全文获取类型
收费全文 | 69145篇 |
免费 | 6150篇 |
国内免费 | 13187篇 |
专业分类
化学 | 60108篇 |
晶体学 | 2267篇 |
力学 | 882篇 |
综合类 | 704篇 |
数学 | 6687篇 |
物理学 | 17834篇 |
出版年
2024年 | 110篇 |
2023年 | 1075篇 |
2022年 | 1114篇 |
2021年 | 2331篇 |
2020年 | 2255篇 |
2019年 | 2087篇 |
2018年 | 1712篇 |
2017年 | 2205篇 |
2016年 | 2298篇 |
2015年 | 2007篇 |
2014年 | 2752篇 |
2013年 | 6001篇 |
2012年 | 4225篇 |
2011年 | 4521篇 |
2010年 | 3948篇 |
2009年 | 4716篇 |
2008年 | 4590篇 |
2007年 | 4796篇 |
2006年 | 4456篇 |
2005年 | 3887篇 |
2004年 | 3723篇 |
2003年 | 3072篇 |
2002年 | 2610篇 |
2001年 | 2081篇 |
2000年 | 2741篇 |
1999年 | 2155篇 |
1998年 | 1942篇 |
1997年 | 1240篇 |
1996年 | 1095篇 |
1995年 | 1079篇 |
1994年 | 951篇 |
1993年 | 789篇 |
1992年 | 743篇 |
1991年 | 555篇 |
1990年 | 371篇 |
1989年 | 353篇 |
1988年 | 306篇 |
1987年 | 206篇 |
1986年 | 153篇 |
1985年 | 185篇 |
1984年 | 169篇 |
1983年 | 61篇 |
1982年 | 108篇 |
1981年 | 152篇 |
1980年 | 123篇 |
1979年 | 105篇 |
1978年 | 81篇 |
1977年 | 75篇 |
1976年 | 45篇 |
1973年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
961.
962.
本文介绍了一种分析裂化气中杂质烃的方法。所建分析系统由填充玻璃毛细管柱和填充预柱各一根、阀和FID各一个组成。采用一次进样、外标法定量,约30分钟即可完成测定裂化气中低至0.5ppm含量的痕量C_2~C_4炔和二烯。方法适用围范至少是0.5ppm~1%。当样品含量为1~5 ppm时,相对标准偏差小于50%。 相似文献
963.
本文研究了以玻碳电极为基体的1:12硅钼杂多酸根(SiMo_(12)O_(40)~(4-)简称12-MSA)修饰电极的制备及其电化学行为,将12-MSA电极应用于线性扫描伏安法测定天然水中可溶性硅酸盐,结果满意.硅浓度在8.0×10~(-7)~1.7×10~(-3)mol/L,相对标准偏差(n=7)为1.85%,加标回收率为98.2%~103.6%,SiMo_(12)电极具有优良的选择性和稳定性。 相似文献
964.
新显色剂1-(对-偶氮苯)-3-(2-噻唑)-三氮烯与锌显色反应的研究及应用 总被引:3,自引:0,他引:3
本文研究了新显色剂1-(对-偶氮苯)-3-(2-噻唑)-三氮烯在非离子表面活性剂TritonX-100存在下,与锌(Ⅱ)显色反应的适宜条件。结果表明,在pH13.0~14.0范围内,锌(Ⅱ)与题示试剂形成1:3的稳定粉红色配合物,其最大吸收波长为545nm,试剂的最大吸收波长为463nm。配合物表现摩尔吸光系数在545nm处为9.6×10 ̄4L·mol ̄(-1)*cm ̄(-1);0~24μg/25mLZn符合比耳定律。该试剂对锌的选择性良好。方法用于铝合金中锌的直接测定,结果令人满意。 相似文献
965.
SAPO-5,-11和-34分子筛模板剂的脱除及其结构的热致变化 总被引:2,自引:0,他引:2
用水热法合成了SAPO-5,-11和-34硅磷酸铝类分子筛。并对经不同温度焙烧处理的样品进行表征,结果表明:分子筛中模块剂的热分解过程与其结构和孔大小有较大的关系。在SAPO-5和SAPO-34分子筛中,外部联接的T-O-T键的反对称伸缩振动峰(1240cm~(-1)-1038cm~(-1))和其对称伸缩振动(734cm~(-1)-600cm~(-1))及双环的变形振动(600-500cm~(-1))的吸收峰对样品的结晶度比较敏感。但在SAPO-11中,分子筛骨架中T-O-T反对称伸缩振动(1225cm~(-1)和1038cm~(-1))随结晶度的下降而基本保持不变,但内四面体(TO_4)的对称伸缩振动,反对称伸缩振动峰及T-O弯曲振动峰随样品焙烧温度的升高而减弱并分裂。红外吸收带向低频方向移动。SEM-EDAX结果表明:随样品焙烧温度的升高,结晶度下降,分子筛晶形变得不规则,晶面越来越粗糙,但结晶度的下降并非由于其中PO_4四面体的热分解所引起,而是由外部联结的T-O-T键的断裂,双环被破坏及骨架元素间的重排所引起。 相似文献
966.
