全文获取类型
收费全文 | 2257篇 |
免费 | 440篇 |
国内免费 | 1134篇 |
专业分类
化学 | 1871篇 |
晶体学 | 111篇 |
力学 | 248篇 |
综合类 | 71篇 |
数学 | 420篇 |
物理学 | 1110篇 |
出版年
2024年 | 1篇 |
2023年 | 20篇 |
2022年 | 97篇 |
2021年 | 90篇 |
2020年 | 87篇 |
2019年 | 68篇 |
2018年 | 79篇 |
2017年 | 120篇 |
2016年 | 88篇 |
2015年 | 115篇 |
2014年 | 163篇 |
2013年 | 217篇 |
2012年 | 194篇 |
2011年 | 230篇 |
2010年 | 195篇 |
2009年 | 237篇 |
2008年 | 224篇 |
2007年 | 220篇 |
2006年 | 224篇 |
2005年 | 204篇 |
2004年 | 147篇 |
2003年 | 98篇 |
2002年 | 125篇 |
2001年 | 111篇 |
2000年 | 110篇 |
1999年 | 53篇 |
1998年 | 32篇 |
1997年 | 27篇 |
1996年 | 16篇 |
1995年 | 12篇 |
1994年 | 21篇 |
1993年 | 32篇 |
1992年 | 24篇 |
1991年 | 16篇 |
1990年 | 11篇 |
1989年 | 25篇 |
1988年 | 19篇 |
1987年 | 13篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 9篇 |
1983年 | 14篇 |
1982年 | 2篇 |
1981年 | 10篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1971年 | 1篇 |
排序方式: 共有3831条查询结果,搜索用时 15 毫秒
61.
Preparation, Characterization and Catalytic Properties of S2O8^2-/ZrO2 Supported by Tungsten Carbide
XUEHua-xin CHENJian-min 《高等学校化学研究》2004,20(1):68-72
A WC-supported S2O8^2-/ZrO2(PSZ) catalyst was prepared and characterized by means of XRD, BET, FTIR and XPS. The isomerization of n-pentane over the catalyst was investigated as well. The results show that the skeletal isomerization and the crack of n-pentane proceed simultaneously on WC-supported S2O8^2-/ZrO2 catalyst. The addition of tungsten carbide showed a significant enhancement in the activity and stability of the catalyst for n-pentane isomerization. The catalyst showed evidently a better activity than S2O8^2-/ZrO2 supported by Pt and WO3. The results can be interpreted by the existence of the tungsten oxycarbide compound(WCxOy) with carbidic, oxide and acidic sites. 相似文献
62.
正丁烷氧化制顺酐流化床催化剂的性能 总被引:1,自引:0,他引:1
考察了前驱体制备过程中水合肼/五氧化二钒摩尔比(以下简记\r\n为n(N2H4)/n(V2O5))对催化剂性能的影响.由较高的n(N2H4)\r\n/n(V2O5)比制得的催化剂,其(VO)2P2O7相含量较多,正丁烷转化\r\n率较高.随着n(N2H4)/n(V2O5)比的减小,δ-VOPO4相的含量逐\r\n渐增多,正丁烷转化率随之升高;而顺酐选择性开始时升高,达到最大\r\n值后逐渐降低.在n(N2H4)/n(V2O5)=0.34时制备的催化剂最佳\r\n,在丁烷浓度为4.0%、空速为500h-1及反应温度为420℃的反应条件\r\n下,顺酐收率可达49.74%.本文中细粒床催化剂有较大的操作弹性. 相似文献
63.
YanXUE HaiYingYANG YongTanYANG 《中国化学快报》2005,16(1):67-70
A new method was developed for the determination of the size distribution of nano-particles by capillary zone electrophoresis (CZE). Scattering effect of nanoparticles was studied. This method for the determination of size distribution was statistical. 相似文献
64.
65.
A large specific surface area perovskite-type mixed oxide PbTiO3 supported cupric oxide was synthesized as a catalyst for NO decomposition and characterized by techniques such as XPS, XRD, H2-TPR before and after NO decomposition reactions. The catalytic properties were tested with a fix-bed micro-reactor. The results showed that the PbTiO3 was inactive for the reactions, but 1wt % Cu/PbTiO3 catalyst gave fairly good activities for NO decomposition at temperature as low as 473 K. Copper species were found well-dispersed but weakly interacted with the support before NO decomposition, and the NO decomposition caused significant change in the environment of the copper species, which became Cu(Ⅰ) and most probably incorporated into surface crystal lattice of the nano-sized PbTiO3. In NO reaction, a large amount of oxygen atoms from the decomposition of NO penetrated into the nano-sized PbTiO3 support and caused small expansion of crystal lattice. The transport of oxygen between the copper species and the catalyst support may be helpful to speed up the kinetic regeneration of active metal sites from oxygen occupancy and resulted in good catalytic performance. 相似文献
66.
