全文获取类型
收费全文 | 249641篇 |
免费 | 3402篇 |
国内免费 | 438篇 |
专业分类
电工技术 | 4399篇 |
综合类 | 295篇 |
化学工业 | 41579篇 |
金属工艺 | 9607篇 |
机械仪表 | 7579篇 |
建筑科学 | 6040篇 |
矿业工程 | 1411篇 |
能源动力 | 5441篇 |
轻工业 | 25721篇 |
水利工程 | 2558篇 |
石油天然气 | 5134篇 |
武器工业 | 33篇 |
无线电 | 25683篇 |
一般工业技术 | 46197篇 |
冶金工业 | 45892篇 |
原子能技术 | 6011篇 |
自动化技术 | 19901篇 |
出版年
2021年 | 2438篇 |
2019年 | 2211篇 |
2018年 | 3761篇 |
2017年 | 3659篇 |
2016年 | 4003篇 |
2015年 | 2469篇 |
2014年 | 4229篇 |
2013年 | 10817篇 |
2012年 | 6633篇 |
2011年 | 8912篇 |
2010年 | 6982篇 |
2009年 | 7718篇 |
2008年 | 8019篇 |
2007年 | 8108篇 |
2006年 | 7235篇 |
2005年 | 6337篇 |
2004年 | 6082篇 |
2003年 | 5845篇 |
2002年 | 5856篇 |
2001年 | 5694篇 |
2000年 | 5518篇 |
1999年 | 5414篇 |
1998年 | 12711篇 |
1997年 | 9034篇 |
1996年 | 6955篇 |
1995年 | 5408篇 |
1994年 | 4813篇 |
1993年 | 4709篇 |
1992年 | 3723篇 |
1991年 | 3446篇 |
1990年 | 3657篇 |
1989年 | 3539篇 |
1988年 | 3350篇 |
1987年 | 2941篇 |
1986年 | 2983篇 |
1985年 | 3428篇 |
1984年 | 3287篇 |
1983年 | 2937篇 |
1982年 | 2775篇 |
1981年 | 2856篇 |
1980年 | 2757篇 |
1979年 | 2655篇 |
1978年 | 2714篇 |
1977年 | 2980篇 |
1976年 | 3843篇 |
1975年 | 2405篇 |
1974年 | 2268篇 |
1973年 | 2409篇 |
1972年 | 1982篇 |
1971年 | 1873篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
R Mechoulam E Fride S Ben-Shabat U Meiri M Horowitz 《Canadian Metallurgical Quarterly》1998,362(1):R1-R3
Orphanin FQ/nociceptin binds with high affinity to the orphan opioid receptor-like/K-3 (ORL1/KOR-3) clone, and stimulates feeding. The present study demonstrated that antisense oligodeoxynucleotides directed against either exons 1, 2 or 3 of the ORL1/KOR-3 clone reduced orphanin FQ/nociceptin-induced hyperphagia. A missense probe was ineffective. Naltrexone dose-dependently reduced orphanin FQ/nociceptin-induced hyperphagia. These data suggest that the receptor responsible for orphanin FQ/nociceptin-induced hyperphagia is encoded by the ORL1/KOR-3 clone. 相似文献
992.
ON direction-selective (DS) ganglion cells were identified by electrophysiological recordings in DAPI labeled, isolated rabbit retinas. Their responses to a flashing spot were sustained. Their responses to moving stimuli were strong in the preferred direction and weak in the null direction. Injection of the recorded cells with Lucifer yellow revealed that the cells had a distinct dendritic morphology, consistent with that described previously (Buhl & Peichl, 1986; Amthor et al., 1989; Famiglietti, 1992a). When neighboring cells were injected, an extensive dendritic co-fasciculation was observed. The pattern of fasciculation restricts the possible synaptic connections of the ON DS cell. 相似文献
993.
C Sissi L Bolgan S Moro G Zagotto C Bailly E Menta G Capranico M Palumbo 《Canadian Metallurgical Quarterly》1998,54(6):1036-1045
To elucidate structure-activity relationships for drugs that are able to poison or inhibit topoisomerase II, we investigated the thermodynamics and stereochemistry of the DNA binding of a number of anthracene derivatives bearing one or two 4, 5-dihydro-1H-imidazol-2-yl-hydrazone side chains (characteristic of bisantrene) at different positions of the planar aromatic system. An aza-bioisostere, which can be considered a bisantrene-amsacrine hybrid, was also tested. The affinity for nucleic acids in different sequence contexts was evaluated by spectroscopic techniques, using various experimental conditions. DNA-melting and DNase I footprinting experiments were also performed. The location and number of the otherwise identical side chains dramatically affected the affinity of the test compounds for the nucleic acid. In addition, the new compounds exhibited different DNA sequence preferences, depending on the locations of the dihydroimidazolyl-hydrazone groups, which indicates a major role for the side-chain position in generating specific contacts with the nucleic acid. Molecular modeling studies of the intercalative binding of the 1- or 9-substituted isomers to DNA fully supported the experimental data, because a substantially more favorable recognition of A-T steps, compared with G-C steps, was found for the 9-substituted derivative, whereas a much closer energy balance was found for the 1-substituted isomer. These results compare well with the alteration of base specificity found for the topoisomerase II-mediated DNA cleavage stimulated by the isomeric drugs. Therefore, DNA-binding specificity appears to represent an important determinant for the recognition of the topoisomerase-DNA cleavable complex by the drug, at least for poisons belonging to the amsacrine-bisantrene family. 相似文献
994.
