全文获取类型
收费全文 | 4793篇 |
免费 | 67篇 |
国内免费 | 72篇 |
专业分类
电工技术 | 28篇 |
综合类 | 52篇 |
化学工业 | 83篇 |
金属工艺 | 20篇 |
机械仪表 | 42篇 |
建筑科学 | 51篇 |
矿业工程 | 13篇 |
能源动力 | 2篇 |
轻工业 | 97篇 |
水利工程 | 15篇 |
石油天然气 | 49篇 |
武器工业 | 6篇 |
无线电 | 76篇 |
一般工业技术 | 50篇 |
冶金工业 | 4222篇 |
原子能技术 | 1篇 |
自动化技术 | 125篇 |
出版年
2024年 | 4篇 |
2023年 | 7篇 |
2022年 | 10篇 |
2021年 | 22篇 |
2020年 | 15篇 |
2019年 | 12篇 |
2018年 | 24篇 |
2017年 | 21篇 |
2016年 | 18篇 |
2015年 | 26篇 |
2014年 | 28篇 |
2013年 | 34篇 |
2012年 | 46篇 |
2011年 | 53篇 |
2010年 | 46篇 |
2009年 | 47篇 |
2008年 | 46篇 |
2007年 | 41篇 |
2006年 | 46篇 |
2005年 | 45篇 |
2004年 | 25篇 |
2003年 | 24篇 |
2002年 | 13篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 142篇 |
1998年 | 1376篇 |
1997年 | 731篇 |
1996年 | 465篇 |
1995年 | 263篇 |
1994年 | 221篇 |
1993年 | 257篇 |
1992年 | 41篇 |
1991年 | 52篇 |
1990年 | 62篇 |
1989年 | 51篇 |
1988年 | 56篇 |
1987年 | 60篇 |
1986年 | 47篇 |
1985年 | 57篇 |
1983年 | 5篇 |
1982年 | 17篇 |
1981年 | 24篇 |
1980年 | 26篇 |
1979年 | 3篇 |
1978年 | 7篇 |
1977年 | 82篇 |
1976年 | 196篇 |
1975年 | 4篇 |
1955年 | 3篇 |
排序方式: 共有4932条查询结果,搜索用时 0 毫秒
991.
SS Yeap M Beaumont A Bennett NA Keating DA White DJ Hosking 《Canadian Metallurgical Quarterly》1998,74(872):349-354
This study assessed whether relatives with low bone mineral density (BMD) could be identified in five large families using historical, biochemical, and genetic markers for osteoporosis. Fifty of 65 relatives had their bone density and bone turnover markers measured, together with an assessment of their risk factors for osteoporosis. Only 33% (5/15) of siblings, 50% (6/12) of children and 43% (10/23) of nephews and nieces had entirely normal BMD. There was no difference in life-style risk factors for osteoporosis, history of previous fractures or body mass index between normal subjects and those with osteopenia or osteoporosis. Osteopenic individuals had a significantly higher than normal osteocalcin value. Within families, there was no clear association between BMD and any of the genetic markers (vitamin D receptor gene polymorphisms, COL 1A1 and COL 1A2 polymorphisms of the collagen gene), either alone or in combination. The addition of genetic markers to the other risk factors for low BMD did not improve the prediction of BMD. In conclusion, we suggest that the presence of osteoporosis in a first degree relative should be one of the clinical indications for bone density measurement as the individuals at risk would not be picked up by other methods. 相似文献
992.
993.
BT Heniford DA Iannitti P Evans M Gagner JM Henderson 《Canadian Metallurgical Quarterly》1998,64(12):1165-1169
Primary duodenal adenocarcinoma not involving the ampullary region is rare. Our aim was to review the outcome of these patients and determine the factors that affect survival. We performed a retrospective review of all patients with primary, nonampullary duodenal adenocarcinoma at the Cleveland Clinic Foundation from January 1986 through December 1996. Twenty-six patients with primary, nonampullary duodenal malignancies were identified. There were 16 adenocarcinomas, 3 gastrinomas, 3 stromal tumors, 3 leiomyosarcomas, and 1 carcinoid tumor. Patients with adenocarcinoma had symptoms present an average of 6.1 months. Tumors were identified by upper gastrointestinal contrast study and esophagogastroduodenoscopy in 90 per cent and 87 per cent of patients, respectively. Twelve of 13 (93%) cancers found in the third or fourth portion of the duodenum were adenocarcinomas. Seven of the 16 adenocarcinomas were resectable on exploration. Those that were contained within the serosa have not recurred (mean, 6 years); one of the two patients with locally invasive adenocarcinoma remains disease free. The average survival for patients with unresectable disease was 6.7 months. The 5-year survival rates were: all adenocarcinoma, 38 per cent; resectable, 86 per cent; and unresectable, 0 per cent. All patients presenting with weight loss or obstructive symptoms died of disease; those with melena survived long term. Patients with tumors other than adenocarcinoma had a 90 per cent 5-year survival. We conclude that patients typically present with a long history of symptoms. Distal duodenal malignancies are most frequently adenocarcinomas. Upper gastrointestinal contrast study or endoscopy is often diagnostic. Patients with weight loss and/or obstructive symptoms had invasive disease and a morbid prognosis. Aggressive surgery is warranted, and most with resectable disease (86%) had long-term survival. 相似文献
994.
