全文获取类型
收费全文 | 32999篇 |
免费 | 5965篇 |
国内免费 | 6738篇 |
学科分类
数理化 | 45702篇 |
出版年
2024年 | 70篇 |
2023年 | 515篇 |
2022年 | 902篇 |
2021年 | 1127篇 |
2020年 | 1278篇 |
2019年 | 1291篇 |
2018年 | 1129篇 |
2017年 | 1289篇 |
2016年 | 1463篇 |
2015年 | 1634篇 |
2014年 | 2049篇 |
2013年 | 2582篇 |
2012年 | 2980篇 |
2011年 | 3162篇 |
2010年 | 2555篇 |
2009年 | 2439篇 |
2008年 | 2688篇 |
2007年 | 2436篇 |
2006年 | 2353篇 |
2005年 | 1964篇 |
2004年 | 1437篇 |
2003年 | 1103篇 |
2002年 | 1044篇 |
2001年 | 1026篇 |
2000年 | 975篇 |
1999年 | 722篇 |
1998年 | 461篇 |
1997年 | 420篇 |
1996年 | 349篇 |
1995年 | 321篇 |
1994年 | 314篇 |
1993年 | 250篇 |
1992年 | 216篇 |
1991年 | 190篇 |
1990年 | 177篇 |
1989年 | 168篇 |
1988年 | 122篇 |
1987年 | 92篇 |
1986年 | 93篇 |
1985年 | 80篇 |
1984年 | 43篇 |
1983年 | 38篇 |
1982年 | 40篇 |
1981年 | 36篇 |
1980年 | 17篇 |
1979年 | 18篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1971年 | 6篇 |
1936年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
微波等离子体炬发射光谱法去溶系统性能的研究 总被引:7,自引:0,他引:7
用微波等离子体矩(MPT)作激发光源,等离子体的工作气体为氩气,研究了气动雾化进样去溶系统的工作参数对分析性能的影响,探讨了水冷凝与浓硫酸吸收二者协同去溶的相关性,结果表明,OH (Q1带,带头谱线为308.520nm)的发射强度即可判别样品去溶效果。 相似文献
992.
Mg-based hydrogen storage alloys Mg0.9Ti0.1-xZrxNi (x=0.02, 0.04, 0.06) were successfully prepared by means of mechanical alloying (MA). The effects of Zr addition on the discharge capacity and the cycle performance of the Mg-based electrodes were also studied. It was found that the discharge capacities were improved with addition of a small amount of Zr and the cycle performances of the alloy were stabilized with the addition of Ti. The effects of surface modification or coating on the properties of Mg0.9Ti0.06Zr0.04Ni were also studied. The results indicated that coating with graphite improved both the discharge capacity and cycle life of the amorphous Mg0.9Ti0.06Zr0.04Ni electrode. 相似文献
993.
Shape apprearance and crystallite characterization of magnesium hydroxide at different hydrothermal tempera-tures have been investigated by SEM, XRD and DSC methods. The grain size, cell parameter and lattice distortion rate of powders under each condition were obtained. The results show that the grains had grown up, especially along the (001) surface as the hydrothermal temperature was raised, accompanying the shrinkage of lattice and the de-scending of distortion rate. These could be relate to the re-crystallization of Mg(OH)2, as well as the thermal dif-fusion of OH- with lower charges, resulting in good dispersion, high thermal decomposition temperature and en-thalpy of Mg(OH)2. Moreover, it is helpful for the increase of the retardant behavior of Mg(OH)2. 相似文献
994.
新型水溶性膦铑络合物催化烯烃的氢甲酰化反应研究 总被引:10,自引:1,他引:10
新型水溶性膦铑络合物催化烯烃的氢甲酰化反应研究燕远勇,左焕培,金子林(大连理工大学化工学院,大连116012)关键词烯烃,水溶性膦铑络合催化剂,氢甲酰化,醛.1.前言为克服催化剂的流失和与反应产物的分离困难,近年来均相催化的一个重要进展是开发了以磺化... 相似文献
995.
996.
Cerium-doped mesoporous TiO2 nanoparticles with high surface area and thermal stable anatase wall were synthesized via hydrothermal process in a cetyltrimethylammonium bromide (CTAB)/Ti(SO4)2/Ce(NO3)4/H2O system. The obtained materials were characterized by XRD, FESEM, HRTEM, FTIR spectroscopy, nitrogen adsorption and DRS spectra. Experimental results indicated that the doping of cerium not only increased the surface area of mesoporous TiO2 nanoparticles, but also inhibited the mesopores collapse and the anatase-to-rutile phase transformation. Moreover, the undoped, doped anatase mesoporous nanoparticles exhibit higher photocatalytic activity than commercial photocatalyst (Degussa, P25), but the maximum photodegradation rate corresponds to the undoped mesoporous TiO2 nanoparticles. The lower photocatalytic activities of cerium-doped samples compared with undoped one may be ascribed to that the doped cerium partially blocks titania's surface sites available for the photodegradation and absorption of Rhodamine B (RB). 相似文献
997.
