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   3791篇
  免费   640篇
  国内免费   586篇
化学   2351篇
晶体学   44篇
力学   364篇
综合类   42篇
数学   742篇
物理学   1474篇
  2024年   9篇
  2023年   73篇
  2022年   116篇
  2021年   115篇
  2020年   183篇
  2019年   127篇
  2018年   143篇
  2017年   145篇
  2016年   178篇
  2015年   141篇
  2014年   171篇
  2013年   305篇
  2012年   233篇
  2011年   234篇
  2010年   207篇
  2009年   263篇
  2008年   245篇
  2007年   286篇
  2006年   240篇
  2005年   212篇
  2004年   212篇
  2003年   187篇
  2002年   168篇
  2001年   143篇
  2000年   101篇
  1999年   87篇
  1998年   95篇
  1997年   72篇
  1996年   72篇
  1995年   35篇
  1994年   46篇
  1993年   24篇
  1992年   24篇
  1991年   31篇
  1990年   11篇
  1989年   10篇
  1988年   7篇
  1987年   9篇
  1986年   6篇
  1985年   11篇
  1984年   11篇
  1982年   7篇
  1981年   2篇
  1980年   4篇
  1979年   6篇
  1978年   2篇
  1977年   3篇
  1973年   1篇
  1971年   1篇
  1957年   1篇
排序方式: 共有5017条查询结果,搜索用时 15 毫秒
141.
One of the most common problems in wounds is delayed healing and complications such as infection. Therefore, the need for novel materials accelerates the healing of wounds especially abdominal wounds after surgery besides high efficiency and safety is mandatory. The rate of wound healing, anti-inflammatory and biocompatibility of Zn-Al LDH (Zn-Al layer double hydroxide) alone and loaded with Curcumin (Zn-Al LDH/Curcumin) was screened via in-vivo assays through intramuscular implantation in rat abdominal wall with intact peritoneum cavity. The implanted drugs were formed through Curcumin loaded into LDH of Zn-Al with drug release of 56.78 ± 1.51% within 24 h. The synthesized nanocomposite was characterized by (TGA/DTA) thermal analysis, (XRD) X-ray diffraction, (FESEM) Field emission scanning electron microscopy, (HRTEM) high resolution transmission electron microscope, energy dispersive X-ray (EDX) and low-temperature N2 adsorption, pore volume and average pore size distribution. The integrity of blood circulation, inflammatory signs, wound healing rate, capacity of tissue integration, antigenicity and composite biocompatibility, auto fluorescence ability of collagen bundles and the tensile strength of the muscle were assessed histopathologically after 7 and 30 days’ post-implantation. Excellent wound healing ability was achieved with shortest length between the wound gap edges and higher tensile strength of the muscle. Besides emit florescence very well followed by good healing and tensile muscles strength in Curcumin while very low strength with scar formation in Zn-Al LDH/Curcumin in both acute and chronic wound. No signs of inflammation in Curcumin & Zn-Al LDH. No vessels obstruction or bleeding observed in both Zn-Al LDH and Curcumin more than Zn-Al LDH/Curcumin and control which examined through candling. Good healing & infiltrated immune cells in same groups through histopathological examination. This work supports the anti-inflammatory, wound healing and biocompatibility of both LDH and Curcumin with living matter, increasing their biomedical applications in this era with safety and increasing efficacy with prolonged drug release.  相似文献   
142.
