全文获取类型
收费全文 | 2344篇 |
免费 | 60篇 |
国内免费 | 32篇 |
学科分类
工业技术 | 2436篇 |
出版年
2022年 | 11篇 |
2021年 | 11篇 |
2020年 | 6篇 |
2019年 | 14篇 |
2018年 | 14篇 |
2017年 | 13篇 |
2016年 | 11篇 |
2015年 | 13篇 |
2014年 | 15篇 |
2013年 | 45篇 |
2012年 | 37篇 |
2011年 | 68篇 |
2010年 | 60篇 |
2009年 | 51篇 |
2008年 | 60篇 |
2007年 | 57篇 |
2006年 | 42篇 |
2005年 | 57篇 |
2004年 | 28篇 |
2003年 | 16篇 |
2002年 | 22篇 |
2001年 | 24篇 |
2000年 | 20篇 |
1999年 | 41篇 |
1998年 | 474篇 |
1997年 | 293篇 |
1996年 | 184篇 |
1995年 | 115篇 |
1994年 | 83篇 |
1993年 | 85篇 |
1992年 | 19篇 |
1991年 | 16篇 |
1990年 | 21篇 |
1989年 | 31篇 |
1988年 | 35篇 |
1987年 | 19篇 |
1986年 | 16篇 |
1985年 | 20篇 |
1984年 | 5篇 |
1983年 | 8篇 |
1982年 | 17篇 |
1981年 | 15篇 |
1980年 | 22篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 59篇 |
1976年 | 121篇 |
1975年 | 6篇 |
1973年 | 5篇 |
1955年 | 3篇 |
排序方式: 共有2436条查询结果,搜索用时 15 毫秒
1.
研究区位于新疆东天山觉罗塔格造山带吐哈盆地以南百灵山以北,多年来对于研究区古生代构造背景的认识受前人热议,存在争议。通过对研究区新发现基性岩体的岩石、地球化学特征与构造背景进行探讨,以期为大区域构造背景的研究提供一定的数据支持。主量元素研究表明,康南岩体具有高Mg、Fe,低Al的特征,说明研究区基性侵入岩为碱性岩石系列。微量元素测试结果表明具有同源演化的特征,Rb、Hf元素亏损较为严重。K、Ba、P、Sm富集明显,Tb、Nb、Ti表现为弱异常,在稀土元素球粒陨石标准化配分模式图中Eu的负异常不明显,表现为弱的正异常, Yb+Nb-Rb构造环境判别图解中样品落在“后碰撞”环境内,以上特征说明具有岛弧花岗岩的基本特征。岩性为深灰绿色弱蚀变细粒橄榄辉绿岩。 相似文献
2.
A cost comparison of balloon angioplasty and stenting versus endarterectomy for the treatment of carotid artery stenosis 总被引:1,自引:0,他引:1
WD Jordan GD Roye WS Fisher D Redden HA McDowell 《Canadian Metallurgical Quarterly》1998,27(1):16-22; discussion 22-4
PURPOSE: Percutaneous transluminal angioplasty with stenting (PTAS) of the carotid artery has been advocated as an alternative treatment for high-grade stenosis. Rationale for this approach includes less morbidity, shorter recovery, and lower cost when compared with carotid endarterectomy (CEA). METHODS: The clinical results and hospital charges of patients who underwent elective treatment for carotid stenosis were reviewed. During a concurrent 14-month period, 218 patients were admitted 229 times for 234 procedures for the treatment of 239 carotid bifurcation stenoses, 109 by PTAS and 130 by CEA. Hospital charges were reviewed for each hospitalization and were categorized according to radiology, operating room, cardiac catheterization laboratory, and all other hospital charges. RESULTS: The combined incidence of postprocedure strokes and deaths were: PTAS, eight strokes (7.7%) and one death (0.9%); CEA, two strokes (1.5%) and two deaths (1.5%). Total hospital charges per admission for the two groups were $30,140 for PTAS and $21,670 for CEA. The average postprocedure length of stay for PTAS was 2.9 days (median, 2 days) and for CEA was 3.1 days (median, 3 days). Cardiac catheterization laboratory charges for the PTAS group were $12,968, whereas the operating room charges for the CEA group were $4263. When hospitalizations that were extended by complications were excluded, the average total charges for the PTAS group (n = 84) dropped to $24,848 (mean length of stay, 1.9 days) and for the CEA group (n = 111) to $19,247 (mean length of stay, 2.6 days). CONCLUSIONS: After evaluating hospital charges, PTAS for the treatment of carotid stenosis cannot currently be justified on the basis of reduced costs alone. With future cost-containing measures, total hospital charges can be reduced in both groups. 相似文献
3.
