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   198468篇
  免费   2147篇
  国内免费   632篇
电工技术   4063篇
综合类   123篇
化学工业   26964篇
金属工艺   7516篇
机械仪表   5473篇
建筑科学   4052篇
矿业工程   747篇
能源动力   4685篇
轻工业   14463篇
水利工程   1698篇
石油天然气   2846篇
武器工业   7篇
无线电   27346篇
一般工业技术   37474篇
冶金工业   45396篇
原子能技术   3563篇
自动化技术   14831篇
  2021年   1298篇
  2019年   1305篇
  2018年   2148篇
  2017年   2126篇
  2016年   2207篇
  2015年   1511篇
  2014年   2656篇
  2013年   7996篇
  2012年   4508篇
  2011年   6151篇
  2010年   4968篇
  2009年   5807篇
  2008年   6168篇
  2007年   6219篇
  2006年   5651篇
  2005年   5318篇
  2004年   5259篇
  2003年   5101篇
  2002年   4909篇
  2001年   5312篇
  2000年   4919篇
  1999年   5439篇
  1998年   15623篇
  1997年   10314篇
  1996年   7933篇
  1995年   5785篇
  1994年   5016篇
  1993年   5024篇
  1992年   3389篇
  1991年   3266篇
  1990年   3218篇
  1989年   3041篇
  1988年   2785篇
  1987年   2239篇
  1986年   2303篇
  1985年   2613篇
  1984年   2316篇
  1983年   2067篇
  1982年   1904篇
  1981年   2044篇
  1980年   1794篇
  1979年   1661篇
  1978年   1657篇
  1977年   2022篇
  1976年   2702篇
  1975年   1422篇
  1974年   1370篇
  1973年   1316篇
  1972年   1125篇
  1971年   959篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
M-CSF is a macrophage-lineage-specific growth factor that causes proliferation and differentiation of progenitor cells in the bone marrow. To investigate the effects of M-CSF on more matured cells, human monocytes were cultured in the presence or absence of M-CSF for 6 days. Addition of M-CSF at more than 10(2) U/ml resulted in higher viability and caused morphological differentiation to large macrophage-like cells. LPS-induced mediator production was also compared between M-CSF-treated and control cell. Monocytes were incubated with or without M-CSF for 3 days, and were stimulated with 1 microgram/ml of LPS for 2 days. IL-1 beta was not detected in the both culture supernatants, and PGE2 production was not influenced by M-CSF. However, amounts of G-CSF, GM-CSF, IL-6, and TNF-alpha produced in response to 1 microgram/ml of LPS were 1.5 to 2 times greater from monocytes treated with 10(4) U/ml of M-CSF than from control cells. The priming effect of M-CSF on LPS-induced cytokine production was found to require 3-day preincubation, and reached a maximum at the concentration of 10(4) U/ml. M-CSF-treated cells responded to a 10 times lower concentration of LPS than control cells in terms of cytokine production. M-CSF was also shown by flowcytometric analysis to influence the expression of CD14, a receptor for LPS, which might render monocytes more sensitive to LPS.  相似文献   
992.
The problem of detonation-wave structure is studied on the basis of a mathematical model for the detonation of aluminum particles in oxygen within the framework of a single-velocity two-temperature continuum. An analysis of flow types in the form of the Chapman-Jouguet strong-detonation and weak-detonation regimes is given. A chart of the mixture flow regimes in the plane of the Mach number of the detonation wave and the ratio of the characteristic times of thermal relaxation and combustion is constructed using the results of numerical experiments. The domain of realization of only strong detonation regimes, the manifold of existence of weak and strong detonation regimes, and the domain of nonexistence of stationary solutions are determined. The structural properties of the solutions with an internal singular point and weak structurally unstable regimes with a saddle singularity in the final state are described. The stability of all types of stationary regimes against small and finite perturbations that retain the detonation-wave (DW) velocity and the final state is shown by numerical modeling of nonstationary detonation flows.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 2, pp. 74–85, March–April, 1996.  相似文献   
993.
