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   471篇
  免费   25篇
化学   378篇
力学   9篇
数学   45篇
物理学   64篇
  2023年   3篇
  2022年   8篇
  2021年   9篇
  2020年   10篇
  2019年   17篇
  2018年   5篇
  2017年   6篇
  2016年   12篇
  2015年   14篇
  2014年   33篇
  2013年   31篇
  2012年   22篇
  2011年   48篇
  2010年   19篇
  2009年   20篇
  2008年   17篇
  2007年   19篇
  2006年   23篇
  2005年   16篇
  2004年   20篇
  2003年   12篇
  2002年   25篇
  2001年   11篇
  1999年   3篇
  1998年   6篇
  1996年   2篇
  1993年   3篇
  1991年   3篇
  1988年   2篇
  1987年   5篇
  1985年   2篇
  1984年   4篇
  1982年   8篇
  1981年   4篇
  1980年   3篇
  1979年   5篇
  1978年   3篇
  1977年   2篇
  1976年   3篇
  1975年   3篇
  1974年   4篇
  1973年   4篇
  1972年   2篇
  1971年   3篇
  1936年   2篇
  1935年   2篇
  1932年   3篇
  1928年   2篇
  1921年   2篇
  1920年   1篇
排序方式: 共有496条查询结果,搜索用时 0 毫秒
31.
32.
33.
This paper describes an IBM-PC menu-driven and user-friendly procedure which can help sales management in setting up optimal quota-bonus plans. For a given sales force, the resulting quotabonus plan maximizes the firm's current profits subject to (1) keeping every individual sales-force member at least at his/her current level of satisfaction (or eventually, increasing this level), and (2) being consistent and harmonious across sales representatives. The QUOPLAN system is composed of two submodels. The first submodel is used by salespersons for eliciting their own utility functions, and is essentially based upon the principles of conjoint analysis. The second submodel is used by management for reconciling all the individuals' judgements and the company's objectives into a consistent quota-bonus plan.  相似文献   
34.
Summary We give a general construction of infinite dimensional Ornstein-Uhlenbeck processes. We present expansions of these processes based on the expansion of the covariance operators in generalized eigenfunctions, and we answer positively some question left open by J.B. Walsh on properties of the hitting times of these processes.Partially supported by NSF-Grant MCS 8202045  相似文献   
35.
We demonstrate industrially feasible large‐area solar cells with passivated homogeneous emitter and rear achieving energy conversion efficiencies of up to 19.4% on 125 × 125 mm2 p‐type 2–3 Ω cm boron‐doped Czochralski silicon wafers. Front and rear metal contacts are fabricated by screen‐printing of silver and aluminum paste and firing in a conventional belt furnace. We implement two different dielectric rear surface passivation stacks: (i) a thermally grown silicon dioxide/silicon nitride stack and (ii) an atomic‐layer‐deposited aluminum oxide/silicon nitride stack. The dielectrics at the rear result in a decreased surface recombination velocity of Srear = 70 cm/s and 80 cm/s, and an increased internal IR reflectance of up to 91% corresponding to an improved Jsc of up to 38.9 mA/cm2 and Voc of up to 664 mV. We observe an increase in cell efficiency of 0.8% absolute for the cells compared to 18.6% efficient reference solar cells featuring a full‐area aluminum back surface field. To our knowledge, the energy conversion efficiency of 19.4% is the best value reported so far for large area screen‐printed solar cells. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
36.
37.
Prostate-specific antigen (PSA) is a routinely used marker of prostate cancer; however, the cut-off values for unambiguous positive/negative prostate cancer diagnoses are not defined. Therefore, despite the best effort, certain percentage of misdiagnosed cases is being recorded every year. For this reason, search for more specific diagnostic markers is of great interest. In this study, systematic comparison of PSA and metallothionein (MT) levels in blood serum of 46 prostate cancer-diagnosed patients is presented. It is clearly demonstrated that PSA levels vary significantly and despite normal total PSA values in the range of 0 - 4?ng/mL were obtained in over 36.9% of cases, positive prostate cancer was diagnosed by biopsy. In contrary, MT levels were considerably elevated in all tested samples and no significant variations were observed. These results are indicating the potential of MT as an additional prostate cancer marker reducing, in combination with PSA, the probability of false positive/negative diagnosis. To increase the throughput of the screening, chip-based capillary electrophoresis was suggested as a rapid and effective method for the fingerprinting analysis of prostate cancer from diseased blood sera.  相似文献   
38.
Let (A, D(A)) denote the infinitesimal generator of some strongly continuous sub-Markovian contraction semigroup onL p (m), p1 andm not necessarily -finite. We show under mild regularity conditions thatA is a Dirichlet operator in all spacesL q (m), qp. It turns out that, in the limitq,A satisfies the positive maximum principle. If the test functionsC c D(A), then the positive maximum principle implies thatA is a pseudo-differential operator associated with a negative definite symbol, i.e., a Lévy-type operator. Conversely, we provide sufficient criteria for an operator (A, D(A)) onL p(m) satisfying the positive maximum principle to be a Dirichlet operator. If, in particular,A onL 2 (m) is a symmetric integro-differential operator associated with a negative definite symbol, thenA extends to a generator of a regular (symmetric) Dirichlet form onL 2 (m) with explicitly given Beurling-Deny formula.  相似文献   
39.
Major JL  Boiteau RM  Meade TJ 《Inorganic chemistry》2008,47(22):10788-10795
We report on the mechanism of a series of Zn (II)-activated magnetic resonance contrast agents that modulate the access of water to a paramagnetic Gd (III) ion to create an increase in relaxivity upon binding of Zn (II). In the absence and presence of Zn (II), the coordination at the Gd (III) center is modulated by appended Zn (II) binding groups. These groups were systematically varied to optimize the change in coordination upon Zn (II) binding. We observe that at least one appended aminoacetate must be present as a coordinating group to bind Gd (III) and effectively inhibit access of water. At least two binding groups are required to efficiently bind Zn (II), creating an unsaturated complex and allowing access of water. (13)C isotopic labeling of the acetate binding groups for NMR spectroscopy provides evidence of a change in the metal coordination of these groups upon the addition of Zn (II) supporting our proposed mechanism of activation as presented.  相似文献   
40.
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