Instead of environmentally toxic chromium oxidant, singlet oxygen generated photcchemically was used as environmentally friendly and benign oxidizing agent to accomplish the transformation of pseudodiosgenin diacetate to diosone efficiently in a low toxic and less expensive solvent acetone. Accordingly 16-dehydropregnenolone acetate(16-DPA), an important intermediate for preparation of steroidal drugs, was prepared in good yield (75%) when photoreaction was run in acetone/acetic anhydride/pyridine system. The mild reaction condition as well as simple and environmentally friendly process made the method commercially viable and important for production of 16-DPA in industrial scale. 相似文献
998.
The interactions between drug molecules and membrane were studied using the new chromatography stationary phase of liposome coated zirconia–magnesia. log Ks(ZrO2–MgO) on this new chromatography for some drugs, compared with that on liposome coated silica chromatography and other reported data, fair correlations were observed between them when excluding effect of special adsorption. log Ks(ZrO2–MgO) values for barbitalum, diazepam, benzene, benzocaine and toluene correlated well with corresponding values on liposome coated silica chromatography (R = 0.99778, P < 0.001; R = 0.98229, P < 0.003; R = 0.9985, P < 0.0001; R = 0.99925, P < 0.0001, pH value of mobile phase at pH 7.4, 7.0, 6.4 and 5.4, respectively). They also correlated well with the literature data on immobilized artificial membrane chromatography (R = 0.99999, P < 0.004 at pH 7.4) and liposome chromatography (R = 0.99994, P < 0.008) for procaine, lidocaine and bupivacaine. Liposome coated zirconia–magnesia chromatography can thus be used for studying drug–membrane interaction and prediction of drug absorption as another liposome chromatography method. 相似文献
999.
考察了阳离子、非离子和阴离子表面活性剂存在下水杨酸-2’-乙基己基酯(EHS)的吸收光谱和激发态分子内质子转移(ESIPT)荧光光谱. 结果表明, EHS可增溶在胶束中, 2’-乙基己基碳链朝向胶束内核, 而水杨酸基朝向胶束-水界面; 胶束环境有利于EHS分子对紫外光的吸收和分子内氢键的形成, 从而使ESIPT 荧光显著增强, 激发态分子以发射可见光和非辐射去活化方式衰减; 并根据EHS和表面活性剂分子的结构和大小, 解释了EHS分子在胶束中的结合位点, 荧光猝灭和酯水解的光谱测量进一步为此结合位点提供了佐证. 相似文献
1000.
Ping Wang Yi-Zeng Liang Ben-Mei Chen Neng Zhou Lun-Zhao Yi Yan Yu Zhi-Biao Yi 《Chromatographia》2007,65(3-4):209-215
A rapid, simple and specific liquid chromatography-electrospray ionization mass spectrometry (LC-ESI-MS) method has been developed
and validated for the simultaneous determination of enalapril and its major active metabolite enalaprilat in human plasma.
Benazepril hydrochloride was used as the internal standard. Plasma was deproteinized with acetone and centrifuged. The supernatant
was transferred and evaporated to dryness and the residue dissolved in mobile phase. Samples were separated on a C18 column
with a mobile phase of methanol–20 mM ammonium acetate (53:47, v/v) containing 0.15% trifluoracetic acid (v/v) with a pH of 3.0. Enalapril, enalaprilat and the internal standard were measured by electrospray positive selective ion
monitoring mode. The method was validated over a linear range of 1.56–400 ng mL−1 and the limits of quantification were 1.56 ng mL−1 for both enalapril and enalaprilat using 0.1 mL plasma. Extraction efficiency was more than 84% and recoveries were in range
of 93.65–101.17%. The intra-day relative standard deviations (RSD) were less than 8.16 and 7.05% and inter-day RSDs were within
8.42 and 5.72% for enalapril and enalaprilat, respectively. The storage stability of QC samples was investigated under various
conditions. The method was successfully applied for the evaluation of the pharmacokinetics and bioequivalence of enalapril
and enalaprilat in 20 healthy volunteers after an oral dose of 20 mg enalapril maleate. 相似文献