全文获取类型
收费全文 | 54542篇 |
免费 | 3723篇 |
国内免费 | 4155篇 |
专业分类
化学 | 26192篇 |
晶体学 | 412篇 |
力学 | 4080篇 |
综合类 | 547篇 |
数学 | 10761篇 |
物理学 | 20428篇 |
出版年
2024年 | 68篇 |
2023年 | 476篇 |
2022年 | 892篇 |
2021年 | 1234篇 |
2020年 | 1353篇 |
2019年 | 1308篇 |
2018年 | 1161篇 |
2017年 | 1334篇 |
2016年 | 1567篇 |
2015年 | 1229篇 |
2014年 | 1840篇 |
2013年 | 3475篇 |
2012年 | 2313篇 |
2011年 | 2427篇 |
2010年 | 1997篇 |
2009年 | 3162篇 |
2008年 | 3368篇 |
2007年 | 3761篇 |
2006年 | 3207篇 |
2005年 | 2577篇 |
2004年 | 2205篇 |
2003年 | 2435篇 |
2002年 | 2768篇 |
2001年 | 2079篇 |
2000年 | 2011篇 |
1999年 | 1700篇 |
1998年 | 1660篇 |
1997年 | 1005篇 |
1996年 | 953篇 |
1995年 | 836篇 |
1994年 | 878篇 |
1993年 | 648篇 |
1992年 | 721篇 |
1991年 | 506篇 |
1990年 | 466篇 |
1989年 | 354篇 |
1988年 | 300篇 |
1987年 | 288篇 |
1986年 | 255篇 |
1985年 | 222篇 |
1984年 | 233篇 |
1983年 | 133篇 |
1982年 | 193篇 |
1981年 | 184篇 |
1980年 | 119篇 |
1979年 | 150篇 |
1978年 | 106篇 |
1977年 | 85篇 |
1976年 | 39篇 |
1973年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
DEHYDROGENATION AND AROMATIZATION OF METHANE IN THE ABSENCE OF OXYGEN OVER DOPED Mo/HZSM-5 CATALYSTS 下载免费PDF全文
研究了在Mo/HZSM-5催化剂上添加助剂以及不同的反应预处理温度对甲烷无氧脱氢芳构化反应的影响。实验结果表明,由于第二组分的添加,Mo/HZSM-5催化剂的活性和选择性都得到了较大程度的改善。预处理温度是影响催化剂反应性能的关键因素。Mo-Ru/HZSM-5催化剂经过873K空气预处理后,甲烷在973K的转化率约为10%,催化剂的稳定性也得到较大程度的提高。TPSR实验结果表明,Ru的加入降低了芳烃生成的温度。TPO和DTA实验结果表明,在Mo-Ru/HZSM-5催化剂上可生成较多的碳物种,结合反应结果,可以认为反应过程中生成的碳物种对甲烷的无氧脱氢芳构化反应是起积极作用的 相似文献
992.
This paper presents a new simple and sensitive method for the micro-determination of protein containing SH– and –S–S– groups based on the single sweep polarographic wave of an electroactive derivative. In 0.04molL–1 Na3PO4 and 0.2% ascorbic acid solution, protein is heated in a boiling water bath for 15min, the reaction product giving a sensitive reduction wave at –0.70V (vs. SCE). The wave height is linearly proportional to the concentration of protein. The calibration curves of bovine serum albumin (BSA), human serum albumin (HSA), ovalbumin (OVA) and lysozyme (Lyso) are constructed under the optimal conditions. For BSA and HSA, the linear ranges and detection limits are 0.05–24mgL–1 and 0.02mgL–1, respectively. The method has been applied to the determination of protein in human serum samples with satisfactory results. The mechanism of the polarographic wave was also studied, and the results show that S2– ion is released from the protein molecule during the derivatization reaction, the wave being attributed to the reduction of HgS. 相似文献
993.
L. Cherif F-Z. El-Berrichi A. Bengueddach P. Tougne J. Fraissard 《Colloids and surfaces. A, Physicochemical and engineering aspects》2003,220(1-3):83-89
The structural evolution of Y zeolite (Si/Al 2.17) weakly dealuminated by hexafluorosilicate (Si/Al 3.13), denoted YD, and exchanged with calcium (CaYD), has been studied after acid–base treatments at 80 °C close to the cation exchange conditions. The stability of the samples was followed by X-ray diffraction and solid-state NMR of 29Si and 27Al; YD zeolite was completely destroyed by treatment with acid pH 2.5 and suffered serious degradation on treatment with alkali at pH 11.8. The introduction of calcium improved the stability of the zeolite in acid and base. In acid CaYD was not destroyed until pH 1. At pH 2, silicon and aluminium were extracted and an amorphous phase was formed. Base treatment at pH 13 did not affect the calcium-exchanged zeolite. 相似文献
994.
