全文获取类型
收费全文 | 2877篇 |
免费 | 32篇 |
国内免费 | 3篇 |
专业分类
电工技术 | 15篇 |
综合类 | 2篇 |
化学工业 | 243篇 |
金属工艺 | 15篇 |
机械仪表 | 9篇 |
建筑科学 | 9篇 |
矿业工程 | 2篇 |
能源动力 | 24篇 |
轻工业 | 33篇 |
石油天然气 | 2篇 |
无线电 | 52篇 |
一般工业技术 | 232篇 |
冶金工业 | 2182篇 |
原子能技术 | 3篇 |
自动化技术 | 89篇 |
出版年
2023年 | 6篇 |
2022年 | 27篇 |
2021年 | 28篇 |
2020年 | 13篇 |
2019年 | 16篇 |
2018年 | 39篇 |
2017年 | 32篇 |
2016年 | 38篇 |
2015年 | 27篇 |
2014年 | 32篇 |
2013年 | 65篇 |
2012年 | 28篇 |
2011年 | 26篇 |
2010年 | 18篇 |
2009年 | 26篇 |
2008年 | 34篇 |
2007年 | 22篇 |
2006年 | 22篇 |
2005年 | 11篇 |
2004年 | 23篇 |
2003年 | 17篇 |
2002年 | 25篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 85篇 |
1998年 | 637篇 |
1997年 | 372篇 |
1996年 | 274篇 |
1995年 | 165篇 |
1994年 | 123篇 |
1993年 | 144篇 |
1992年 | 32篇 |
1991年 | 40篇 |
1990年 | 35篇 |
1989年 | 44篇 |
1988年 | 49篇 |
1987年 | 44篇 |
1986年 | 28篇 |
1985年 | 40篇 |
1984年 | 2篇 |
1983年 | 10篇 |
1982年 | 11篇 |
1981年 | 16篇 |
1980年 | 17篇 |
1978年 | 3篇 |
1977年 | 44篇 |
1976年 | 89篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1955年 | 1篇 |
排序方式: 共有2912条查询结果,搜索用时 812 毫秒
1.
In this communication, the structural, micro-structural, dielectric, electrical, magnetic, and leakage-current characteristics of a double perovskite (Y2CoMnO6) ceramic material have been reported. The material was synthesized via a high-temperature mixed-oxide route. The compound crystallizes in a monoclinic structure which is confirmed from preliminary X-ray structural study. The morphological study by using scanning electron micrograph reveals the almost homogeneous distribution of grains throughout the surface of the sample. The nature of frequency-dependence of dielectric constant has been described by the Maxwell-Wagner model. The occurrence of a dielectric anomaly in the temperature dependence of dielectric permittivity study demonstrates the ferroelectric-paraelectric phase transition in the material. From the Nyquist plots, we found the existence of both grain and grain boundary effects. The frequency dependence of conductivity was studied by the Jonscher’s Power law, and the conduction phenomenon obeys the large overlapping polaron tunneling model. By using the Arrhenius equation, the activation energy has been calculated which is nearly equal to the energy required for the hoping of the electron. Both impedance and conductivity analysis demonstrate that the sample exhibits negative temperature coefficient of resistance (NTCR) properties indicating the semiconducting type of material at high temperatures. The anti-ferromagnetic character of the material is observed from the nature of magnetic hysteresis loop. The leakage current analysis suggests that the conduction process in the material follows the space charge limited conduction phenomenon. Such material will be helpful for modern electronic devices and spintronic applications. 相似文献
2.
3.
