全文获取类型
收费全文 | 84659篇 |
免费 | 17438篇 |
国内免费 | 8049篇 |
专业分类
化学 | 75964篇 |
晶体学 | 1044篇 |
力学 | 3358篇 |
综合类 | 443篇 |
数学 | 7994篇 |
物理学 | 21343篇 |
出版年
2024年 | 164篇 |
2023年 | 968篇 |
2022年 | 1891篇 |
2021年 | 2020篇 |
2020年 | 3095篇 |
2019年 | 4350篇 |
2018年 | 2682篇 |
2017年 | 2327篇 |
2016年 | 5557篇 |
2015年 | 5770篇 |
2014年 | 6191篇 |
2013年 | 7576篇 |
2012年 | 7518篇 |
2011年 | 6933篇 |
2010年 | 6156篇 |
2009年 | 5969篇 |
2008年 | 5725篇 |
2007年 | 4924篇 |
2006年 | 4445篇 |
2005年 | 4033篇 |
2004年 | 3224篇 |
2003年 | 2726篇 |
2002年 | 3459篇 |
2001年 | 2542篇 |
2000年 | 2278篇 |
1999年 | 1392篇 |
1998年 | 848篇 |
1997年 | 795篇 |
1996年 | 729篇 |
1995年 | 627篇 |
1994年 | 559篇 |
1993年 | 410篇 |
1992年 | 393篇 |
1991年 | 346篇 |
1990年 | 302篇 |
1989年 | 234篇 |
1988年 | 172篇 |
1987年 | 163篇 |
1986年 | 149篇 |
1985年 | 140篇 |
1984年 | 73篇 |
1983年 | 67篇 |
1982年 | 59篇 |
1981年 | 28篇 |
1980年 | 22篇 |
1979年 | 21篇 |
1978年 | 12篇 |
1976年 | 16篇 |
1975年 | 12篇 |
1957年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Polynuclear mixed‐valent nickel oxide and nickel hexacyanoferrate hybrid film was prepared on glassy carbon electrode by multiple scan cyclic voltammetry. The film growth was monitored using electrochemical quartz crystal microbalance (EQCM). The cyclic voltammogram of the nickel hexacyanoferrate film is characterized by single redox couple whereas nickel oxide/nickel hexacyanoferrate hybrid film exhibits two redox couples. Cyclic voltammetric features suggest that the charge transfer process in both films resembles that of surface‐confined redox species. In stronger basic solution (pH ≥9), nickel hexacyanoferrate film was gradually converted into nickel oxide film during potentiodynamic cycling. The peak potential of nickel oxide redox couple moved into more negative side with increasing pH of contacting solution whereas the peak potential of nickel hexacyanoferrate redox couple remains the same. Electrocatalytic behavior of hybrid film coated electrodes toward ascorbic acid, hydrazine and hydroxylamine was investigated using cyclic voltammetry technique. Analytical application of nickel oxide/nickel hexacyanoferrate hybrid film electrode was tested in amperometry and flow injection analysis. 相似文献
992.
Fathy Abdel‐Kader Amer Metwally Hammouda Abdel‐Aziz Sayed El‐Ahl Bakr Fathy Abdel‐Wahab 《中国化学会会志》2007,54(6):1543-1552
Treatment of heterocyclic β‐ketonitriles 1a,b with hydrazine hydrate and phenylhydrazine afforded the hydrazine derivatives 2a‐d which cyclized in PPA into pyrrolo[3,4‐c]pyrazoles 3a‐d. Reaction of 1a,b with cyanoacetohydrazide furnished the cyanoacetyl pyrrolo[3,4‐c]pyrazoles 4a,b. The hydrazine 2c reacted with β‐diketone and β‐ketoesters to afford pyrazolyl‐pyrrolines 5‐7. Also the later hydrazine reacted with some D‐aldoses and aceteophenone to give the corresponding hydrazones 10‐12 and hydrazine carboxamide derivatives 15a,b respectively. 相似文献
993.
C-peptide radioimmunoassay (C-peptide RIA) is widely used in determination of pancreatic B-cell secretion activity.125I labeled TyrC-peptide is indispensable in C-peptide RIA kit. Herein we discuss a way of obtaining recombinant Tyr-C-peptide.
Arg32Tyr human proinsulin mutant (R32Y-proinsulin) gene was constructed by site-directed mutagenesis and overexpressed inEscherichia coli. Purified R32Y-proinsulin was converted to insulin and Tyr-C-peptide by trypsin and carboxypeptidase B codigestion. Tyr-C-peptide
was isolated through reverse-phase HPLC (RP-HPLC) and identified by C-peptide RIA and amino acid analysis. 相似文献
994.
