全文获取类型
收费全文 | 3621篇 |
免费 | 760篇 |
国内免费 | 552篇 |
专业分类
化学 | 1759篇 |
晶体学 | 158篇 |
力学 | 123篇 |
综合类 | 19篇 |
数学 | 150篇 |
物理学 | 1320篇 |
无线电 | 1404篇 |
出版年
2024年 | 28篇 |
2023年 | 91篇 |
2022年 | 134篇 |
2021年 | 164篇 |
2020年 | 156篇 |
2019年 | 156篇 |
2018年 | 124篇 |
2017年 | 132篇 |
2016年 | 192篇 |
2015年 | 181篇 |
2014年 | 228篇 |
2013年 | 403篇 |
2012年 | 234篇 |
2011年 | 238篇 |
2010年 | 182篇 |
2009年 | 230篇 |
2008年 | 241篇 |
2007年 | 210篇 |
2006年 | 234篇 |
2005年 | 205篇 |
2004年 | 190篇 |
2003年 | 144篇 |
2002年 | 124篇 |
2001年 | 85篇 |
2000年 | 87篇 |
1999年 | 73篇 |
1998年 | 69篇 |
1997年 | 63篇 |
1996年 | 54篇 |
1995年 | 42篇 |
1994年 | 40篇 |
1993年 | 35篇 |
1992年 | 34篇 |
1991年 | 18篇 |
1990年 | 17篇 |
1989年 | 21篇 |
1988年 | 16篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 11篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1978年 | 1篇 |
1977年 | 4篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有4933条查询结果,搜索用时 15 毫秒
91.
92.
王承娟 《中国无机分析化学》2019,9(1):39-42
建立了交流电弧发射光谱法测定化学样品中锡的方法。以氧化锌、氟化钠、硫粉为缓冲剂,改善元素蒸发行为;以锗为内标,样品与缓冲剂比例为1∶1。根据不同含量的锡选择不同级别灵敏线进行分析,采取内标法,建立浓度与相对强度之间的线性关系,计算出样品中锡的含量。实验中可以将锡的分析上限提高到x%,同时避免由稀释样品带来的分析误差。方法相对标准偏差(RSD,n=12)小于7%,测定了国家一级地球化学标准物质,结果与标准值相符。 相似文献
93.
94.
G. A. Zachariadis N. M. Tzollas M. Nikolaou E. Rosenberg 《Biomedical chromatography : BMC》2013,27(3):299-305
A headspace solid‐phase micro‐extraction (HS‐SPME) method was employed in order to study the effect of storage conditions of human urine samples spiked with tributyltin (TBT) using gas chromatography and mass spectrometry. To render the analyte more volatile, the derivatization (ethylation) was made in situ by sodium tetraethylborate (NaBEt4), which was added directly to dilute unpreserved urine samples and in buffers of similar acidity. The stability of TBT in human urine matrix was compared with the stability of TBT in buffer solutions of similar pH value. Critical parameters of storage conditions such as temperature and time, which affect the stability of TBT in this kind of matrix, were examined extensively. The tests showed that the stability of TBT remains practically satisfactory for a maximum of 2 days of storage either at +4 or 20°C. Greater variations were observed in the concentration of TBT in human urine samples at +4°C and lower ones at ?20°C over a month's storage. The freeze–thaw cycles have negative effect on the stability and should be kept to a minimum. The results from spiked urine samples are also discussed in comparison to those acquired from buffer solutions of equal TBT concentration. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
95.
Dr. Yumei Lin Dewang Xie Prof. Dr. Werner Massa Dr. Leonhard Mayrhofer Sina Lippert Benjamin Ewers Dr. Alexey Chernikov Prof. Dr. Martin Koch Prof. Dr. Stefanie Dehnen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(27):8806-8813
In situ transformations of selenidostannate frameworks in ionic liquids (ILs) were initiated by treatment of the starting phase K2[Sn2Se5] and the consecutive reaction products by means of temperature increase and/or amine addition. Along the reaction pathway, the framework dimensionalities of the five involved selenidostannate anions develop from 3D to 1D and back, both in top‐down and bottom‐up style. Addition of ethane‐1,2‐diamine (en) led to the reversion of the 2D→1D step from 2D‐{[Sn24Se56]16?} to 1D‐{[Sn6Se14]4?}. As rationalized by DFT investigations, the 2D anion is thermodynamically favored. Photoconductivity measurements reveal that all samples show Schottky contact behavior with absolute thresholds below 10 V. One of the samples exhibits conductive states within the energy range of visible photons. 相似文献
96.