The reaction of UO3 and TeO3 with a KCl flux at 800 °C for 3 days yields single crystals of K4[(UO2)5(TeO3)2O5]. The structure of the title compound consists of layered, two-dimensional sheets arranged in a stair-like topology separated by potassium cations. Contained within these sheets are one-dimensional uranium oxide ribbons consisting of UO7 pentagonal bipyramids and UO6 tetragonal bipyramids. The ribbons are in turn linked by corner-sharing with trigonal pyramidal TeO3 units to form sheets. The lone-pair of electrons from the TeO3 groups are oriented in opposite directions with respect to one another on each side of the sheets rendering each individual sheet nonpolar. The potassium cations form contacts with nearby tellurite units and axial uranyl oxygen atoms. Crystallographic data (193 K, MoKα, ): triclinic, space group , , , , α=99.642(1)°, β=93.591(1)°, γ=100.506(1)°, , Z=1,R(F)=4.19% for 149 parameters and 2583 reflections with I>2σ(I).  相似文献   
143.
Fuyuhiko Inagaki 《Tetrahedron》2007,63(24):5154-5160
The Rh(I)-catalyzed PKR of 1-phenylsulfonyl-1,2-octadien-7-ynes and their aza derivatives exclusively produced the corresponding 9-phenylsulfonylbicyclo[4.3.0]nona-1,6-dien-8-ones and no 4-(phenylsulfonylmethylidene)bicyclo[3.3.0]oct-1-en-3-ones could be detected. Thus, the ring-closing pattern was found to be the same as those of the previously reported 3-phenylsulfonyl-1,2-octadien-7-yne derivatives. However, the formation of 4-(phenylsulfonylmethylidene)-7-oxabicyclo[3.3.0]oct-1-en-3-ones was observed as a minor product when the 5-oxa congeners were used. In addition, a larger ring-sized product, 10-phenylsulfonyl-5-azabicyclo[5.3.0]deca-1,7-dien-9-one derivative, was obtained from the 6-aza derivative of 1-phenylsulfonyl-1,2-nonadien-8-yne.  相似文献   
144.
The response of a single TE102 and double TE104 rectangular cavity to the insertion of samples contained in tubes with variable wall thickness and a quartz Dewar into the cavity has been analyzed. A direct, indirect, and concurrent (positive or negative) “lens effect” inside the double TE104 rectangular cavity is discussed. The experimental dependence of the EPR signal intensity on the wall thickness of the sample tube, δ, for the line-like samples with identical length of the sample material column, L=30 mm, recorded in the microwave cavity showed a directly proportional increase of the relative “lens effect” with the increase of the wall thickness of the tube in the interval, δ∈<0.1 mm, >0.5 mm. The insertion of the variable-temperature double-wall quartz Dewar (home-built, resonant frequency shift, ca. −300 MHz) into the single TE102 rectangular cavity showed the same relative “lens effect”, with ca. 1.5-time increase of the EPR signal intensity, for a point-like sample and the line-like samples with material columns of diameter of 1 and 1.3 mm, and wall thickness of the sample tubes, δ∈<0.1 mm, >0.5 mm. The increased effect of the Dewar arises because the active volume of the quartz Dewar tube walls is always much more larger than the active volume of the sample tube wall. In the case of the double TE104 rectangular cavity, the insertion of the quartz Dewar: (i) into the same cavity, in which the sample is present, caused a direct “lens effect”, with ca. 1.8-fold increase of the EPR signal intensity; however, (ii) into the complementary cavity, in which the sample is absent, caused an indirect “lens effect”, with ca. 0.6-fold decrease of the EPR signal intensity. With the Dewar and sample in one cavity and a large empty sample tube in the complementary cavity, a concurrent (positive or negative) “lens effect” can be observed. Thus, the possible increase/decrease of the EPR signal intensity depends on the volume ratio of the quartz Dewar tube walls and large sample tube wall inserted into the double TE104 rectangular cavity. Each of the above phenomena may be a significant source error in quantitative EPR spectrometry unless the samples to be compared in the quantitative EPR analysis are contained in sample tubes having the same wall thickness and each EPR spectra should be recorded inside an identical quartz Dewar.  相似文献   
145.