V. V. Nedel'ko B. L. Korsounskii T. S. Larikova V. R. Stepanov N. V. Chukanov I. V. Nedel'ko 《Russian Chemical Bulletin》1994,43(11):1812-1815
The thermal decomposition of 1-ethyl-5-iodotetrazole in a melt and in solutions was studied using thermogravimetry, manometry, pyrolytic mass spectrometry, and IR spectroscopy. The kinetic and activation parameters of the process and the nature of the decomposition products were determined. The reaction mechanism is assumed to involve equilibrium tautomeric rearrangement of the tetrazole to azidoazomethine form followed by homolytic cleavage of the C-I bond.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1923–1926, November, 1994.The authors are grateful to O. A. Ivashkevich and P. N. Gaponic (Belarussian State University), who provided the samples of the tetrazole. 相似文献
967.
The stoichiometric pK
1
*
and pK
2
*
for the ionization of sulfurous acid has been determined from emf measurements in NaCl solutions with varying concentrations of added MgCl2 (m=0.1, 0.2 and 0.3) from I=0.5 to 6.0 molal at 25°C. These experimental results have been treated using both the ion pairing and Pitzer's specific ion-interaction models. The Pitzer parameters for the interaction of Mg2+ with SO2 and HSO
3
–
yielded =0.085±0.004, (0) = 0.35±0.02, (1) = 1.2±0.04, and C = –0.072±0.007. The Pitzer parameters (0) = –2.8±0.4, (1) = 12.9±2.9 and (2) = –2071±57 have been determined for the interactions of Mg2+ with SO
3
2–
. The calculated values of pK
1
*
and pK
2
*
using Pitzer's equations reproduce the measured values to within ±0.04 pK units. The ion pairing model with log KMgSO3=2.36±0.02 and logMgSO3 = 0.1021, reproduces the experimental values of pK
2
*
to ±0.01. These results demonstrate that treating the data by considering the formation of MgSO3 yields a better fit of the experimental measurements with fewer adjustable parameters. With these derived coefficients obtained from the Pitzer equations and the ion pairing model, it is possible to make reliable estimates of the activity coefficients of HSO
3
–
and SO
3
2–
in seawater, brines and marine aerosols containing Mg2+ ions. 相似文献
968.
969.
Solutions of 1-hexanol and 1,2-hexanediol in heptane have been investigated tigated by means of dielectric time domain spectroscopy (TDS). The permittivity spectrum of 1-hexanol in heptane is characterized by a model function containing a sum of three elementary Debye dispersions, while 1,2-hexanediol in heptane is best described by a Cole-Davidson model function. It is shown that dilute solutions of 1-hexanol in heptane have a completely different behavior to that of 1,2-hexanediol. For the diol, the relaxation time levels off at a high value indicating an existence of higher hydrogen bonded complexes. It is possible to quantify the relative amount of monomeric 1-alcohol molecules from the dielectric spectrum. The monomerization rate for 1-hexanol upon dilution with heptane is initially low, but increases rapidly for mole fractions of heptane exceeding 0.4. 相似文献
970.
J. Howard Rytting Danny R. McHan Takeru Higuchi David J. W. Grant 《Journal of solution chemistry》1986,15(8):693-703
Henry's law constants have been determined for -butyrolactone (BL), ethyl acetate (EA), and 2-methyl-3-pentanol (MEP) in mixtures of iso-octane (ISO) and toluene (TOL), for BL, EA, TOL, and ISO in cinnamaldehyde (CIN) and for TOL and ISO in each other and in BL. From these data and published vapor pressures, the activity coefficients at infinite dilution and the standard molar Gibbs free energy of transfer, G
2
0
of the solutes from dilute solution in ISO to dilute solution in each solvent medium have been calculated. The different behavior patterns of BL and EA are attributed to differences in their abilities to exist in different conformations possessing different dipole moments. For polar solutes, G
2
0
decreases with increasing polarizability of the solvent and with increasing dipole moment of the solute, suggesting increased contributions from dipole-induced dipole (Debye) interactions. The sigmoidal plot of G
2
0
against the change in pair potential energy calculated from the classical expressions suggests that G
2
0
seriously underestimates the strength of the Debye interactions in comparison with the London interactions. 相似文献