RNA tertiary structure mediation by adenosine platforms 总被引:4,自引:0,他引:4
JH Cate AR Gooding E Podell K Zhou BL Golden AA Szewczak CE Kundrot TR Cech JA Doudna 《Canadian Metallurgical Quarterly》1996,273(5282):1696-1699
The crystal structure of a group I intron domain reveals an unexpected motif that mediates both intra- and intermolecular interactions. At three separate locations in the 160-nucleotide domain, adjacent adenosines in the sequence lie side-by-side and form a pseudo-base pair within a helix. This adenosine platform opens the minor groove for base stacking or base pairing with nucleotides from a noncontiguous RNA strand. The platform motif has a distinctive chemical modification signature that may enable its detection in other structured RNAs. The ability of this motif to facilitate higher order folding provides one explanation for the abundance of adenosine residues in internal loops of many RNAs. 相似文献
995.
996.
Investigation of optical losses in photoelastic and ridge waveguides in GaAs-AlGaAs heterostructures
Q.Z. Liu L.S. Yu S.S. Lau H.P. Zappe J.E. Epler 《Photonics Technology Letters, IEEE》1996,8(6):806-808
Two approaches have been used to fabricate stable photoelastic waveguides with planarized surfaces on GaAs-AlGaAs heterostructures. The first approach uses tensile Ni/sub 3/GaAs stressors formed by metal-semiconductor reactions. The second approach uses inert, refractory and compressive stressors, such as RF sputtered W and RF co-sputtered WNi films. For comparison purposes, ridge waveguides have also been fabricated using the same heterostructure by a dry etching technique. Optical losses of photoelastic waveguides, measured by Fabry-Perot (FP) method at a wavelength of 1.53 /spl mu/m, are comparable to or better than those of the ridge waveguides. Material loss appears to be the primary loss mechanism in both photoelastic and ridge waveguides. These results indicate that the photoelastic waveguide processing technique reported in this study is a promising alternative to commonly used dry etching techniques for planarization. 相似文献
997.
The reactions of HCOOH and CH3COOH on CeO2(111) and CeO2(100) were studied using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The carboxylic acids were found to dissociate on both surfaces to form carboxylates. Adsorbed formates and acetates decomposed near 600 K to produce primarily the dehydration products CO and CH2CO, respectively. This result is consistent with previous studies which have indicated that the ease of reduction of the oxide is the primary factor in determining the selectivity for dehydration versus dehydrogenation during carboxylate decomposition. In addition to CO and CO2, small amounts of formaldehyde were produced during formate decomposition on both CeO2(111) and CeO2(100). In contrast, acetone was observed as a product during acetate decomposition only on the CeO2(111) surface. 相似文献
998.
MC Bueno Jayme Gallani RC Rodrigues Colombo E de Araújo Cintra S Brunelli Rigacci 《Canadian Metallurgical Quarterly》1996,4(2):179-199
The overall purpose of this study was to explore nurses' feelings about the applicability and adequacy of a pilot model of nursing assessment (PMNA) developed for coronary care units (CCU) in order to obtain data that could help in establishing a definitive model. The evaluation, performed by 11 CCU nurses, showed that they considered the development and implementation of PMNA as valuable, and that its design was adequate for interviewing cardiac patients. These results will be employed in the elaboration of a definitive model of nursing assessment. 相似文献
999.
Michael E. G. Ferguson Peter L. Spedding 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1995,63(3):262-278
Experimental data for air–water two-phase co-current flow in two different pipe diameters were used to test the prediction of pressure drop by a number of existing theories and correlations. Several models are shown to be useful for prediction, particularly with the stratified regimes which have proved difficult to handle in the past. The model suggested by Olujic proved to be of particular value. 相似文献
1000.
P Leterme A Théwis E Fran?ois P Van Leeuwen B Wathelet J Huisman 《Canadian Metallurgical Quarterly》1996,126(9):2188-2198
We assessed the use of 15N-labeled dietary proteins as a possible tool for the determination of the true ileal amino acid (AA) digestibility in pigs. The first experiment was designed to study the dietary N excretion pattern at the ileum subsequent to the ingestion of a single 15N-labeled meal. In a second experiment, we compared ileal endogenous AA outputs and true AA digestibility estimates obtained in pigs ingesting 15N-labeled dietary proteins in a single meal vs. intravenous infusion of [15N]leucine for 10 d during the ingestion of a pea-based diet and a protein-free starch diet. The proportion of endogenous N found in the ileal digesta differed when the label was delivered orally (50%) vs. intravenously (72%) and changed with time. As a consequence, the true ileal AA digestibilities measured with labeled diets were lower. A third experiment demonstrated that this was due to the rapid recycling of labeled dietary N in endogenous moieties, because 15N was found in blood within 10 min of consuming the labeled meal, within 50 min of consumption in pancreatic enzymes, 90 min in bile and 4 h in ileal mucins. We conclude that the use of 15N-labeled meals for determination of true ileal AA digestibilities is limited by the fast recycling of dietary N in endogenous secretions following a single 15N-labeled meal. The accuracy of results will depend on meaningful estimates of AA flow during a limited period and accurate estimates of 15N in AA. 相似文献