首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   772篇
  免费   10篇
电工技术   10篇
化学工业   73篇
金属工艺   17篇
机械仪表   11篇
建筑科学   45篇
矿业工程   11篇
能源动力   21篇
轻工业   38篇
水利工程   9篇
石油天然气   3篇
无线电   124篇
一般工业技术   95篇
冶金工业   260篇
原子能技术   1篇
自动化技术   64篇
  2023年   4篇
  2021年   10篇
  2019年   5篇
  2018年   7篇
  2015年   10篇
  2014年   5篇
  2013年   30篇
  2012年   20篇
  2011年   29篇
  2010年   23篇
  2009年   17篇
  2008年   19篇
  2007年   19篇
  2006年   14篇
  2005年   17篇
  2004年   12篇
  2003年   13篇
  2002年   10篇
  2001年   6篇
  2000年   5篇
  1999年   17篇
  1998年   79篇
  1997年   47篇
  1996年   34篇
  1995年   34篇
  1994年   24篇
  1993年   21篇
  1992年   13篇
  1991年   10篇
  1990年   7篇
  1989年   13篇
  1988年   18篇
  1987年   13篇
  1986年   8篇
  1985年   16篇
  1984年   8篇
  1983年   8篇
  1982年   10篇
  1981年   9篇
  1980年   11篇
  1979年   8篇
  1978年   7篇
  1977年   16篇
  1976年   19篇
  1975年   4篇
  1974年   9篇
  1973年   4篇
  1972年   6篇
  1970年   3篇
  1968年   3篇
排序方式: 共有782条查询结果,搜索用时 0 毫秒
21.
A 50-question survey was developed by the land grant universities in the Pacific Northwest to document public awareness, aptitudes, attitudes and actions toward water issues in the states of Alaska, Idaho, Oregon and Washington, USA. Demographic data were also collected about the survey respondents. The statistically designed survey was completed by over 50% of the 1800 residents who were solicited for the study. The public consider clean groundwater a high priority issue. Survey respondents currently consider groundwater quality to be good; however, most people are unaware of groundwater quantity issues. In the last 5 years, a majority of survey respondents have taken voluntary actions both to protect groundwater quality and to conserve the use of this resource. People want the government to focus more attention on groundwater issues; however, they are unwilling to pay increased taxes to achieve this end. Based on this survey, the land grant universities need to increase groundwater education programmes for the public using television, radio, newspapers and the Internet rather than by the traditional workshop delivery methods.  相似文献   
22.
Daily subcutaneous administration of bovine parathyroid hormone (PTH)(1-34) stimulates bone formation and increases bone mass in rat tibiae, femora and lumbar spine. However, the effects of PTH on the whole body bone mineral content and density determined by dual energy x-ray absortiometry (DEXA) have not been previously reported in rats. Eighteen-month-old intact female rats were subcutaneously injected daily with 0, 40, 80 or 160 micrograms/kg/day of bovine PTH (1-34) for either 15 or 60 days. Whole body DEXA was performed at 1 day before autopsy, and bone area, bone mineral content (BMC) and bone mineral density (BMD) of the total body were determined. Total femoral, tibial and lumbar spine BMD was also determined ex vivo. Cancellous bone histomorphometry was performed on sections of double-labeled proximal tibial metaphyses. Whole body bone mineral content and density were significantly increased by 60 days, but not by 15 days, of PTH treatment at all dose groups compared with vehicle controls. Lumbar vertebral and total femoral BMD was significantly increased at all doses of PTH by 15 days of administration and further increased by 60 days. All doses of PTH increased trabecular bone area in proximal tibial metaphyses by 15 days and further increased by 60 days. All doses of PTH increased trabecular bone area in proximal tibial metaphyses by 15 days and further increased by 60 days. In proximal tibial cancellous bone, dose-dependent increases in percent labeled perimeter, mineral apposition rate and bone formation rate-bone volume referent were found between 40 and 160 micrograms/kg of PTH treatment by 15 days, and no further increases were found by 60 days. Our results showed that in aged female rats, bovine PTH(1-34) increased bone formation and total body bone mass.  相似文献   
23.
Three modes of combustion are possible in an aircraft engine fire, and these have been simulated separately in a small wind tunnel.  相似文献   
24.
The transient current of an MIS (metal-insulator-silicon) capacitor which is subjected to a voltage step (or pulse) has been investigated. It is shown that the different magnitudes and shapes exhibited by the transient current depend on the initial and final biasing conditions. By varying these conditions the three main contributing current components, namely, surface emission, surface generation and bulk generation can be identified.  相似文献   
25.
