全文获取类型
收费全文 | 9641篇 |
免费 | 416篇 |
国内免费 | 296篇 |
专业分类
化学 | 5641篇 |
晶体学 | 46篇 |
力学 | 325篇 |
综合类 | 68篇 |
数学 | 2163篇 |
物理学 | 2110篇 |
出版年
2023年 | 48篇 |
2022年 | 73篇 |
2021年 | 83篇 |
2020年 | 86篇 |
2019年 | 287篇 |
2018年 | 121篇 |
2017年 | 158篇 |
2016年 | 131篇 |
2015年 | 109篇 |
2014年 | 173篇 |
2013年 | 277篇 |
2012年 | 194篇 |
2011年 | 391篇 |
2010年 | 356篇 |
2009年 | 523篇 |
2008年 | 421篇 |
2007年 | 368篇 |
2006年 | 341篇 |
2005年 | 229篇 |
2004年 | 143篇 |
2003年 | 185篇 |
2002年 | 195篇 |
2001年 | 106篇 |
2000年 | 121篇 |
1999年 | 111篇 |
1998年 | 102篇 |
1997年 | 75篇 |
1996年 | 71篇 |
1995年 | 63篇 |
1994年 | 49篇 |
1993年 | 46篇 |
1992年 | 39篇 |
1991年 | 34篇 |
1990年 | 29篇 |
1989年 | 32篇 |
1987年 | 24篇 |
1986年 | 22篇 |
1985年 | 683篇 |
1984年 | 472篇 |
1983年 | 405篇 |
1982年 | 462篇 |
1981年 | 363篇 |
1980年 | 295篇 |
1979年 | 308篇 |
1978年 | 230篇 |
1977年 | 251篇 |
1976年 | 273篇 |
1975年 | 243篇 |
1974年 | 242篇 |
1973年 | 279篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
A new titrimetric method for determination of tin (4%) in aluminium alloys, tin- and lead-base alloys, solders, white bearing alloys, special bronzes and silver brazing alloys is proposed. HEDTA, Semi-Xylenol Orange and bismuth perchlorate are used as titrant, indicator and back-titrant respectively. Measures are taken to overcome the hydrolysis of Sn(IV). Monochloroacetic acid and ethylene glycol are added as auxiliary agents. The standard deviation of this method was found to be 0.2 mg and its coefficient of variation to vary from 0.25 to 2%, according to amount of tin. A novel method of sample decomposition and a modified method for separating Sn(IV) are also suggested. 相似文献
52.
The objective of the work described in this paper was to produce dispersions of small spherical carbon particles, having particle diameters in the region of 0.1 μm. To this end, the dehydrochlorination of poly(vinylidene chloride) (PVDC) latex particles was attempted. The PVDC latex was prepared by a dispersion polymerization route. Both chemical and thermal dehydrochlorination routes were attempted. Chemical dehydrochlorination, using a variety of base/solvent systems, led to nonporous, spherical black particles of the required size, but which contained only 60% carbon; most of the remainder was oxygen, introduced by nucleophilic substitution reactions. Thermal dehydrochlorination, at 700°C under a nitrogen atmosphere, using a fluidized bed arrangement, on the other hand, led to black particles, having 90% carbon and which retained their sphericity, but which were highly porous. Initial chemical dehydrochlorination, prior to thermal treatment, did not seem to reduce the porosity of the final carbons. Dispersions of the carbon particles in a variety of solvents were readily achieved. 相似文献
53.
Zhengping Hao Lidun An Junling Zhou Hongli Wang 《Reaction Kinetics and Catalysis Letters》1996,59(2):295-300
Supported gold catalyst for eliminating hydrogen from CO2 feed gas in the production of urea is found to be superior to other industrial catalysts (e.g. Pt/Al2O3 and PdPt/Al2O3) in catalytic activity and resistance to sulfur poisoning. 相似文献
54.