PE Sipila VJ Wiebe GB Hubbard SK Koester VD Emshoff JU Maenpaa GT Wurz RC Seymour MW DeGregorio 《Canadian Metallurgical Quarterly》1993,(15):2138-2144
The effects of long-term tamoxifen exposure on cell growth and cell cycle kinetics were compared between oestrogen receptor (ER)-positive (MCF-7) and ER-negative (MDA-MB-231) cell lines. In the MCF-7 cell line, prolonged tamoxifen exposure (0.5 mumol/l for > 100 days) blocked cells in G0-G1 of the cell cycle, and slowed the doubling time of cells from 30 to 59 h. These effects corresponded to an increase in the cellular accumulation of tamoxifen over time [mean area under concentration curve (AUC) = 77.92 mumoles/10(6)/cells/day]. In contrast, in the MDA-MB-231 cell line, long-term tamoxifen exposure had no obvious effect on the doubling time, and reduced cellular tamoxifen accumulation (mean AUC = 50.50 mumoles/10(6)/cells/day) compared to the MCF-7 cells. Flow cytometric analysis of MDA-MB-231 cells demonstrated that a new tetraploid clone emerged following 56 days of tamoxifen exposure. Inoculation of the MDA-MB-231 tetraploid clone and MDA-MB-231 wildtype cells into the opposite flanks of athymic nude mice resulted in the rapid growth of tetraploid tumours. The tetraploid tumours maintained their ploidy following tamoxifen treatment for nine consecutive serial transplantations. Histological examination of the fifth transplant generation xenografts revealed that the tetraploid tumour had a 25-30 times greater mass, area of haemorrhage and necrosis, a slightly higher mitotic index and was more anaplastic than the control neoplasm. The control wildtype MDA-MB-231 tumours maintained a stable ploidy following tamoxifen treatment until the eighth and ninth transplantation, when a tetraploid population appeared, suggesting that tamoxifen treatment may select for this clone in vivo. These studies suggest that prolonged tamoxifen exposure may select for new, stable, fast growing cell clones in vitro as well as in vivo. 相似文献
4.
5.
Pyrolysis experiments were performed in high vacuum and under reduced air pressure (100 Pa). The volatile products of pure cellulose and cellulose containing various amounts of flame retardant 2,2′-oxybis (5,5-dimethyl-1,3,2-dioxaphosphorinane-2,2′-disulfide), i.e., Sandoflam 5060 of Sandoz AG, were studied by means of gas chromatography in combination with mass spectroscopy. The volatile products were characterized with infrared spectroscopy. The studied revealed that the incorporation of the flame retardant enhanced the water release and shifted the onset of this reaction to lower temperature. On the basis of these findings an explanation for the mechanism of flame retardancy in generated cellulose fibers modified with this particular flame retardant is attempted. From experiments with different residual air pressure the influence of oxygen on the primary processes of the pyrolytic degradation of cellulose is being discussed. 相似文献
6.
7.
8.
9.
10.
The purpose of the present study was to evaluate the accuracy of data regarding diagnoses of spinal disorders in administrative databases at eight different institutions. The records of 189 patients who had been managed for a disorder of the lumbar spine were independently reviewed by a physician who assigned the appropriate diagnostic codes according to the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM). The age range of the 189 patients was seventeen to eighty-four years. The six major diagnostic categories studied were herniation of a lumbar disc, a previous operation on the lumbar spine, spinal stenosis, cauda equina syndrome, acquired spondylolisthesis, and congenital spondylolisthesis. The diagnostic codes assigned by the physician were compared with the codes that had been assigned during the ordinary course of events by personnel in the medical records department of each of the eight hospitals. The accuracy of coding was also compared among the eight hospitals, and it was found to vary depending on the diagnosis. Although there were both false-negative and false-positive codes at each institution, most errors were related to the low sensitivity of coding for previous spinal operations: only seventeen (28 per cent) of sixty-one such diagnoses were coded correctly. Other errors in coding were less frequent, but their implications for conclusions drawn from the information in administrative databases depend on the frequency of a diagnosis and its importance in an analysis. This study demonstrated that the accuracy of a diagnosis of a spinal disorder recorded in an administrative database varies according to the specific condition being evaluated. It is necessary to document the relative accuracy of specific ICD-9-CM diagnostic codes in order to improve the ability to validate the conclusions derived from investigations based on administrative databases. 相似文献