Theoretical studies have been made on the dynamic characteristics of the metal-insulator-semiconductor (MIS) capacitor containing distributed surface traps. It has been shown that when the surface traps are in dynamic equilibrium with the voltage ramp, the device exhibits steady-state charge, current and capacitance characteristics. When the surface traps are out of equilibrium with the voltage ramp, then the emission of trapped charge is a function of time only and not of voltage. Under such conditions, the characteristics are considered to be non-steady-state in nature.In the steady state, the emission of electrons from a continuum of surface traps accounts for the reduction in the slopes of the CV curves from the ideal ones. Kinks are manifested when the traps just empty the last of their electrons.In the non-steady state, electron emission can be described by the non-steady-state (time-dependent) occupancy function derived herein, which is shown to be similar in shape to the Fermi function. This means that electron emission takes place from a narrow band of energy positioned near the uppermost-filled traps. Hysteresis effects are manifested in the CV characteristics due to the non-steady-state emission of trapped charge. At the transition from the steady state to the non-steady state and vice versa, kinks are exhibited in the charge and capacitance characteristics, while step changes in current components are also predicted.The physical processes involved have been stressed and closed-form expressions have been obtained for the charge, current and capacitance in terms of the trapping parameters, sweep-rate and temperature.  相似文献   
26.
Despite its concentration on technical developments in library science, this survey is offered here because it is relevant to humanists for two reasons: first, that humanists all use libraries and therefore need to encourage their evolution to higher levels of efficiency, and, second, that the procedures outlined here may be of help to humanists in establishing procedures for their own research.  相似文献   
27.
28.
To study the effect of re-immunization against inhibin on ovarian response and hormonal profiles, Japanese beef heifers (n = 5) were re-immunized three times with inhibin vaccine (recombinant ovine inhibin alpha-subunit in oil emulsion, 125 microg ml(-1)) one year after the primary immunization. Control heifers (n = 5) were injected with placebo (Montanide: Marcol adjuvant alone). Oestrous cycles were synchronized by using prostaglandin F2alpha (PGF2alpha) and ovarian response was monitored daily by ultrasonography. Blood samples were collected by jugular venipuncture for assessment of hormonal levels and inhibin antibody titres. In contrast to controls, inhibin re-immunized heifers generated antibodies against inhibin rapidly reaching a peak level 9 days after the first booster injection. The mean concentrations of FSH in re-immunized cows increased significantly in comparison with controls. In addition, there was a significant increase in oestradiol-17beta and progesterone levels in re-immunized cows compared with controls. Inhibin re-immunized heifers had a significant increase in small (> or =4 < 7 mm), medium (> or =7 < 10 mm) and large (> or =10 mm in diameter) sized follicles. Moreover, the mean ovulation rate was 5.0 +/- 1.1 after the third booster injection in re-immunized heifers compared with control heifers (single ovulation). These results clearly demonstrate that re-immunization of inhibin can be used to enhance ovarian follicular development and ovulation rate. Furthermore, the great number of follicles is a potential source of oocytes that could be harvested for in vitro fertilization and embryo transfer programmes.  相似文献   
29.
Under certain conditions, traditional hypothesis-testing techniques may be used as a management tool by software developers or software purchasers who wish to insure that their packages have some specified reliability level. These conditions are: (1) the existence of independent collections of test data, (2) a way of determining the correctness of processing of these collections, and (3) a way of randomly selecting test data.Two basic approaches have been described. In a fixed sample size test, the user decides on the reliability desired. He can then determine the number of test cases which must be examined and the acceptance/rejection criteria. In a sequential test, the desired reliability level is again pre-determined, but samples are tested one at a time until an accept/reject decision can be made.Experiments with a large amount of error data derived from six separate systems indicate that reliability results derived from these models are consistent with actual reliability figures.Most current acceptance procedures are based on a naive assumption that a large program can be exhaustively tested and delivered in an error-free condition. Because these expectations cannot be fulfilled, the manager of a software development project or the purchaser of a software product is provided with no quantitative information on which to base an acceptance decision and is thus forced to make these decisions based mostly on intuition and his own experience in similar situations. These models allow one to replace these intuition-based decisions with quantitatively-based decisions and thus constitute an important contribution to the science of management of software development efforts.  相似文献   
30.
Effects of water and a variety of organic liquids on crack-growth rates in soda-lime-silica glass was investigated. When water is present in organic liquids, it is usually the principal agent that promotes subcritical crack growth in glass. In region I, subcritical crack growth is controlled primarily by the chemical potential of the water in the liquid; whereas in region II, crack growth is controlled by the concentration of water and the viscosity of the solution formed by the water and the organic liquid. In region III, where water does not affect crack growth, the slope of the crack-growth curves can be correlated with the dielectric constant of the liquid. It is suggested that these latter results can be explained by electrostatic interactions between the environment and charges that form during the rupture of Si-O bonds.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号