AP West PJ Bjorkman DA Dougherty HA Lester 《Canadian Metallurgical Quarterly》1997,272(41):25468-25473
To provide material suitable for structural studies of the nicotinic acetylcholine receptor, we have expressed and purified the NH2-terminal extracellular domain of the mouse muscle alpha subunit. Several constructs were initially investigated using Xenopus oocytes as a convenient small scale expression system. A fusion protein (alpha210GPI) consisting of the 210 NH2-terminal amino acids of the alpha subunit and a glycosylphosphatidylinositol anchorage sequence conferred surface alpha-bungarotoxin binding in oocytes. Coexpression of alpha210GPI with an analogous construct made from the delta subunit showed no evidence of heterodimer formation. The alpha210GPI protein was chosen for large scale expression in transfected Chinese hamster ovary cells. The alpha210GPI protein was cleaved from these cells and purified on an immunoaffinity column. Gel and column chromatography show that the purified protein is processed as expected and exists as a monomer. The purified protein also retains the two distinct, conformation-specific binding sites expected for the correctly folded alpha subunit. Circular dichroism studies of alpha210GPI suggest that this region of the receptor includes considerable beta-sheet secondary structure, with a small proportion of alpha-helix. 相似文献
995.
997.
Plasma ferritin (F) concentrations were measured (range 12-245 ng.ml-1) as an indicator of iron status in 24 male and 45 female healthy, nonanemic athletes. Usual food intake was assessed using a self-completed but supervised food frequency questionnaire. Linear regression was applied to determine the relationship between log F (as data was skewed) and average daily intake of iron (Fe), meat (M), protein (Pr), carbohydrate (CHO), fat (Fa), fiber (Fi), and kilojoules (Kj). There were negative correlations between F and each of Kj (P < 0.01) and CHO (P < 0.001), and a positive correlation between F and Pr/Kj (P < 0.01). No significant relationship existed between F and any of Fe, M, Fa, or Fi. These data suggest that iron or meat intake may not be important determinants of iron status in these athletes. Instead, the percentage of protein in the diet may be more influential on F, as may the (negative) effect of Kj and CHO intake, or at least their reflection of energy expenditure. 相似文献
998.
999.
DA Imoedemhe AB Sigue EL Pacpaco AB Olazo EC Luciano 《Canadian Metallurgical Quarterly》1995,10(11):2970-2975
Embryos obtained from patients undergoing routine in-vitro fertilization (IVF) and embryo transfer were compared with those undergoing subzonal microinsemination (SUZI) for male factor infertility. Overall, the proportion of cleaved embryos was significantly higher in the IVF group in comparison with the SUZI group at 48 h post-insemination [1533 out of 1609 (95.3%) versus 776 out of 952 (81.5%)]. The mean +/- SD grading score of the IVF-derived embryos of 3.61 +/- 0.50 was significantly better than that for SUZI of 2.97 +/- 0.86 (P < 0.0005) at the same time. The implantation rates following the replacement of IVF or SUZI embryos at 48 h were comparable: 14.3 and 10.0% respectively. However, the IVF embryo implantation rate of 15.1% at 72 h was significantly better than that following the replacement of SUZI embryos at either 48 (10.0%) or 72 h (8.0%). The replacement of SUZI-derived embryos at 48 h resulted in significantly higher pregnancy (25.0%) and implantation rates (10.0%) than at 72 h, with rates of 10.8 and 8.0% respectively. Similarly, the overall embryo quality deteriorated following in-vitro culture for up to 72 h. The clinical pregnancy loss rate (33.0%) was highest following the replacement of SUZI embryos at 72 h, although the data were limited. It is suggested that these data indicate that a combination of in-vitro manipulation, the injection of multiple spermatozoa into the subzonal space and probably the genomic capacity of spermatozoa derived from poor-quality semen may contribute to the poorer outcome of embryo development following SUZI. Prolonged in-vitro culture beyond 48 h appears to be deleterious to the development of SUZI cleaved embryos and the subsequent outcome of treatment, and hence should be avoided. 相似文献
1000.
BACKGROUND: Previous studies suggest differences in the fixation disparity curves obtained with the Sheedy Disparometer and the Wesson Fixation Disparity Card, the two most commonly used methods for measuring fixation disparity. In one study the investigators proposed that the differences do not exist for subgroups divided by phoria. The purpose of this paper is to try to clarify this issue by use of two large sets of data. METHODS: Dissociated phorias were measured by the von Graefe method. Fixation disparity curves were plotted using the Disparometer and the Wesson card. RESULTS: Type I fixation disparity curves were most common with the Wesson card. Type II curves were found more often with the Disparometer than with the Wesson card. The x-intercepts were shifted in the base-in (BI) direction with the Wesson card compared to the Disparometer. The y-intercepts were shifted in the exo direction with the Wesson card compared to the Disparometer. The differences were statistically significant regardless of whether the dissociated phoria was exo or eso. The slope of the fixation disparity curve was steeper with the Wesson card than with the Disparometer. The difference was statistically significant for exophores but not for esophores. The differences between results obtained with the two instruments are not consistent from one subject to another as shown by high standard deviations of the differences. CONCLUSIONS: The fixation disparity curves measured with these two instruments are different. Fixation disparity parameters obtained from one of these instruments cannot be used with normative findings from the other. 相似文献