4.
5.
Estrogen treatment affects the hepatic synthesis and/or secretion of several proteins involved in clinically important pathological processes such as atherosclerosis, hypertension, and thrombosis. The endocrine regulation of the estrogen receptor (ER) concentration in primary cultures of rat hepatocytes was studied. Human growth hormone (hGH) and dexamethasone (DEX) in combination increased ER concentration 6-fold and ER mRNA levels 2.5-fold. These effects were not significantly different from those observed after treatment with the purely somatogenic bovine growth hormone (GH) in combination with DEX. Treatment with the lactogen ovine prolactin in the presence or absence of DEX did not significantly affect ER or ER mRNA concentrations. Triiodothyronine treatment at the most effective concentration (50 nM) increased ER and ER mRNA levels twofold. Medium supplementation with estradiol (0.1 nM) throughout the experiment did not affect the response to treatment with hGH and DEX. Treatment with high concentrations of ethinylestradiol in combination with hGH and DEX, however, increased the ER level twice as much as hGH and DEX without addition of estradiol or ethinylestradiol, whereas the ER mRNA concentration was the same in both the GH+DEX group and GH+ DEX+ (estradiol or ethinylestradiol) groups. These data indicate the importance of GH in combination with glucocorticoids for the maintenance of ER concentrations in the rat liver. Thyroid hormones may be of some, although minor importance, whereas the data suggest that prolactin is not directly involved in hepatic ER regulation. 相似文献
6.
OBJECTIVE: The most common indication for electroconvulsive therapy (ECT) is major depression. It is less recognized that ECT is effective also in the treatment of acute mania. This article aims to provide a comprehensive and critical review of the literature on the use of ECT for manic patients. METHOD: All published papers in the English language on the use of ECT in acute mania that could be found were reviewed with regard to efficacy, frequency and number of treatments, bilateral versus unilateral electrode placement, predictors of antimanic response, stability of therapeutic response, cognitive consequences, and other relevant issues. RESULTS: The evidence indicates that ECT is associated with remission or marked clinical improvement in 80% of manic patients and that it is an effective treatment for patients whose manic episodes have responded poorly to pharmacotherapy. Manic patients do not require a high frequency or prolonged course of treatments to respond to ECT. The seizure threshold appears to be lower in manic patients than in depressed patients. The issues of relapse following response to ECT, cognitive consequences of ECT, and the relative merits of unilateral versus bilateral ECT in manic patients require further study. CONCLUSIONS: ECT is an effective and safe treatment for acute mania. Remission of mania following ECT reflects a primary therapeutic effect rather than a secondary consequence of an ECT-induced organic brain syndrome. 相似文献
7.
Diethylstilbestrol (DES), a synthetic estrogen, is known to be a carcinogen in human and in animals. This study was designed to examine the interaction of DES with yeast RNA in aqueous solution at physiological pH with drug/RNA-phosphate (P) molar ratios of 1/80, 1/40, 1/20, 1/10, 1/4, and 1/2. Fourier transform infrared (FTIR) difference spectroscopy was used to determine the drug binding mode, the binding constant, the sequence selectivity, and RNA secondary structure in the RNA.DES complexes. Spectroscopic evidence showed that at low drug concentration (1/80 and 1/40), DES is intercalating through both Gua-Cyt and Ade-Urd base pairs with minor interaction with the backbone PO2 group (external binding). The calculated binding constant of K approximately 8.5 x 10(4) M-1 at a drug concentration of 3.12 x 10(-4) M shows that DES is a weaker intercalator than those of the methylene blue, acridine orange, and ethidium bromide. At high drug content (r > 1/40, where r represents the DES/RNA-phosphate molar ratio), a partial helix destabilization occurs with no alteration of RNA conformation upon drug complexation. However, a comparison with DNA.DES complexes showed that drug intercalation causes major reduction of the B-DNA structure in favor of A-DNA with no participation of the backbone PO2 group in the DES. DNA complexation. 相似文献
8.