995.
W/O/W乳液的渗透溶胀与夹带溶胀 总被引:6,自引:0,他引:6
研究了W/O/W乳液的溶胀,实验结果表明,渗透溶胀随内外相溶液间的渗透压差、表面活性剂及载体浓度的增大而增加,但随膜粘度的增加而降低,渗透压差较高时,水渗透的影响大于夹带的影响;膜相中含氧化合物对溶胀的影响大于含氮化合物,采用Span 80作乳化剂时,比采用E 644渗透溶胀约高6倍,夹带溶胀也较高;重复聚结再分散使夹带溶胀急剧增加,因而多级混合澄清槽对液膜操作似不适用。 相似文献
996.
应用改进了的旋转全电极上的电势阶跃计时库仑法,分别测定了不同电极电势下氧还原过程中电极吸附中间物和溶解中间物氧化所需的电量,实验证明,在此过程中电极上确有吸附中间物存在,而且是电极电势的函数,但其分子属性尚需进一步鉴别。 相似文献
997.
Q. Zhong L. He T. E. Beesley W. S. Trahanovsky P. Sun C. Wang D. W. Armstrong 《Chromatographia》2006,64(3-4):147-155
The comparisons of five different chiral stationary phases (CSPs) based on 2,6-dinitro-4-trifluoromethylphenyl (DNP-TFM) ether substituted β-cyclodextrin are presented. The five CSPs differ from each other in the linkage/spacer chemistry, or on the position of the substituents on β-cyclodextrin, or in the sequence of the synthetic procedure. The results show that there are two optimum combinations: (1) DNP-TFM randomly substituted on the β-cyclodextrin as the chiral selector along with a carbamate linkage chain bonding it to the silica support; and (2) β-cyclodextrin derivatized by DNP-TFM substituents only on the C-2 and C-3 positions of the cyclodextrin with an ether linkage chain anchoring it to the silica gel. These two combinations show complementary separations for some enantiomers. The spacer chain effect is much more pronounced for the CSP based on the β-cyclodextrin derivatives with DNP-TFM substituents only on C-2 and C-3 positions than its randomly substituted counterpart. The sequence of derivatizing the cyclodextrin and attaching it to silica gel also affects its selectivity and efficiency. The β-cyclodextrin should be derivatized before it is linked to the silica gel. 相似文献
998.
无机胶体法制备Pt/C催化剂及其性能表征 总被引:3,自引:0,他引:3
采用无机胶体法制备用于质子交换膜燃料电池(PEMFC)的Pt/C催化剂。研究了影响PtO2胶体生成和稳定性的因素(溶液的pH值、浓度和温度条件等)以及不同还原剂浓度对Pt/C催化剂性能的影响。透射电子显微镜测试结果表明,采用经优化的工艺条件所制备的Pt/C催化剂平均粒径为3 nm,且分散性好、粒度均匀。X-射线衍射分析表明,催化剂中Pt(111)晶面的相对含量较高,有利于加速氧还原反应。单体PEMFC的电压/电流密度曲线测试表明,所制备的Pt/C催化剂具有良好的电化学性能。 相似文献
999.
A highly sensitive and selective catalytic adsorptive cathodic striping procedure for the determination of trace germanium is presented. The method is based on adsorptive accumulation of the Ge(IV)‐gallic acid (GA) complex onto a hanging mercury drop electrode, followed by reduction of the adsorbed species. The reduction current is enhanced catalytically by addition of vanadium(IV)‐EDTA. The optimal experimental conditions include the use of 0.03 mol/L HClO4 (pH1.6), 6.0×10?3 mol/L GA, 3.0×10?3 mol/L V(IV), 4.0×10?3 mol/L EDTA, an accumulation potential of ?0.10 V(vs. Ag/AgCl), an accumulation time of 120 s and a differential pulse potential scan mode. The peak current is proportional to the concentration of Ge(IV) over the range of 3.0×10?11 to 1.0×10?8 mol/L and the detection limit is 2×10?11 mol/L for a 120 s adsorption time. The relative standard deviation at 5.0×10?10 mol/L level is 3.1%. No serious interferences were found. The method was applied to the determination of germanium in ore, mineral water and vegetable samples with satisfactory results. 相似文献
1000.
Howard C. Clark Anil B. Goel Chun S. Wong 《Journal of organometallic chemistry》1980,190(4):C101-C104
Simple synthetic routes to the mixed ligand complexes PtLL'X2 and PtLL'XY (L' = PEt3; L = phosphine, arsine, etc.; X = Cl and Y = Cl, H or Me) are described; unexpectedly, these display an extensive chemistry without disproportionation, although in some cases ligand scrambling does occur. 相似文献