全文获取类型
收费全文 | 178598篇 |
免费 | 2488篇 |
国内免费 | 707篇 |
专业分类
电工技术 | 3307篇 |
综合类 | 101篇 |
化学工业 | 28311篇 |
金属工艺 | 7814篇 |
机械仪表 | 5144篇 |
建筑科学 | 4508篇 |
矿业工程 | 878篇 |
能源动力 | 4705篇 |
轻工业 | 16581篇 |
水利工程 | 1717篇 |
石油天然气 | 3128篇 |
武器工业 | 2篇 |
无线电 | 19973篇 |
一般工业技术 | 33622篇 |
冶金工业 | 33503篇 |
原子能技术 | 4313篇 |
自动化技术 | 14186篇 |
出版年
2021年 | 1352篇 |
2019年 | 1279篇 |
2018年 | 2160篇 |
2017年 | 2129篇 |
2016年 | 2243篇 |
2015年 | 1611篇 |
2014年 | 2769篇 |
2013年 | 7898篇 |
2012年 | 4645篇 |
2011年 | 6485篇 |
2010年 | 5138篇 |
2009年 | 5960篇 |
2008年 | 5958篇 |
2007年 | 5928篇 |
2006年 | 5078篇 |
2005年 | 4757篇 |
2004年 | 4544篇 |
2003年 | 4199篇 |
2002年 | 4151篇 |
2001年 | 4161篇 |
2000年 | 3948篇 |
1999年 | 4083篇 |
1998年 | 10427篇 |
1997年 | 7421篇 |
1996年 | 5688篇 |
1995年 | 4308篇 |
1994年 | 3652篇 |
1993年 | 3597篇 |
1992年 | 2633篇 |
1991年 | 2559篇 |
1990年 | 2440篇 |
1989年 | 2460篇 |
1988年 | 2392篇 |
1987年 | 2142篇 |
1986年 | 2078篇 |
1985年 | 2384篇 |
1984年 | 2204篇 |
1983年 | 2029篇 |
1982年 | 1899篇 |
1981年 | 1960篇 |
1980年 | 1809篇 |
1979年 | 1838篇 |
1978年 | 1783篇 |
1977年 | 2114篇 |
1976年 | 2692篇 |
1975年 | 1562篇 |
1974年 | 1551篇 |
1973年 | 1610篇 |
1972年 | 1352篇 |
1971年 | 1263篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
171.
Laparoscopic retroperitoneal lymph node dissection is a new surgical procedure used to enhance staging in men with clinical stage I nonseminomatous germ cell tumors of the testis. The procedure has been performed in a limited number of patients at several centers with extensive laparoscopic experience. Laparoscopic retroperitoneal lymphadenectomy is a technically demanding procedure which can be successfully completed in the majority of patients. However, the risk of complications is greater than in patients who undergo standard open retroperitoneal lymph node dissection. The primary advantage of a laparoscopic approach is shortened hospitalization and rapid return to normal activity. The role of laparoscopy in the management of patients with testis malignancy has not been defined. The use of this staging procedure may help minimize the need for surveillance studies following surgery and may be best utilized in men with a lower likelihood of nodal metastases. Ultimately, prospective study in large groups of patients will be necessary to determine the role of laparoscopic retroperitoneal lymph node dissection in patients with testis cancer. 相似文献
172.
F.D. Lydon 《Construction and Building Materials》1993,7(4):213-220
A modified version of a test, originally proposed by BRE (UK), was used to assess relative permeability to nitrogen gas of concretes cured in various ways. Three simple parameters from test plots clearly showed the effects of duration of water curing, followed by drying to constant mass at 50°C; test specimen moisture content was dominant and drying at 105°C reduced differences between concretes of very different quality. Convincing evidence was not found of differences between upper and lower ‘halves’ of cylinders nor between cast and cored ones. 相似文献
173.
D Sternberg A Peled E Shezen O Abramsky W Jiang F Bertolero D Zipori 《Canadian Metallurgical Quarterly》1996,2(1):29-38
The immunological agglutination reactions of physically absorbed F(ab')2 molecules onto anionic and cationic latex particles have been investigated by means of optical absorbance measurements. These measurements have been conducted under different conditions to determine the most influential factors. Surface F(ab')2 and BSA densities, particle concentration in the reaction medium and polyethylene glycol concentration are some of these factors. Sensitized cationic and anionic latexes differ considerably with respect to their colloidal stability and reactivity. As a general rule, the sensitized cationic latex has a relatively higher colloidal stability and hence, it provides reagents with a better optical response. Less than 0.025 microgram/ml of C-reactive protein has been detected using this particle enhanced optical immunoassay. 相似文献
174.
