全文获取类型
收费全文 | 10436篇 |
免费 | 106篇 |
国内免费 | 9篇 |
专业分类
电工技术 | 98篇 |
综合类 | 3篇 |
化学工业 | 1012篇 |
金属工艺 | 101篇 |
机械仪表 | 300篇 |
建筑科学 | 216篇 |
矿业工程 | 28篇 |
能源动力 | 129篇 |
轻工业 | 875篇 |
水利工程 | 107篇 |
石油天然气 | 15篇 |
无线电 | 867篇 |
一般工业技术 | 1035篇 |
冶金工业 | 4909篇 |
原子能技术 | 108篇 |
自动化技术 | 748篇 |
出版年
2021年 | 68篇 |
2020年 | 52篇 |
2019年 | 61篇 |
2018年 | 60篇 |
2017年 | 82篇 |
2016年 | 65篇 |
2015年 | 47篇 |
2014年 | 104篇 |
2013年 | 315篇 |
2012年 | 160篇 |
2011年 | 237篇 |
2010年 | 212篇 |
2009年 | 232篇 |
2008年 | 260篇 |
2007年 | 240篇 |
2006年 | 245篇 |
2005年 | 219篇 |
2004年 | 178篇 |
2003年 | 199篇 |
2002年 | 200篇 |
2001年 | 162篇 |
2000年 | 161篇 |
1999年 | 254篇 |
1998年 | 1277篇 |
1997年 | 769篇 |
1996年 | 637篇 |
1995年 | 406篇 |
1994年 | 337篇 |
1993年 | 370篇 |
1992年 | 132篇 |
1991年 | 152篇 |
1990年 | 154篇 |
1989年 | 169篇 |
1988年 | 156篇 |
1987年 | 158篇 |
1986年 | 122篇 |
1985年 | 145篇 |
1984年 | 94篇 |
1983年 | 102篇 |
1982年 | 82篇 |
1981年 | 101篇 |
1980年 | 85篇 |
1979年 | 79篇 |
1978年 | 74篇 |
1977年 | 168篇 |
1976年 | 312篇 |
1975年 | 75篇 |
1974年 | 73篇 |
1973年 | 64篇 |
1972年 | 54篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Brian F.G. Johnson 《Topics in Catalysis》2003,24(1-4):147-159
In this article, we report studies of two new forms of highly active supported catalysts. First, those derived from supported carbonylate clusters—nanocatalysts and second, those produced from the heterogenization of known chiral homogeneous systems. The utilization of established cluster compounds of precisely known composition and structure have proved invaluable in the preparation of mixed metal nanoparticles of well-defined composition. The attachment of these nanoparticles to the inner walls of mesoporous silica has led to the development of highly active and effective catalysts for a series of hydrogenation reactions, emphasizing the enhanced reactivity of these metal systems as a consequence of their size and of the low coordination numbers of the metal atoms involved. These attributes combined with the relative ease of characterization of both the active sites and their location has led to a detailed examination of the role of these nanosystems in a new approach to clean technology. In an alternative strategy, the use of heterogenized homogeneous chiral catalysts based on the ferrocenyl moiety and diamino ligands and linked to the inner surface of mesoporous materials either by a direct chemical bond or by an ionic interaction has also been explored. These catalysts have been shown to be highly effective in the enantioselective synthesis of organic compounds. Significantly, we have found that the mesopore (usually MCM-41) imposes spatial restrictions arising from the concavity of the inner surface and leads to greatly enhanced enantioselective (ee) performance. 相似文献
102.
103.
JJ Rankine DG Fortune CE Hutchinson DG Hughes CJ Main 《Canadian Metallurgical Quarterly》1998,23(15):1668-1676
STUDY DESIGN: Prospective comparative study of pain drawings with findings on lumbar spine magnetic resonance imaging. OBJECTIVES: To assess the ability of the pain drawing to predict the presence of nerve root compression. SUMMARY OF BACKGROUND DATA: Most research work has concentrated on the ability of the pain drawing to act as a screening method for psychological distress with less work directed at the influence the anatomic abnormality has on the pain drawing. METHODS: One hundred thirty-four consecutive outpatients attending for lumbar magnetic resonance imaging in the investigation of back and leg pain completed pain drawings and psychological testing immediately before the examination. The pain drawing was analyzed by previously reported criteria, and the magnetic resonance imaging was assessed independently for the presence of nerve compression by three radiologists. Multivariate stepwise discriminant analysis was used to identify patients with nerve compression on the basis of their pain drawing. RESULTS: Nerve compression was predicted by numbness in the anterolateral aspect of the foot. There was considerable overlap in the appearances of the pain drawings between patients with and without nerve compression, and the pain drawing correctly classified only 58% of patients with nerve compression. CONCLUSIONS: The pain drawing is not a good predictor of nerve compression on magnetic resonance imaging in a group of patients investigated for back and leg pain. It should be interpreted with caution and in light of the full clinical picture. 相似文献
104.