A 13-week subchronic toxicity study of josamycin was performed in male and female F344 rats to determine the maximum tolerable dose (MTD) for subsequent investigation of the carcinogenicity. As animals refused to take diet containing 5.0% josamycin in our preliminary study, dose levels in the present study were determined as 0, 0.16, 0.32, 0.63, 125 and 2.5% in diet. Rats were randomly allocated to 6 groups, each consisting of 10 males and 10 females. No animal died during the administration period and no group showed significant changes in body weight gain. Definite toxicity of josamycin was not noted in hematological and serum biochemical examinations. Histopathological examinations revealed no particular findings related to josamycin administration except cecal enlargement in the 1.25 and 2.5% groups. based on the results of the present study, it was concluded that the MTD of josamycin in 2.5% in diet, because the dietary dose level of 2.5% proved to exert no significant toxicological signs.  相似文献   
994.
Dissimilar-alloy welds have been produced between Ti---6Al---2Sn---Z4---2Mo---0.1Si (wt. %) and Ti---13.5Al---21.5 NB (wt.%) titanium aluminde using the three different solid-phase welding processes that create significantly different thermo-mechanical conditions at the weld interface. Exposure to supertransus temperatures, appreciable deformation and rapid cooling of the weld interface region during linear-friction welding promote dynamic recrystallization of beta grains and beta decomposition to fine martensitic products. In contrast, diffusion welding at temperatures below the base metal beta transus temperatures and at relatively low pressures minimizes deformation and microstructural variations in the weld interface region relative to the unaffected base metal. During capacitor-discharge resistance spot welding, extremely rapid heating of the weld interface region to near-solidus temperatures, and subsequent rapid cooling, result in the formation of a metastable, ordered-beta microstructure in the Ti---s13.5Al---21.6Nb and fine alpha-prime martensite in the Ti---6Al---2Sn---4Zr---2Mo-0.1Si.  相似文献   
995.
Translated from Steklo i Keramika, No. 2, pp. 13–16, February, 1994.  相似文献   
996.
OBJECTIVES: This study examined US trends in nutrient intake, using almost identical methods and nutrient databases in two time periods. METHODS: An extensive dietary intake questionnaire was included in supplements to the 1987 and 1992 National Health Interview Surveys. Dietary data from approximately 11,000 persons in each of those years were analyzed. RESULTS: The total and saturated fat intake and the percentage of energy from fat declined among Whites and Hispanics, but only minimal changes were seen in Black Americans. The changes in fat intake were attributable principally to behavioral changes in frequency and type of fat-containing foods consumed rather than to the increased availability of leaner cuts of meat. Dietary cholesterol showed one of the largest declines of the nutrients examined. Less desirable changes were also seen. Cereal fortification played an important role in the observed changes in several micronutrients. CONCLUSIONS: Educational campaigns on dietary fat and cholesterol have been moderately effective, but not in all racial/ethnic groups. Future campaigns should emphasize maintaining or increasing micronutrient intake.  相似文献   
997.
Recent advances in control systems analysis and design have implied new uses for the Lyapunov equation of the form AX+XAT+Q=0. Implementation requirements for the incorporation of the use of Lyapunov equations in practical design, however, point out the need for a set of specialized numerical procedures. This special set of numerical procedures must efficiently solve large, sparse Lyapunov equations, solve sets of Lyapunov equations that differ only in the coefficient matrix Q, and provide good low rank estimates of the Lyapunov equation solution X in the case where low rank approximations are applicable. Discussions of the motivations for the solution of these problems and of candidate solution approaches are provided  相似文献   
998.
999.
The specific heat of 4He condensed on an evaporated gold surface has been measured for coverages between 0.007 and 0.096 Å?2 and temperatures between 0.4 and 3.0 K. There are at least two types of physisorption site on the gold surface with binding energies ?83 ± 7 and ?94 ± 7 K. For coverages below 0.06 Å?2 (~0.5 monolayers), the 4He is a two-dimensional classical gas at sufficiently high temperatures. At lower temperatures and higher coverages, the 4He forms condensed phases, including liquids, solids commensurate with the gold lattice, and incommensurate solids. The phase diagram of 4He/evaporated gold is similar to that for 4He/graphite, but the phase boundaries are shifted and not well defined, nor is the diagram as rich in structure.  相似文献   
1000.
Taking into consideration the knock-on cascade mechanism and the free volume theory, the cooling condition dependentR s (ion etching rate) is investigated for a liquid-quenched Fe-10at%Si-15at%B alloy glass. A highR s is found in a slow cooled glass.  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