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   161336篇
  免费   2743篇
  国内免费   1402篇
化学   88236篇
晶体学   2593篇
力学   7155篇
综合类   64篇
数学   15298篇
物理学   52135篇
  2021年   1345篇
  2020年   1545篇
  2019年   1618篇
  2018年   1966篇
  2017年   1966篇
  2016年   2965篇
  2015年   1989篇
  2014年   2901篇
  2013年   6839篇
  2012年   5848篇
  2011年   7147篇
  2010年   5036篇
  2009年   5072篇
  2008年   6635篇
  2007年   6321篇
  2006年   6187篇
  2005年   5624篇
  2004年   5007篇
  2003年   4591篇
  2002年   4428篇
  2001年   6109篇
  2000年   4436篇
  1999年   3431篇
  1998年   2419篇
  1997年   2484篇
  1996年   2348篇
  1995年   2178篇
  1994年   2132篇
  1993年   1895篇
  1992年   2361篇
  1991年   2475篇
  1990年   2266篇
  1989年   2219篇
  1988年   2169篇
  1987年   2149篇
  1986年   1958篇
  1985年   2424篇
  1984年   2435篇
  1983年   1939篇
  1982年   1947篇
  1981年   1782篇
  1980年   1760篇
  1979年   2046篇
  1978年   2123篇
  1977年   2063篇
  1976年   1992篇
  1975年   1868篇
  1974年   1845篇
  1973年   1874篇
  1972年   1232篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
A new method for high-quality hydroxyapatite (HAp) coating is developed, the laser-assisted laser ablation method. In this method, two lasers are used. One is used for ablation of a HAp target. The other, the assist laser, is used to irradiate a Ti substrate surface. The effects of the assist laser irradiation are to anneal the HAp coating and to improve its adhesive strength to the Ti substrate. The quality of the HAp coating depends on the delay time of the assist-laser irradiation. HAp coatings obtained at a delay time of 10 microseconds or more are more amorphous. It was confirmed that the amorphous part of a coating dissolves in simulated body fluid, while the crystalline part does not. The value of the Ca/P ratio in a coating obtained at a delay time of a few microseconds is close to the stoichiometric value. PACS 81.16.Mk  相似文献   
982.
The solubility diagrams of the potassium iodide-water-N,N-dimethylformamide (DMFA) and iodine-iodide-water-N,N-dimethylformamide systems were studied at 25°C by the method of sections. The systems are characterized by three-phase equilibria of the peritonic and eutonic types, respectively. Potassium iodide in the I2-KI-/H2O-DMFA (95: 5 wt %)/ system was found to salt in crystalline iodine. The selected composition of the H2O-DMFA mixed solvent possessed a higher iodine-solving ability compared with pure mixture components.  相似文献   
983.
The flow induced by a finite disc rotating near horizontal ground is considered, including the effects of an underbody. This paper concentrates on determining the shape of the free layer beyond the rim of the disc which is horizontal in the absence of the underbody and ground but forced to deform to ensure that conditions across the layer are satisfied when the underbody or ground is added. The far-field behaviour, the inviscid flow produced by a nominally infinite disc near the ground and the global solution for small ground clearances are considered analytically, and the full problem is posed as an integral problem. This is then solved numerically and analytically. Results are presented for various heights of the disc above the ground and for discs with an axisymmetric underbody present. A universal form is found for the farfield shape (which is controlled by entrainment into the free layer) but both the underbody and the ground effects are found to increase very significantly for reduced clearances.  相似文献   
984.
We describe one convenient synthesis route to boron nitride (BN) nanotube by the reaction of boron powder, iron oxide, and ammonium chloride at 600 °C for 12 h. Characterized by XRD, FTIR, XPS, TEM and SAED, the composition and morphology of the products are confirmed. The possible reaction mechanism is also discussed.  相似文献   
985.
986.
Specific features of the chemical behavior and structure of copper(II) trimethylacetate complexes with 3,5-dimethylpyrazole have been considered based on X-ray crystallographic data.  相似文献   
987.