FM Johnson 《Canadian Metallurgical Quarterly》1998,410(2):123-140
Distribution and metabolism of the thyroid hormone 3,5, 3'-l-triiodothyronine (T3) were studied in several ways to gain insights into these processes in the warm water fish tilapia Oreochromis mossambicus. Trace doses of 125I-labeled T3 (T*3)1 were injected intraarterially, extraarterially, or intraperitoneally in freshwater-reared male tilapia to explore plasma clearance kinetic responses to these different input modalities. Multicompartmental analysis of the plasma clearance data indicated a kinetic distribution of T*3 much like that reported for the rat and human, with about 2% of total body T*3 in plasma, 5% in rapidly exchanging tissues such as kidney and liver, and 93% in slowly exchanging tissues such as muscle. However, plasma clearance rates (PCR, 5.37 mL/h . 100 g body wt) and plasma appearance rates (PAR3 = PCR x [T3] plasma = 36.3 ng/h . 100 g body wt) were quite different than these indices in rat and human and 5 to 50 times larger than values reported for rainbow trout. On a whole-body basis, normalized for body weight, the tilapia we studied produced and accumulated much more T3 than rat, human, or rainbow trout. Enzymatic and chromatographic analyses of the plasma clearance data samples indicated substantial production of labeled glucuronide, but not sulfate, conjugates of iodothyronines (TiG) of unknown origin appearing in plasma. The TiG appeared beginning a few hours postinjection, peaked at 6 hours, and yielded a predicted steady-state TiG level of 8.3% of the T3 level in plasma. In contrast, in published studies, no conjugates were detected in rainbow trout plasma from 2 to 24 h after iv injection of T*3, T*4, or reverse-T*3, although conjugates of all were present in bile. To our knowledge, although T3 and T4 sulfate conjugates are present in the sera of several mammals, this is the first quantification of iodothyronine glucuronides reported in blood of any species under normal conditions. This might have physiological significance for the tilapia, with T3G providing a reversible storage form of T3 in blood, as has been suggested for sulfate conjugates of T3 and T4 in blood of several mammals. 相似文献
105.
106.
K Shannon P Stapleton X Xiang A Johnson H Beattie F El Bakri B Cookson G French 《Canadian Metallurgical Quarterly》1998,36(10):3105-3110
Representative isolates from 10 distinct extended-spectrum beta-lactamase-producing strains of Klebsiella pneumoniae that caused hospital outbreaks in the United Kingdom from 1991 to 1994 were examined for relationships between their enzymes and plasmids. The beta-lactamases were identified by a combination of isoelectric focusing and gene sequencing. SHV-2 beta-lactamase was produced by isolates from four outbreaks, SHV-5 was involved in three, and SHV-4, TEM-15, and TEM-26 were involved in one outbreak each. All of the extended-spectrum beta-lactamases were encoded by self-transmissible plasmids, with sizes ranging from about 70 to 160 kb. No similarities between the restriction digest patterns of the extended-spectrum beta-lactamase-encoding plasmids were detected, except to some extent between those that produced TEM-15 and TEM-26. Thus, outbreaks of hospital infection with these organisms in the United Kingdom from 1991 to 1994 involved distinct organisms and resistance plasmids and appeared to be unrelated. 相似文献
107.
P Barrett G Davidson DG Hazlerigg MA Morris AW Ross PJ Morgan 《Canadian Metallurgical Quarterly》1998,139(1):163-171
108.
A six-day-old Missouri foxtrotter colt was examined because it had had diarrhoea since it was 24 hours old. A diagnosis of colitis, septicaemia, and disruption of the arterial blood flow to the pelvic limbs was made on the basis of clinical and laboratory findings. Despite intensive medical therapy, the foal died 13 hours after being examined. Postmortem examination revealed diffuse fibrinous enteritis with lymphoid necrosis, multifocal fibrinonecrotic typhlocolitis, disseminated intravascular coagulation, and a large occluding thrombus at the aortic termination. The results of bacteriological culturing supported the diagnosis of septicaemia leading to activation of the clotting cascade, disseminated intravascular coagulation, aorto-iliac thrombosis and infarction of the pelvic limbs. 相似文献
109.
Two HPLC-UV assays are reported here: one is a rapid assay for mycophenolic acid (MPA) and the other is a simultaneous assay for MPA and its metabolite mycophenolic acid glucuronide (MPAG). For both methods, plasma samples (500 microl) with added internal standard were acidified and extracted using C18 solid-phase extraction cartridges. Chromatographic separation was achieved on a C18 Novapak column using a mobile phase consisting of methanol-0.05% orthophosphoric acid (40:60, v/v) for the rapid MPA assay and 30:70 for the simultaneous MPA and MPAG assay. The assays were linear over the ranges 0.1 to 50.0 mg/l for MPA and 2.8 to 225.8 mg/l for MPAG. Mean absolute recovery for all analytes was >99%. These methods are suitable for therapeutic drug monitoring and pharmacokinetic studies. 相似文献
110.
MS Lefcoe GA Fox DJ Leasa RK Sparrow DG McCormack 《Canadian Metallurgical Quarterly》1994,105(3):885-887
Sixty-six supine portable chest radiographs done on the day of bronchoscopy in 62 critical care unit patients suspected of having pneumonia were examined in a blinded fashion by two radiologists. Quantitative culture results obtained from protected brush catheter (PBC) specimens were compared with chest radiograph scores. For one observer, the sensitivity of the chest radiograph for predicting the presence of positive culture results was 0.60, specificity was 0.29, overall agreement was 0.41, positive predictive value was 0.34, and negative predictive value was 0.55. For the second observer, the values were as follows: sensitivity, 0.64; specificity, 0.27; overall agreement, 0.41; positive predictive value, 0.35; and negative predictive value, 0.55. The kappa statistic was calculated at 0.27 indicating marginal interobserver reproducibility. We conclude the portable chest radiograph in the critical care setting is not accurate in predicting the presence of pneumonia when the diagnosis is based on quantitative cultures obtained from protected brush catheter specimens. 相似文献