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51.
Yi‐Jun Huang Guo‐Rong Qi Yu‐Hua Wang 《Journal of polymer science. Part A, Polymer chemistry》2002,40(8):1142-1150
A double metal‐cyanide catalyst based on Zn3[Co(CN)6]2 was prepared. This catalyst is very effective for the ring‐opening polymerization of propylene oxide. Polyether polyols of moderate molecular weight having low unsaturation (<0.015 meq/g) can be prepared under mild conditions. The molecular weight of polymer is entirely controlled by a reacted monomer‐to‐initiator ratio. The polymers prepared with stepwise addition of monomer exhibit a narrower molecular weight distribution as compared with those prepared with one‐step addition of monomer. Various compounds containing active hydrogen, except basic compounds and low‐carbon carboxylic acid, may be used as initiators. The reaction rate increases with increasing catalyst amount and decreases with rising initiator concentration. Polymerization involves a rapid exchange reaction between the active species and the dormant species. It was also proven that, to a certain extent, the chain termination of this catalytic system is reversible or temporary. 13C NMR analysis showed that the polymer has a random distribution of the configurational sequences and head‐to‐tail regiosequence. It is assumed that the polymerization proceeds via a cationic coordination mechanism. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1142–1150, 2002 相似文献
52.
We present a microstructural model of aerogels that includes the effect of particle necks, tortuosity and dangling ends on the scaling of elastic moduli with density. Relative neck radii can be determined for sintering series of silica aerogels and for Resorcinol Formaldehyde (RF) aerogels produced with different catalyst concentrations. The density of elastically ineffective dangling ends and the tortuosity can be estimated using information from thermal conductivity and elastic modulus measurements in silica aerogels. Typical values for the load bearing mass range from >50% for high density and heat treated aerogels to <10% of the total mass for low density wet-gels. 相似文献
53.
Ag(TCNQ)准一维微米结构的制备及表征 总被引:1,自引:0,他引:1
利用溶液化学反应法制备了准一维结构的金属有机配合物Ag(TCNQ). X射线衍射(XRD)表明,所制备的Ag(TCNQ)为晶态结构;扫描电子显微镜(SEM)的观察证明,Ag(TCNQ)为准一维的微米管或线;Raman 测试结果表明,单根的Ag(TCNQ)形成时,Ag原子与TCNQ分子之间发生了电荷转移.对样品的制备工艺,即 Ag膜厚度和浸入溶液的反应时间对生成Ag(TCNQ)晶体形貌的影响进行了研究.结果表明,Ag膜越薄,生长出的晶体越稀疏;Ag膜与TCNQ乙腈溶液的反应时间影响其形貌的变化.反应历经三个阶段,晶体形成和长大阶段、反应完全阶段及溶解阶段. 相似文献
54.
Mojumdar S. C. Kozánková J. Chocholoušek J. Majling J. Nemecek V. 《Journal of Thermal Analysis and Calorimetry》2004,78(1):145-152
The conditions to fabricate the bulk porous specimens have been studied on account of sodium borosilicate (NBS) glasses. Glass
composition, heat treatment at phase separation and TiO2 addition have been considered in this study. Original glass samples of composition in mol%: sample A: 9.19 Na2O - 23.58 B2O3 - 67.23 SiO2, sample B: 9.29 Na2O - 3.17 TiO2 - 23.82 B2O3 - 63.72 SiO2 were prepared by melting reagent grade chemicals (Na2CO3, HBO3, SiO2 and AgNO3) in platinum crucibles at 1480°C for 1 h in air. The melts were poured onto stainless steel plates and were annealed at 500°C
for 0.5 h after cooling. Thus, obtained samples were phase separated at 700°C for 2, 15, 25 and 50 h to study their microstructure
by scanning electron microscope (SEM). Besides the direct study of the microstructure by SEM, information on glass structural
changes of samples are provided by measuring in situ changes by the optical transmittance thermal analysis. The isothermal
measurements were carried out at 700, 720 and 740°C. The temperature of phase separation, the leaching and nucleator addition
(TiO2), significantly influence the microstructure of the resulting leached product. TiO2 additive seems to suppress crystallization of cristobalite: especially at the extended above heat treatment phase separation
runs. The phase-separated domains of glasses containing above 80 moles of SiO2 are so small that it is very hard to observe them by SEM. The glass composition in our case was selected in a way to have
relatively large phase separated areas easily observed by SEM at magnification 20 000·. The influence of TiO2 is not too pronounced. It seems to suppress the cristobalite crystallization, especially of longer heating runs. The image
analysis of leached glasses shows the prevailing content of the skeletal phase in a comparison to pores. The TiO2 content diminishes the content of the skeletal phase.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
55.
