA metabolism in slices was decreased by all PGs (PGA1, PGB1, PGE1, PGE2, PGF1alpha, PGF2alpha), ranging in concentration from 1 mu M to 1 mM. PGA1, at 1 mM, produced the greatest degree of inhibition, 39%. PG addition to microsomes, however, failed to alter PCMA metabolism. In contrast to PCMA biotransformation, Hechi metabolism was increased by all PGs in slices but not in liver subfractions. PGs of the E and F series were the most potent, producing a two-fold increase in Hechi metabolism at 1 mM after a 20 min preincubation. The increased effect was observed as early as 10 min and lasted for 4 hr. The relationship of PG metabolism and binding to cytochrome P-450 is presented as a possible mechanism to account for the opposite effects on Hechi and PCMA, type I and II substrates respectively, metabolism. 相似文献
996.
JB Hursh MG Cherian TW Clarkson JJ Vostal RV Mallie 《Canadian Metallurgical Quarterly》1976,31(6):302-309
Five human subjects inhaled a mixture of stable and radioactive mercury vapor for periods of 14 to 24 minutes. The subjects retained an average of 74% of that inhaled. Evidence is submitted to show that the retention occurred almost entirely in the alveoli. For 3 days after exposure, the exhaled breath was passed at intervals through activated charcoal traps for sampling periods of 10 to 35 minutes. The data indicated that an average of 7% of the retained mercury was lost in the expired breath, with a half time of 18 hours. Examination of the subjects in a whole body counter yielded average half times for mercury clearance from different parts of the body as follows: lung, 1.7 days; head, 21 days; kidney region, 64 days; chest, 43 days; and whole body, 58 days. 相似文献
997.
998.
A Krompier R Howard A Macewen C Natoli JB Wear 《Canadian Metallurgical Quarterly》1976,115(6):664-666
Diverticulitis is a disease affecting patients in the fifth to sixth decades. The charts of 12 patients with cesicocolonic fistulas secondary to diverticulitis were reviewed. Their presentation was primarily urological with pneumaturia present in 10 of 12 patients and fecaluria present in 5 of 12. The process of fistulization occurs in stages. If the incipient stage can be identified early fistulization can be prevented and the morbidity and mortality rates will be reduced significantly. In older patients with bladder irritation (with or without positive urine cultures) a high degree of suspicion for diverticular disease will ensure an accurate diagnosis earlier. Patients with penumaturia or fecaluria present less of a diagnostic challenge. Cystoscopy is a reliable procedure and we recommend its use in discovering a vesicocolonic fistula. Surgical therapy should be individualized for each patient and good results were obtained with primary resection as well as staged repair. 相似文献
999.
Array detector-based instruments are now fundamental to measurements of ozone and other atmospheric trace gases from space in the ultraviolet, visible, and infrared. The present generation of such instruments suffers, to a greater or lesser degree, from undersampling of the spectra, leading to difficulties in the analysis of atmospheric radiances. We provide extended analysis of the undersampling suffered by modern satellite spectrometers, which include the Global Ozone Monitoring Experiment, Scanning Imaging Absorption Spectrometer for Atmospheric Chartography, Ozone Monitoring Instrument, and Ozone Mapping and Profiler Suite. The analysis includes basic undersampling, the effects of binning into separate detector pixels, and the application of high-resolution Fraunhofer spectral data to correct for undersampling in many useful cases. 相似文献
1000.
The optical fractional Fourier transform (OFRT) in combination with speckle photography has previously been used to measure the magnitude of surface tilting and translation. Previous OFRT techniques used to determine motion have not been able to discern the direction of the tilt and translation. A simple new approach involving use of correlation is presented to overcome this limitation. Controlled variation of the minimum resolution and dynamical range of measurement is demonstrated. It is then experimentally confirmed that if a rigid body's motion is captured by two OFRT systems of different orders, the direction and magnitude of both the tilting and the in-plane translation motion of the body can be independently determined without a priori knowledge. The experimental results confirm the validity of previous theoretical predictions. 相似文献