Different experimental methods including ellipsometry, zeta potential measurements, imbibition studies, and contact angle measurements were used to study the mechanism and influencing factors of wettability alteration of water-wet sandstone surface caused by CTAB (hexadecyl trimethyl ammonium bromide). Results show that when the concentration of CTAB reaches a certain level (below CMC), due to the electrostatic attraction between the positively charged head groups of CTAB and the negatively charged sandstone surface, the monolayer of CTAB is formed and hydrophobic chains of CTAB molecules are toward the aqueous phase, making the solid surface oil-wet. When the concentration of CTAB continues to increase (above CMC), due to the hydrophobic interaction, the compact bilayer of CTAB is formed and hydrophilic head groups of CTAB molecules are toward the aqueous phase, rendering the solid surface water-wet. The contact angles between the oil–water interface and the surface treated with CTAB increase with the increase of the concentration of NaCl and CaCl2. Compared to NaCl, the inorganic salt CaCl2 has a greater impact on the contact angle. In addition, the contact angles increase with the increase of temperature and decrease with the increase of pH value of the aqueous solution. 相似文献
97.
SnO2 rods have been synthesized by using tin (II) chain‐like complexes as precursors in which oxalate and ethylene glycol act as co‐ligands. On the basis of experimental analysis, a reasonable formation mechanism of rods is discussed, which is further supported by Fourier transform infrared spectroscopy (FT‐IR). These results show that the synergistic effect of oxalate group and ethylene glycol plays an important role in the formation of long SnO2 rods. 相似文献
98.
Jing Zhang Li Zhang Xiangqing Li Shi-Zhao Kang 《Journal of Dispersion Science and Technology》2013,34(7):943-947
A stable metalloporphyrin sensitized TiO2 (Degussa P25) photocatalyst was prepared by using trans-dihydroxo[5,10,15,20-tetraphenylporphyrin]tin(IV) (SnP) as a sensitizer in a simple impregnation process. The solid diffuse reflectance ultraviolet-visible (UV-vis) spectrum of the SnP sensitized TiO2 photocatalyst (SnP-TiO2) indicated that there existed interaction between SnP and TiO2. It was found that SnP-TiO2 exhibited an enhanced visible light photocatalytic activity as compared with that over P25 for the degradation of 4-nitrophenol (4-NP) and methyl orange (MO) in aqueous solutions. The mechanism exploration showed that the degradation of MO and 4-NP experienced two different ways, that is, MO was photodegraded by reactive oxygen species and 4-NP was directly photodegraded by the excited state of SnP. Furthermore, it was found that the loading content of SnP had an important influence on the photocatalytic activity of TiO2. The maximum photocatalytic efficiency was achieved when the contents of SnP were 25 mg and 30 mg per gram TiO2 for MO and 4-NP, respectively. Importantly, SnP-TiO2 was particularly stable and the photocatalytic activity was hardly decreased after being recycled seven times in the presence of oxygen, which could be attributed to the easy reductive regeneration of SnP. 相似文献
99.
100.
R.K.B GoverN.D. Withers S. AllenR.L. Withers J.S.O. Evans 《Journal of solid state chemistry》2002,166(1):42-48
SnP2O7 is a member of the ZrP2O7 family of materials, several of which show unusual thermal expansion behavior over certain temperature ranges and which show a number of displacive phase transitions on cooling from high temperature. Here we describe the structural properties of SnP2O7 from 100 to 1243 K as determined by X-ray and neutron powder diffraction. These studies reveal that SnP2O7 shows two phase transitions in this temperature range. At room temperature the material has a pseudo-cubic 3×3×3× superstructure. Electron diffraction studies show that the symmetry of this structure is P213 or lower. On warming to ∼560 K it undergoes a phase transition to a structure in which the subcell reflections show a triclinic distortion; above 830 K the subcell reflections show a rhombohedral distortion. Significant hysteresis in cell parameters is observed between heating and cooling. The structure of SnP2O7 is discussed with references to other members of the AM2O7 family of materials. 相似文献