The title compound N'-tert-butylaminocarbonyl-N-2-chlorophenoxyacetylthiou- rea has been synthesized for the first time. Complete assignments were achieved by IR, 1H NHR and single-crystal X-ray diffraction analyses. The inhibitory rate of the cellular growth of K562 cells (chronic myeloid 1eukemic cells) was measured using MTT [3-(4,5-dimethylthiazo-2-y1)-2,5-di- phenyltetra-zolium bromide] assay. The cell apoptosis was assessed by agarose gel electrophoresis to find that the title compound has antiproliferation and apoptosis inducing effects on K562 cells. In order to investigate the relationship between structure and activity of the target compound, we report its crystal structure and biological behavior in the present paper. Crystallographic data: C14H18- ClN3O3S, Mr = 343.82, orthorhombic, space group Pnma, a = 19.786(6), b = 6.789(2), c = 12.938(4) , V = 1738.0(9) 3, Z = 4, Dc = 1.314 g/cm3, F(000) = 720, μ(MoKα) = 0.354 mm-1, R = 0.0378 and wR = 0.0941. The molecule is a planar structure. 相似文献
67.
由于汇率、利率、商品价格的变化以及双渠道供应链关系的复杂性,企业的现金流存在较大的波动性。本文研究双渠道销售模式下,制造商对现金流进行套期保值的决策问题。重点关注垄断企业在拥有一个分销商和线上销售的双渠道销售模式下,并利用内部融资将现金流用于提高生产效率时,如何根据现金流的波动程度、市场需求、替代产品敏感系数以及批发价格进行套期保值决策。研究了双渠道销售模式下制造商的均衡问题,比较了两种决策下的均衡解,并通过数值分析给出了直观的结论:市场最大需求与替代产品敏感系数增加时,企业更倾向于选择套期保值;现金流波动程度与批发价格增加时,企业更倾向于不选择套期保值。 相似文献
68.
Using the total human/mouse DNA as the probe, screening has been carried out three times with in situ plaque hybridization to obtain the single-copy DNA sequence from the human X chromosome genomic library. The effective rate of screening is 1. 45%. DNAs from clones containing single-copy inserts have been analyzed by a panel of hybrid cells with or without human X chromosome. Three segments, designated by DXFD52,73,75, are mapped to the X chromosome. DXFD52 has been precisely localized on Xq12-q13 with in situ chromosomal hybridization. DXFD52 has been partially sequenced. The results indicate that DXFD52 is a new isolated single-copy segment on the X chromosome. Great progress in the RFLPs study with DXFD52 has been achieved in the population of Chongqing, Sichuan Province. The results show that the DXFD52 can be used to detect the RFLP with Hind Ⅲ, Bgl Ⅱ, and Hinf Ⅰ. DXFD52 will be a potential "landmark" for the construction of the complete linkage map of human genome and the analysis of genomic s 相似文献
69.
不同价态的无机砷在玻碳电极上具有较高的电化学氢化物发生效率,同时又存在较为明显的差异。通过选择不同的电流条件,采用电化学氢化物发生与原子吸收光谱联用技术有效地实现了无机砷的形态分析。对试验参数和干扰情况进行了研究。As(Ⅲ)和As(Ⅴ)检出限分别为0.2μg.L-1和0.4μg.L-1;对砷量为20μg.L-1的条件下作了方法的精密度试验,测定结果的相对标准偏差(n=6)对As(Ⅲ)为1.9%和对As(Ⅴ)为3.1%。应用此方法分析了人发标准物质(GBW 09101),测得总砷量与证书值相符。 相似文献
70.
Effect of microcrystallite preparation conditions on the formation of colloid crystals of cellulose 总被引:7,自引:0,他引:7
Stable colloidal suspensions of cellulose microcrystallites may be prepared from filter paper by sulfuric acid hydrolysis. Above a critical concentration, the suspensions form a chiral nematic ordered phase, or colloid crystal. The preparation conditions govern the properties of the individual cellulose microcrystallites, and hence the liquid crystalline phase separation of the cellulose suspensions. The particle properties and the phase separation of the suspensions were strongly dependent on the hydrolysis temperature and time, and on the intensity of the ultrasonic irradiation used to disperse the particles. The particle size of the microcrystallites was characterized with transmission electron microscopy and photon correlation spectroscopy. The surface charge was determined by conductometric titration. It was possible to fractionate the microcrystallites by size using the partitioning between isotropic and liquid crystalline phases; the longer microcrystallites migrate to the liquid crystalline phase 相似文献