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   27691篇
  免费   4365篇
  国内免费   4096篇
化学   21191篇
晶体学   440篇
力学   1358篇
综合类   307篇
数学   3011篇
物理学   9845篇
  2024年   96篇
  2023年   503篇
  2022年   960篇
  2021年   1060篇
  2020年   1098篇
  2019年   1161篇
  2018年   979篇
  2017年   1011篇
  2016年   1335篇
  2015年   1375篇
  2014年   1709篇
  2013年   2121篇
  2012年   2551篇
  2011年   2603篇
  2010年   1990篇
  2009年   1880篇
  2008年   1987篇
  2007年   1827篇
  2006年   1728篇
  2005年   1374篇
  2004年   1014篇
  2003年   795篇
  2002年   757篇
  2001年   659篇
  2000年   585篇
  1999年   489篇
  1998年   365篇
  1997年   327篇
  1996年   279篇
  1995年   266篇
  1994年   237篇
  1993年   172篇
  1992年   149篇
  1991年   146篇
  1990年   122篇
  1989年   92篇
  1988年   75篇
  1987年   54篇
  1986年   65篇
  1985年   51篇
  1984年   21篇
  1983年   19篇
  1982年   19篇
  1981年   6篇
  1980年   7篇
  1979年   8篇
  1978年   3篇
  1971年   4篇
  1957年   4篇
  1936年   4篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
82.
叙述了用θ-2θ型X射线衍射仪精确测定电解质溶液结构的新实验技术。设计制作了具有恒温功能的超厚液体样品池,并建立了样品池窗口强度的校正方法。优化了液体X射线衍射数据和结构参数精细化的计算机程序,获得了非常令人满意的实验结果。由θ-2θ型X射线衍射仪精确测定的径向分布函数与θ-θ型衍射仪自由散射比较,表明DRF分辨率有所提高。  相似文献   
83.
Three kinds of nylon 10 14 crystals with different perfections were prepared under various crystallization conditions. The Brill transition behavior of these nylon 10 14 crystals was investigated by variable-temperature X-ray diffraction. It was found that the crystallization conditions influence the Brill transition temperature greatly. The Brill transition temperature of the lamellar crystals grown from dilute solution is so high that no Brill transition temperature can be observed before melting. However, for crystals postannealed at 125 °C, the Brill transition temperature is as low as 130 °C. The results show that the Brill transition behavior of nylons is strongly dependent on the crystallization conditions, for example, the perfections of the crystals.  相似文献   
84.
The spectral theory of selfadjoint operators and unitary operators in Hilbert space has been successfully generalized to $\[{\Pi _k}\]$ space. However, there are only a few results for the spectral theory of selfadjoint operators and unitary operators in $\[\Pi \]$ space. One of the important reasons is that the structure of $\[\Pi \]$ space is more complex than that of $\[{\Pi _k}\]$ space. This paper and the forthcoming paper "The structure of $\[\Pi \]$ space (II)" will mainly be dealt with the structure of $\[\Pi \]$ spaces, which will be used to further study the operators in $\[\Pi \]$ spaces.  相似文献   
85.
Three new enmein‐type ent‐kaurenoids, i.e., the two pairs 1 and 2 of 20‐epimers and the (20R)‐isomer 3 , besides the seven known diterpenoids 4 – 10 , were isolated from the aerial parts of Isodon serra. Their structures were elucidated by spectroscopic techniques and X‐ray diffraction. The immunosuppressive effect for T‐lymphocytes proliferation induced by Con A in BALB/c mouse was evaluated for the isolates 1 – 10 . They all displayed a remarkable inhibitory effect, with multi‐glycosides of Tripterygium wilfordii as positive reference substance (Table 3).  相似文献   
86.
A new series of MR-IPN polyacrylamide—poly(vinyl-ethylenediamine)chelatesorbents were prepared by the reaction of ethylenediamine with MR-IPN matrixes.The newchelate sorbent showed good sorption properties for auric and platinic ions.  相似文献   
87.