In this paper interfacial edge crack problems are considered by the application of the finite element method. The stress intensity factors are accurately determined from the ratio of crack-tip-stress value between the target given unknown and reference problems. The reference problem is chosen to produce the singular stress fields proportional to those of the given unknown problem. Here the original proportional method is improved through utilizing very refined meshes and post-processing technique of linear extrapolation. The results for a double-edge interface crack in a bonded strip are newly obtained and compared with those of a single-edge interface crack for different forms of combination of material. It is found that the stress intensity factors should be compared in the three different zones of relative crack lengths. Different from the case of a cracked homogeneous strip, the results for the double edge interface cracks are found to possibly be bigger than those for a single edge interface crack under the same relative crack length.  相似文献   
146.
Photoelectrochemical(PEC) water splitting is a promising approach for renewable hydrogen production.However,the practical PEC solar-to-fuel conversion efficiency is still low owing to poor light absorption and rapid recombination of charge carriers in photoelectrode.In this work,we report a ternary photoanode with simultaneously enhancement of light absorption and water oxidation efficiency by introducing copper phthalocyanine(CuPc) and nickel iron-laye red double hydroxide(NiFe-LDH) on TiO_2(denoted as TiO_2/CuPc/NiFe-LDH).An experimental study reveals that CuPc loading on TiO_2 bring strong visible light absorption;NiFe-LDH as an oxygen evolution reaction catalyst efficiently accelerates the surface water oxidation reaction.This synergistic effect of CuPc and NiFe-LDH gives enhanced photocurrent density(2.10 mA/cm2 at 0.6 V vs.SCE) and excellent stability in the ternary TiO_2/CuPc/NiFeLDH photoanode.  相似文献   
147.
The reductive debromination of an very crowded bis(bromodiaryl- silyl)butadiyne 2 was performed with potassium graphite to give the corresponding 2-allenyl-1-benzosilole 3 was isolated as a main product.  相似文献   
148.
Four Cu2+ complexes of salicylidene-amino acid Schiff base with 1,10-phenanthroline(Phen) or 2,2’- bipyridine(Bipy) were successfully intercalated in interlayer galleries of Mg/Al-NO3-layered double hydroxide(LDH) by the swelling-restored method.The hybrids were characterized by elemental analysis,X-ray diffraction,FT-IR spectra,UV-vis DRS,TG-DTA and SEM observation.Good protection of the complexes by LDH in neutral and weak acidic solutions was revealed by UV spectra,cyclic voltammograms and luminescence spectra.  相似文献   
149.
The high-entropy materials have raised much attention in recent years due to their extraordinary performances in mechanical, catalysis, energy storage fields. Herein, a new type of high-entropy hydroxides (e.g., NiFeCoMnAl(OH)x) that are amorphous and capable of broad solar absorption is reported. A facile one-pot co-precipitation method is employed to synthesize these amorphous high-entropy hydroxides (a-HEHOs) under ambient conditions. The a-HEHOs thus obtained display widely tunable bandgap (e.g., from 2.6 to 1.1 eV) due to their high-entropy and amorphous characteristics, enabling efficient light absorbance and photothermal conversion in the solar regime. Further solar water evaporation measurements show that the a-HEHOs delivered a considerable energy conversion efficiency of 55%, comparable to black titanium oxides that are synthesized using more complex and expensive methods.  相似文献   
150.
The double Suzuki cross‐coupling reaction has successfully been applied for the synthesis of 5,5′‐(5‐butoxy‐1,3‐phenylene)bis(2‐chloropyrimidine) with two reactive chloro groups and an alkoxy side chain starting from 2‐chloropyrimidin‐5‐ylboronic acid and 1,3‐dibromo‐5‐butoxybenzene. The reactivity of this dielectrophile was tested by reaction with aniline and phenol, a nitrogen and oxygen nucleophile, respectively. The new dielectrophile would further provide an ideal platform for the construction of large hetero‐atom bridged macrocycles for desired properties and functions in supramolecular and material chemistry. Copyright © 2012 John Wiley & Sons, Ltd.  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