研究了阿富汗青金石矿石、敦煌石窟青金石颜料、合成群青的呈色机理。实验表明三者均有g=2.029峰,前两者多一个g=2.047峰,400℃时,g=2.047峰变平,600℃时消失。g=2.029峰随温度升高而增强,800℃时增强放慢,颜色也随温度变深。在M0O3/SiO2和M0O3/Al2O3表面上,S2^-的EPR有g1=2.047,g2=2.208,g3=1.998三个分量。因此,群青、青金石和 相似文献
55.
The first evidence that α-acylnitrone is an intermediate in the base-promoted reaction of 4-nitro-N,N-diethylaniline with aceptophenome to give an enaminoketone was obtained.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1645–1647, August, 1998. 相似文献
56.
选用乳化-溶剂挥发法制备乙基纤维素载药微球(EMs), 并通过内部凝胶化法进行包衣制得海藻酸钠-乙基纤维素载药微囊(AEMs), 最后通过离子交联法进一步包衣制得壳聚糖-海藻酸钠-乙基纤维素载药微囊(CAEMs). 研究克拉霉素漂浮\|生物粘附微囊的制备工艺, 并考察微囊的体外漂浮性能、 粘附性能及体内滞留性能. 结果表明, CAEMs球形度较好, 药物包封率为72.3%~78.2%, 载药量为7.1%~12.7%. 在pH=5的醋酸缓冲液中, 6 h时的累积释放率为56.6%~70.6%, 漂浮率大于70%, 4 h时的体内滞留率为60.5%. CAEMs有望通过延长药物胃内滞留时间, 在临床用于根除幽门螺旋杆菌, 从而降低消化道溃疡的复发率. 相似文献
57.
J. Fernandez M.I. Paz Andrade F. Sarmiento R. Bravo 《The Journal of chemical thermodynamics》1983,15(12):1145-1149
The excess molar enthalpies HmE of methyl propanoate or methyl pentanoate + 1-butanol, + 1-hexanol, + 1-octanol, and + 1-decanol have been determined experimentally at 298.15 K using a Calvet microcalorimeter. For all these mixtures HmE > 0; the values increase with the chain length of the alkanol but decrease as the ester chain lengthens. 相似文献
58.
59.
The d.c. polarographic current-potential curves of Cd(II)-EDTA complexes were examined in the pH range 0.5–10.0, to elucidate the mechanism of their electrode processes and to determine the relevant electrochemical kinetic parameters. It was shown that the first wave observed below pH 3 at ?0.58 to ?0.65 V vs. SCE is the reversible reduction wave of Cd(II) aquo-ion with kinetically-controlled limiting current, and the second wave observed above pH 1.5 at ?0.75 to ?1.21 V vs. SCE corresponds to the simultaneous irreversible reduction of four complex species, CdH3L+, CdH2L, CdHL? and CdL2?, where CdHpL(p?2)+ and L4? denote the protonated complex species with p protons and the unprotonated EDTA ion, respectively. Analysis of the dependence of limiting current on the hydrogen ion concentration led to the conclusion that the preceding reaction determining the behaviour of limiting current is CdH3L+?Cd2++H3L? with k3d=6.3×102 s?1 and k3f=3.3×106 s?1M?1, where k3d and k3f are the dissociation and formation rate constants, respectively. On the other hand, from analysis of the dependence of half-wave potentials of the second wave on the hydrogen ion concentration, the kinetic parameters of the four complex species were evaluated, and are given in Table 1. Further, it was shown that the cathodic rate constants of these four charge transfer processes at some reference potential together with those of Cd(II)-HEDTA complexes fulfil the linear free energy relationship. 相似文献
60.
The magnesium-butadiene 1 : 1 (1), 1 : 2 (II) and 1 : 3 addition compounds obtained by the direct metalation of butadiene with metallic magnesium were isolated. These compounds have a polymeric structure. Structural analysis of these compounds by NMR and IR methods was carried out. The process for the formation of II via I and III via II was investigated. The results of protolysis and alkylation of the above magnesium compounds corresponded well to those of crotylmagnesium compounds. 相似文献