Adsorption of ammonia (NH3) onto activated carbons prepared from palm shells impregnated with sulfuric acid (H2SO4) was investigated. The effects of activation temperature and acid concentration on pore surface area development were studied. The relatively large micropore surface areas of the palm-shell activated carbons prepared by H2SO4 activation suggest their potential applications in gas adsorption. Adsorption experiments at a fixed temperature showed that the amounts of NH3 adsorbed onto the chemically activated carbons, unlike those prepared by CO2 thermal activation, were not solely dependent on the specific pore surface areas of the adsorbents. Further adsorption tests for a wide range of temperatures suggested combined physisorption and chemisorption of NH3. Desorption tests at the same temperature as adsorption and at an elevated temperature were carried out to confirm the occurrence of chemisorption due to the interaction between NH3 and some oxygen functional groups via hydrogen bonding. The surface functional groups on the adsorbent surface were detected by Fourier transform infrared spectroscopy. The amounts of NH3 adsorbed by chemisorption were correlated with the contents of elemental oxygen present in the adsorbents. Mechanisms for chemical activation and adsorption processes are proposed based on the observed phenomena.  相似文献   
88.
Allylic hydroxy phosphonates and their derivatives can be interconverted by using cross metathesis with second generation Grubbs catalyst. The absolute stereochemistry of the starting phosphonate is conserved in the product. Cross metathesis reaction of the acrolein-derived phosphonate 2a yields a series of functionalized allylic hydroxy phosphonates. However, the cross metathesis reaction is often accompanied by competing dimerization and alkene migration reactions leading to a reduction in yield. The cinnamaldehyde- and crotonaldehyde-derived phosphonates 2b and 2c were also examined. In general, the metathesis reactions of phosphonates 2b and 2c are considerably slower than those for phosphonate 2a leading to mixtures. Several hydroxyl-protected derivatives of the phosphonate 2a (methyl carbonate 3a, acetate 4a, N-tosyl carbamate 5a, TBDMS 6a, and acetoacetate 7a) undergo metathesis without competing side reactions to give substituted allylic phosphonates in good to excellent yield.  相似文献   
89.
Let H be a Hilbert space, and let A be a linear bounded operator on H. For \(\lambda \in \rho (A)\), the \({U_\lambda } = {(A - \lambda )^{ * - 1}}(A - \lambda )\) is called polar.Produot operator. In this paper, we discuss the properties of \({U_\lambda }\) and the relation between \({U_\lambda }\) and A. We obtain tbe following results. Definition. Let B be a linear bounded operator on H, suppose \(0 \in \rho (B)\). For every \(x,y \in H\), we definite \([x,y] = (Bx,y)(H,B)\)(or (H, [·,·]) is called a non- degenerate bilinear space (it is obvious that if B=B*,then (H,B)is a space with an indefinite metric; and that if B>0, then (H,B) is a Hilbert Space. If an operator U(A) satisfies \[[Ux,Uy] = [x,y]([Ax,y] = [x,Ay]),x,y \in H\] then the operator U(A) is called a wvitary (self adjoint) on (H,B). Theorem I . Suppose A is a linear bounded operator on H, (1) If \(0 \in \rho (A)\), then \(U = {A^{ * - 1}}A\) is a unitary operator on (H,A) or (H, A*), and \(\sigma (U) = \frac{1}{{\sigma (U)}}\). (2) If there is a complex number \(\alpha \), such that \({\mathop{\rm Im}\nolimits} A \ge \alpha > 0\) then a)\(0 \in \rho (A)\), and the operator \(U = {A^{ * - 1}}A\) is a unitary on Hilbert space \((H,{\mathop{\rm Im}\nolimits} A) and 1 \in \rho (U)\);b) there exist two Hilbert spaces \((H,{v_1}),(H,{v_2})\), such that A, A* are all the unitary operator from (H,v1) onto (H,v2), and there are two spectral measures \(\{ E_\lambda ^i,\lambda \in [\alpha ,\chi ] \subset (0,2\pi )\} ,i = 1,2,\), such that \(AE_\Delta ^1H \subset E_\Delta ^2H,{A^ * }E_\Delta ^1H \subset E_\Delta ^2H\) for any \(\Delta = (\lambda ,u] \subset (0,2\pi ]\). (3) If \(0 \in \rho (A) \cap \rho ({\mathop{\rm Im}\nolimits} A)\) then the operator \(U = {A^{ * - 1}}A\) is a unitary on \((H,{\mathop{\rm Im}\nolimits} A)\). with an indefinite met He, and \(1 \in \rho (U)\). (4) For any complex number \(\lambda = r{e^{i\theta }},\left| \lambda \right| > \left\| A \right\|\), then \({U_\lambda }\) must be a unitary operator on the Hilbert space \(\left( {H,{\mathop{\rm Im}\nolimits} (\frac{1}{{i\lambda }}A + iI)} \right),and - {e^{i2\theta }} \in \rho ({U_\lambda })\) Theorem 2. (1) A is a normal operator iff there exists a complex number \(\lambda \), \(\lambda \in \rho (A)\), such that \(\frac{{\partial {U_\lambda }}}{{\partial \lambda }}{U_\lambda } = {U_\lambda }\frac{{\partial {U_\lambda }}}{{\partial \lambda }}\),where \(\frac{\partial }{{\partial \lambda }}\) is the directional derivative. (2) If there exists a complex number \(\alpha ,{\mathop{\rm Im}\nolimits} A \ge \alpha > 0\) then A is a normal operator iff \(U = {A^{ * - 1}}A\) is also. (3) If A is a hyperiwrmal or a subnormal, then for every \(\lambda \in \rho (A),\sigma ({U_\lambda })\) lies on the circle. 3, 4期 关于极?积算子炉一U 499 Theorem 3? Let A be a linear bounded operator on Hm Suppose 0£p(J.)? (1) If U=А*~гА is a unitary operator in a certain non-degenerate bilinear space (H} 1 J |Л|>1 入 J Ш=1 solmble iff (l)cr(ü) =-=i=-, (2) the operotor Ui= f XdE\ is unitarity equivalent to cr(C7) J |л|>1 the operator ül_1 == ШЕ1, 、 J 1Л|>1 If the conditions (1),(2) are satified, then we have (3) the subspace JJi?^2 of H reduces any solution of the equation A*"1 A — U, and 1 AI (Ягея*)1 = ^Uo where Ъ is any in vertible self-adjoint on (i?i?J?2) L, and bv Uo (Uo = Jiai i ЫЕ1 )}⑷41Я1фя, = , Ла = A1ü2, if the operator V is to realize TJt and Ut^unitary equivalence^ then Ai = VSf where S is any invertible on Нг and SvUi. " Corollary. Swpp)se operator U is a normal on a certain Hilbert space(Hy v) (^the U is similar to a certain normal operator on IT), if U satisfies the conditions (1)、(2) of Theorem^ on ?S,v) ? Then the general form of the solution of А*~гА = U is A = vA'y where A' is same as A in Theorem^. Theorem 5. Let U be a linear bounded operator on H. Suppose O?p(J.)and p(ü) is a simply connected region, then the equation А*~гЛ = U is a solvable iff there exists a certain space {H,v) with a indefinite metric, such that U is a unitary operator on (Hf 丨). If is a unitary operator on ?H,v),then there exists a particular solution of I X+$ А*~гА = U: Ar = 2e v [ (?7 —X)_1+X], where eie ? p (JJ), and the general form of the solution is Аж АУ, wliere V is any in vertible self-adjoint on (H, -u)and VvU,  相似文献   
90.
We report here porphodilactol derivatives and their corresponding metal complexes. These systems show promise as “all-in-one” phototheranostics and are predicated on a design strategy that involves controlling the relationship between intersystem crossing (ISC) and photothermal conversion efficiency following photoexcitation. The requisite balance was achieved by tuning the aromaticity of these porphyrinoid derivatives and forming complexes with one of two lanthanide cations, namely Gd3+ and Lu3+. The net result led to a metalloporphodilactol system, Gd-trans-2, with seemingly optimal ISC efficiency, photothermal conversion efficiency and fluorescence properties, as well as good chemical stability. Encapsulation of Gd-trans-2 within mesoporous silica nanoparticles (MSN) allowed its evaluation for tumour diagnosis and therapy. It was found to be effective as an “all-in-one” phototheranostic that allowed for NIR fluorescence/photoacoustic dual-modal imaging while providing an excellent combined PTT/PDT therapeutic efficacy in vitro and in vivo in 4T1-tumour-bearing mice.

We report here porphodilactol derivatives and their corresponding metal complexes as “all-in-one” phototheranostics by controlling the relationship between intersystem crossing (ISC) and photothermal conversion efficiency following photoexcitation.  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