The viscosity of PEG-modified urethane acrylate (PMUA) showed peculiar behavior in the course of soap-free emulsification. Moreover, the viscosity change with added amounts of water was influenced by the reaction molar ratio of polyethylene glycol (PEG). The rate of increase in viscosity slowed and the ratio of increase in viscosity increased as the reaction molar ratio of PEG increased. This peculiar viscosity behavior was due to the microphase separation between hydrophilic and hydrophobic segments of PMUA, and the orientation of polyoxyethylene groups at O/W interface which influenced droplet size of the soap-free PMUA emulsion. The location of polyoxyethylene groups of this resin at O/W interface was confirmed using the adsorption isotherm measurement of PMUA molecules containing polyoxyethylene groups at water/benzene interface. The microphase separation behavior of PMUA between hydrophilic and hydrophobic segments could apply to the preparation of the PMUA gels containing peculiar structure. PMUA gels were prepared using dioxane (UAG) and the swelling behavior of these gels were compared to that of gels prepared using water (UAHG) in the same medium. In the same medium, the swelling behavior of UAHG gels differed from that of UAG gels because of the difference in the microstructure of gel due to the microphase separation between hydrophilic and hydrophobic segments. This phase separation in the course of gelation in water could be confirmed using contact angle measurement. 相似文献
995.
以双[对-酰氯苯基]二甲基硅烷和酚酞、四溴酚酞以及双酚A为原料,合成了三种主链含硅聚芳酯,并对聚合物相关的物理性质进行了表征,制成了均质透明的薄膜.采用低真空法测定这些聚合物对H2、O2、N2、CO2、CH4的气体透过速率,并计算出气体透过系数、扩散系数、溶解系数、分离系数.从气体透过性能与聚合物分子结构关系的角度,按照气体透过的溶解-扩散机制,对聚合物的气体透过性能进行了研究,而且着重讨论了聚合物的堆积密度对气体扩散系数的影响,以及聚合物主链中极性链段的百分含量对气体溶解系数和溶解选择性的影响。 相似文献
996.
Pentacoordinate complex cations of the general formula [(C6F5)2SbL3]3+ stabilized as solid salts in combination with tetraphenylborate (BPh4), tetrafluorobroate (BF4) anions, where L=DMSO, Ph3AsO, PyO, DMF, α-, β- and γ-picoline have been isolated. The newly formed complexes were characterized by elemental analysis, molar conductance measurements, solid-state IR and and NMR. From these results, a five-fold coordination around antimony was required. 相似文献
997.
Kazuyuki MiyashitaHidenobu Murafuji Hiroshi IwakiEito Yoshioka Takeshi Imanishi 《Tetrahedron》2003,59(26):4873-4879
As a second-generation pyridoxal model compound for cystathionine β-synthase, we designed a novel model compound having an ionophore function and an imidazole function, application of which to the β-replacement reaction with various thiols smoothly took place to give S-substituted cysteines. Peptides having a serine-O-carbonate residue at the N-terminal position were also converted to the corresponding peptides having an S-substituted cysteine residue under the catalytic conditions of the novel pyridoxal model compound. 相似文献
998.
999.
Ahmed M. Mayouf Yong-Tae Park 《Journal of photochemistry and photobiology. A, Chemistry》2002,150(1-3):115-123
The UV photolysis of 2′-chloro-4-R-benzanilides in acetonitrile solution under nitrogen atmosphere leads to the formation of intramolecular photocyclization products (9-R-phenanthridin-6(5)-one, 20–30%), along with the minor photoreduction, photo-Fries, and intramolecular photosubstitution products. The photoreaction of 2′-chloro-4-R-benzanilide in acetonitrile solution containing 10% of water or aqueous sodium hydroxide however gives 2-(4-R-phenyl)-1,3-benzoxazole as a major product (30%) along with the photoreduction, photo-Fries and intramolecular photocyclization products. 相似文献
1000.
Lead(II) 2,2'-bipyridine hexachloroplumba tetetrahydrate was synthesized and investigated by DTA, TG and DTG. IR spectroscopy
and other methods enabled the identification of some of the decomposition products. Comparative studies on the corresponding
chlorides: [Pb(bipy)]Cl2 and [Pb(bipy)3]Cl2, which can be considered as precursors of the hexachloroplumbate, were also undertaken. X-ray measurements enabled the tentative
determination of the crystal structure of [Pb(bipy)]Cl2. Hexachloroplumbate decomposes with the liberation of chlorine, water and organic ligands, and the process is accompanied
by the simultaneous transition of Pb(IV)→Pb(II). Chlorides release only ligands upon heating. Residues comprised always PbCl2.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献