Many food algae and red tide algae were cultivated in the f/2 medium, and the nitric oxide (NO) concentration of the medium and the cell density were determined. The experiments on algae were conducted when different concentrations of NO were added into the medium using two methods. The results show that low concentrations of NO were self-produced by marine algae during the exponential growth period and were about nmol/L level. But at the end of the period, i.e., 2 or 3 days before the cell density reaches the maximum, an NO peak appeared, with the NO concentration reaching 10 nmol/L. The NO threshold concentration exists according to the influence of exogenous NO on the marine phytoplankton growth. One type is the threshold concentration that can promote algae growth, and its value is between 10 and 1 nmol level, or even lower. The other type can inhibit the phytoplankton growth, which is about μmol level or higher. The results indicate that red tide algae are far more sensitive to NO than are food algae. The fundamental experimental outcome above may provide a new clue for red tide chemical forecast by inspecting the NO change.  相似文献   
988.
Nucleophilic substitution of 5-methylthio- and 5-cyanomethylthio-7-amino-s-triazolo[1,5-c]pyrimidines has been carried out using sodium hydroxide, ammonia, hydrazine hydrate, and four amines. The cyanomethyl group is particularly reactive under these conditions and 5-cyanomethylthiotriazolopyrimidine can be used to functionalize this heterocycle.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 382–385, March, 1992.  相似文献   
989.
The maximum fluxes (Jmax) of beta-blockers through keratinized membranes were determined in vitro and compared with their physiochemical parameters such as lipophilicity (log k'0) and melting point (mp). Rat abdominal skin and hamster cheek pouch mucosa were used as the model membranes. Propranolol, metoprolol, timolol, pindolol, nadolol and agenolol were used as beta-blockers with a variety of physicochemical characters. Linear relations of Jmax with either log k'0 or mp were observed both in intact rat skin and in intact hamster cheek pouch, suggesting that the lipophilicity and thermodynamic activity of a drug in the crystal state primarily affect the drug's permeation through these membranes. However, the slope, dJmax/d(log k'0), for cheek pouch mucosa was greater than that for rat skin, corresponding to the lack of appendigeal shunt pathways in cheek pouch. Penetration studies using the delipidized membranes and the isolated stratum corneum sheet of hamster cheek pouch mucosa clarified that the primary rate-limiting barrier function might exist in the lipid layer of the stratum corneum. Jmax values for the tape-stripped and delipidized skins correlated with both the solubilities of drugs in the vehicle and with the mp, suggesting the polar porous characteristics of both model membranes. However, a theoretical approach confirmed that the contribution of an intracellular or aqueous pore route in the intact membrane to the permeation of drugs with positive lipophilic indexes is negligible.  相似文献   
990.
We present a review of our recent measurements of total cross sections (Q T's) for the scattering of positrons by Na, K, and Rb, and positronium-formation cross sections (Q Ps's) for Na and K. For our total cross section measurements, a beam transmission technique has been used. For ourQ Ps measurements, our approach involves setting upper and lower limits onQ Ps using a combination of (1) measuring the transmission of the positron beam with the angular discrimination of the apparatus made as poor as possible, and (2) measuring the 511 keV annihilation gamma rays in coincidence produced by the decay of para-Ps formed in the scattering cell. Comparison with theoretical calculations shows that our measuredQ T's andQ Ps's for Na and K agree reasonably well with a close coupling approximation (CCA) calculation which takes into account the formation of Ps in then=1 andn=2 states. In the 3–10 eV energy range, this calculation predicts a peak in theQ T's andQ Ps's for K which also appears in our measurements. The absence of such a peak in our measuredQ T's andQ Ps's (preliminary) for Na in this energy range is also consistent with the same theory. Comparisons with five-state CCA calculations ofQ T which do not take Ps-formation into account also show good agreement with our positron-Na, K, and RbQ T measurements for energies above 20 eV, but show dramatic departures from our measurements below 10 eV for K and Rb.  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