R. S. A. de Lange K. -N. P. Kumar J. H. A. Hekkink G. M. H. van de Velde K. Keizer A. J. Burggraaf W. H. Dokter H. F. van Garderen T. P. M. Beelen 《Journal of Sol-Gel Science and Technology》1994,2(1-3):489-495
Microporous SiO2 and SiO2/MO2 (M=Ti, Zr, Al; 10 mol% MOx) materials for gas separation membrane applications have been prepared from polymeric sols. Characterization of these sols with SAXS showed that the mean fractal dimension of the SiO2 sols is 1.3–1.4 with a radius of gyration of approximately 2.5 nm. The dried and calcined films are microporous and the pore size distribution was bimodal with maxima at diameters of 0.5 nm and 0.75 nm. For the SiO2/TiO2, SiO2/ZrO2 and SiO2/Al2O3 systems, much milder reaction conditions proved to be necessary to obtain sols with comparable fractal dimensions due to the high reactivity of the Ti/Zr/Al-alkoxides. Microporous supported membranes with molecular sieve-like gas transport properties can be prepared from a relatively wide range of sol structures: from polymers too small to characterize with SAXS to structures with fractal dimensions: 1<d
f<2.04. 相似文献
56.
57.
采用金属蒸气真空弧(MEVVA)离子源技术对强流Ti离子束注入到纯铜表面的结构和性能进行了系统的研究.材料表面层的机械性能测试表明:强流Ti离子注入纯铜后材料表面的硬度和耐磨性均有提高,相对于纯铜基体,5×1017 cm-2注量注入可以使材料表面硬度提高2.3倍,使表面摩擦因数下降14%.注入层的X射线结构分析表明:金属离子注入后,在纯铜表面注入层中析出了合金相,合金相的析出是材料的表面硬度和耐磨性提高的主要因素. 相似文献
58.
SBS改性沥青的两种典型亚微观结构 总被引:1,自引:0,他引:1
应用粘度剂、显微镜等方法,测定了不同SBS改性沥青的系列粘度数据和显微照片,通过分析这些数据和照片,研究了改性沥青结构和性质之间的关系。发现在相同SBS剂量下的改性沥青具有不同的亚微观结构,其中有两种差别较大的典型结构。结果表明,改性沥青亚微观结构的差异在很大程度上影响宏观性质,具体表现在延度、稳定性、粘度等指标上有较大差别。 相似文献
59.
热挤压AZ91D镁合金的组织与力学性能 总被引:12,自引:0,他引:12
对直径φ50mm铸态AZ91D合金棒材在603K下进行单次热挤压,获得了直径φ14mm的棒材。用OM、SEM和TEM分析热挤压前后组织的变化,研究热挤压对其组织与性能的影响规律。结果表明,热挤压变形可以细化其微观组织,显著提高AZ91D合金的抗拉强度(σb、σ0.2)、伸长率(δ)及硬度(HB)。在此过程中合金α-Mg相有足够的独立滑移系可以启动,棱面滑移和基面滑移共同作用发生局部大变形,α-Mg沿挤压方向呈细条带状,β—Mg12,Al12相分布于α-Mg条带间。然而,经单次挤压不可能得到均匀的变形组织,从边缘到中心,组织渐变,边缘组织细小,心部组织粗大。 相似文献
60.
为探讨北方山溪鲵和隆肛蛙视网膜结构与生活习性、捕食方式的关系,用光镜观察了这两种动物视网膜的结构,测量各层厚度、3个核层胞核的层数及胞核直径.结果表明,山溪鲵与隆肛蛙视网膜虽然各核层的层数相似,但隆肛蛙的胞核直径明显小于山溪鲵,致使两种动物在3个核层和2个网层的厚度上存在明显差异.从视网膜结构分析其机能,依赖于视觉捕食的隆肛蛙,视网膜具有较高的视敏度;捕食时以嗅觉为主、视觉为辅的山溪鲵,视网膜则具有较高的光敏度.显示了视网膜结构机能与各自的捕食方式的适应性. 相似文献