Least squares is perhaps the most widely used technique for model fitting. In this article, we illustrate the poor performance of least squares when there are spurious values, or outliers, in a sequence of measurements. A brief overview of three well-known classes of robust alternatives to the least-squares mean is presented. For robust regression, a recent proposal called least median squares (LMS) is decribed. LMS regression is compared to least-squares regression in an example involving the estimation of optical fiber geometry. References are provided for software that is available for robust estimation techniques surveyed in this article. 相似文献
175.
Vivien Walsh Robin Roy Margaret Bruce Stephen Potter 《Creativity & Innovation Management》1993,2(2):78-86
A series of studies of top European and other firms has revealed patterns of design management associated with commercial success. Firms that invest resources and professional expertise in product and industrial design in traditional and new industries have been commercially more successful than firms that pay less attention to these aspects of design. As an industry matures there is a shift in emphasis from design associated with technological innovation, to designs supporting technical improvements, and then to supporting user needs, fashion and product variants. These issues are illustrated through the history of the evolution of the bicycle. 相似文献
176.
F Abadía-Molina R Calvente R Carmona R Luján F Abadía-Fenoll 《Canadian Metallurgical Quarterly》1993,8(2):297-304
In this study chick embryo optic cups at HH stage 13 of development were analyzed under normal conditions and after inoculation with colchicine for 1, 2, 4, and 8 h. Several changes were seen after these periods of treatment: 1) modifications of the structure, with thicker regions in the cup and a general decrease in the total volume according to the duration of exposure to the drug (about 4 times less than normal, 5,035 x 10(3) microns 3 vs 1,334 x 10(3) microns 3 after 8 h of treatment); 2) enlargement of the ventricular cavity and its closure, due to failure of approximation of retinal and pigmentary layers; 3) failure of lens development, with delay and impairment of pit formation and deformation of all structures; lens volume was less than normal (about 4 times less, 2,148 x 10(3) microns 3 vs 658 x 10(3) microns 3 after 8 h of treatment); 4) a general segregation of the cells making up the structure, principally in the more active proliferating zones. The local alterations found are described. 相似文献
177.
It is now generally recognized that cell growth conditions in nature are often suboptimal compared to controlled conditions provided in the laboratory. Natural stresses like starvation and acidity are generated by cell growth itself. Other stresses like temperature or osmotic shock, or oxygen, are imposed by the environment. It is now clear that defense mechanisms to withstand different stresses must be present in all organisms. The exploration of stress responses in lactic acid bacteria has just begun. Several stress response genes have been revealed through homologies with known genes in other organisms. While stress response genes appear to be highly conserved, however, their regulation may not be. Thus, search of the regulation of stress response in lactic acid bacteria may reveal new regulatory circuits. The first part of this report addresses the available information on stress response in Lactococcus lactis. Acid stress response may be particularly important in lactic acid bacteria, whose growth and transition to stationary phase is accompanied by the production of lactic acid, which results in acidification of the media, arrest of cell multiplication, and possible cell death. The second part of this report will focus on progress made in acid stress response, particularly in L. lactis and on factors which may affect its regulation. Acid tolerance is presently under study in L. lactis. Our results with strain MG1363 show that it survives a lethal challenge at pH 4.0 if adapted briefly (5 to 15 minutes) at a pH between 4.5 and 6.5. Adaptation requires protein synthesis, indicating that acid conditions induce expression of newly synthesized genes. These results show that L. lactis possesses an inducible response to acid stress in exponential phase. To identify possible regulatory genes involved in acid stress response, we determined low pH conditions in which MG1363 is unable to grow, and selected at 37 degrees C for transposition insertional mutants which were able to survive. About thirty mutants resistant to low pH conditions were characterized. The interrupted genes were identified by sequence homology with known genes. One insertion interrupts ahrC, the putative regulator of arginine metabolism; possibly, increased arginine catabolism in the mutant produces metabolites which increase the pH. Several other mutations putatively map at some step in the pathway of (p)ppGpp synthesis. Our results suggest that the stringent response pathway, which is involved in starvation and stationary phase survival, may also be implicated in acid pH tolerance. 相似文献
178.
Social burden caused by paralyzed poliomyelitis was analysed, using data from a Sampling Survey of Handicapped People in China, in 1987. The study results showed: 29.94% of 1707 handicapped polio victims aged 6 or over were not able to go to school; 44.23% of them could not take care of their daily life; 36.32% of them could not play out-door and do shopping; 2.58% of them could not normally communicate with others. 84.53% of the 1487 handicapped victims caused by polio aged 15 or over wholly or partly lost their working ability. We also compared handicapped people's employment rate and the status of marriage with non-handicapped people. Data showed that the employment rate and the number of married handicapped polio victims were remarkably lower then those non-handicapped people. 相似文献
179.
180.