A new sub-class of binding protein-dependent transporter with specificity for a broad range of polar amino acids has been identified by sequence comparison, in Rhizobium leguminosarum, Rhodobacter capsulatus, Escherichia coli and Pseudomonas fluorescens. Southern blotting and PCR analysis has shown that transporters from this new sub-class are widely distributed in Gram-negative bacteria, including, in addition to the above, Citrobacter freundii, Erwinia carotovorum and Rhizobium meliloti. ABC transporters of polar amino acids can be divided into two groups: those with narrow solute specificity and the newly identified sub-class with broad solute specificity. The binding and inner membrane proteins from transporters with a broad solute specificity are larger by approximately 30% than those with a narrow solute specificity. Multiple alignment of the inner membrane proteins from all sequenced polar amino acid transporters indicates there is an N-terminal conserved region that may be involved in solute specificity. A conserved arginine or lysine at residue 30 of this region is changed to glutamate in arginine transporters. Residue 53 also has a strong correlation with the charge on the transported solute, with basic amino acid transporters replacing an aliphatic amino acid at this position with a negatively charged amino acid. The general amino acid permease from R. leguminosarum, which will transport aliphatic as well as basic and acidic amino acids, juxtaposes two prolines at residues 52 and 53 of the N-terminal conserved region. 相似文献
9.
HA Harvey MR Ketterer A Preston D Lubaroff R Williams MA Apicella 《Canadian Metallurgical Quarterly》1997,65(6):2420-2427
In men with gonococcal urethritis, the urethral epithelial cell is a site of infection. To study the pathogenesis of gonorrhea in this cell type, we have developed a method to culture primary human urethral epithelial cells obtained at the time of urologic surgery. Fluorescent analysis demonstrated that 100% of the cells stained for keratin. Microscopic analyses indicated that these epithelial cells arrayed in a pattern similar to that seen in urethral epithelium. Using immunoelectron and confocal microscopy, we compared the infection process seen in primary cells with events occurring during natural infection of the same cell type in men with gonococcal urethritis. Immunoelectron microscopy studies of cells infected with Neisseria gonorrhoeae 1291 Opa+ P+ showed adherence of organisms to the epithelial cell membrane, pedestal formation with evidence of intimate association between the gonococcal and the epithelial cell membranes, and intracellular gonococci present in vacuoles. Confocal studies of primary urethral epithelial cells showed actin polymerization upon infection. Polyclonal antibodies to the asialoglycoprotein receptor (ASGP-R) demonstrated the presence of this receptor on infected cells in the primary urethral cell culture. In situ hybridization using a fluorescent-labeled probe specific to the ASGP-R mRNA demonstrated this message in uninfected and infected cells. These features were identical to those seen in urethral epithelial cells in exudates from males with gonorrhea. Infection of primary urethral cells in culture mimics events seen in natural infection and will allow detailed molecular analysis of gonococcal pathogenesis in a human epithelial cell which is commonly infected. 相似文献
10.
M Beksa? MS Beksa? VB Tipi HA Duru MU Karakás AN Cakar 《Canadian Metallurgical Quarterly》1997,30(3):145-150
The endothelium participates actively in homeostatic mechanisms such as the regulation of vascular tone and maintenance of a nonthrombotic environment, as well as directing biological responses such as leukocyte trafficking to inflammatory sites. Disruption of these processes leads to disease. In the antiphospholipid antibody syndrome autoantibodies provoke the endothelium to develop a prothrombotic surface. In systemic vasculitides associated with presence of antineutrophil cytoplasm antibodies, it is likely that the autoantibodies incite premature neutrophil activation, disrupted neutrophil-endothelium interactions and endothelial damage. This review considers how normal endothelial functions may be subverted in disease and how active endothelial responses may contribute to disease. 相似文献