全文获取类型
收费全文 | 8040篇 |
免费 | 1377篇 |
国内免费 | 562篇 |
专业分类
化学 | 2818篇 |
晶体学 | 86篇 |
力学 | 1623篇 |
综合类 | 66篇 |
数学 | 516篇 |
物理学 | 4870篇 |
出版年
2024年 | 13篇 |
2023年 | 57篇 |
2022年 | 156篇 |
2021年 | 178篇 |
2020年 | 213篇 |
2019年 | 222篇 |
2018年 | 180篇 |
2017年 | 265篇 |
2016年 | 341篇 |
2015年 | 351篇 |
2014年 | 423篇 |
2013年 | 1081篇 |
2012年 | 487篇 |
2011年 | 568篇 |
2010年 | 411篇 |
2009年 | 447篇 |
2008年 | 456篇 |
2007年 | 459篇 |
2006年 | 400篇 |
2005年 | 379篇 |
2004年 | 362篇 |
2003年 | 291篇 |
2002年 | 255篇 |
2001年 | 230篇 |
2000年 | 189篇 |
1999年 | 151篇 |
1998年 | 191篇 |
1997年 | 153篇 |
1996年 | 138篇 |
1995年 | 138篇 |
1994年 | 114篇 |
1993年 | 117篇 |
1992年 | 96篇 |
1991年 | 72篇 |
1990年 | 51篇 |
1989年 | 60篇 |
1988年 | 69篇 |
1987年 | 34篇 |
1986年 | 35篇 |
1985年 | 28篇 |
1984年 | 24篇 |
1983年 | 4篇 |
1982年 | 20篇 |
1981年 | 18篇 |
1980年 | 5篇 |
1979年 | 19篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1973年 | 4篇 |
1972年 | 5篇 |
排序方式: 共有9979条查询结果,搜索用时 31 毫秒
131.
Reactions in a Mixture of CH4 and CO2 under the Aciton of Microwave Discharge at Atmospheric Pressure 下载免费PDF全文
Reactions between CH4 and CO2 under the action of continuous microwave discharge at atmospheric pressure were studied in a special homemade reactor,The main products were CO and H2,while acetylene and ethylene were also found in the products.Experimental results show that conversions of CH4 and CO2 could be higher than 90% without the presence of any catalyst,Effects of CO2/CH4 molar ratio and total flow rate of the feed gas on the reaction were also investigated. 相似文献
132.
Z. P. Lu L. Stachowicz P. Kong J. Heberlein E. Pfender 《Plasma Chemistry and Plasma Processing》1991,11(3):387-394
Diamond crystals and films have been success full y synthesized by DC thermal plasma jet CVD at a pressure of I atrn. A novel triple torch plasma reactor has been used to generate a convergent plasma volume to entrain the participating gases. Three coalescing plasma jets produces! by this reactor direct the dissociated and ionized gaseous species onto ( 100) silicon wafer substrates where the diamond grows. In a typical 10-min run, depending on the method of .substrate preparation, either microcrystals with sizes up to 8 m or continuous films with thicknesses of 1–2 m have been obtained. X-ray diffraction, scanning electron microscopy, and Raman spectroscopy have been used for the characterization of the crystals and of the films. 相似文献
133.
称取镍基单晶高温合金0.100 0g于聚四氟乙烯烧杯中,先令其与盐酸9mL和硝酸1mL加热反应,待反应缓慢时滴加氢氟酸2mL并继续加热使样品完全溶解。加入500g·L^-1酒石酸溶液2mL,冷却至室温,在塑料容量瓶中加水定容至100.0mL。按仪器工作条件采用电感耦合等离子体原子发射光谱法测定其中5种合金元素(Mo、W、Ta、Re、Ru)的含量,选择分析谱线依次为204.598,207.911,240.063,197.312,240.272nm。结果发现:除了Re外,其余4种元素的测定中均受共存元素的光谱干扰,严重影响了测定结果的准确性。为克服其干扰,除了采用基体匹配法消除镍的基体干扰外,试验采用混合校正系数矩阵法对测定结果进行校准。通过一系列试验计算得到混合校准系数矩阵K,并应用于模拟样品的分析。结果表明:经过矩阵K的校准,所测定的5种元素的准确度显著提高,达到了消除共存元素之间光谱干扰的目的。通过精密度试验,测得上述5种元素测定值的相对标准偏差(n=11)均在1.5%以下,并通过标准加入法进行回收试验,测得5种元素的回收率为97.0%~105%。 相似文献
134.
135.
《Chemphyschem》2003,4(4):359-365
We studied the thermodynamic stability of a small monomeric protein, staphylococcal nuclease (Snase), as a function of both temperature and pressure, and expressed it as a 3D free‐energy surface on the p,T‐plane using a second‐order Taylor expansion of the Gibbs free‐energy change ΔG upon unfolding. We took advantage of a series of different techniques (small‐angle Xray scattering, Fourier‐transform infrared spectroscopy, differential thermal analysis, pressure perturbation calorimetry and densitometry) in the evaluation of the conformation of the protein and in evaluating the changes in the thermodynamic parameters upon unfolding, such as the heat capacity, enthalpy, entropy, volume, isothermal compressibility and expansivity. The calculated results of the free‐energy landscape of the protein are in good agreement with experimental data of the p,T‐stability diagram of the protein over a temperature range from 200 to 400 K and at pressures from ambient pressure to 4000 bar. The results demonstrate that combined temperature–pressure‐dependent studies can help delineate the free‐energy landscape of proteins and hence help elucidate which features and thermodynamic parameters are essential in determining the stability of the native conformational state of proteins. The approach presented may also be used for studying other systems with so‐called re‐entrant or Tamman loop‐shaped phase diagrams. 相似文献
136.
K3MnH5, the First Salt-like Manganese Hydride K3MnH5 and K3MnD5 were synthesized by the reaction of potassium hydride (deuteride) with manganese powder under a hydrogen pressure above 3000 bar at 875 K. X-ray investigations on powdered samples and elastic neutron diffraction experiments on the deuterated compound at the time-of-flight spectrometer LAD in the temperature range 5–600 K led to the complete structure determination. The atomic arrangement is isotypic with that of Cs3[CoCl4]Cl (space group: 14/mcm, Z = 4). The structure of K3MnH5 contains isolated [MnH4]2?-tetrahedra and additional hydrogen ions which are exclusively coordinated by potassium cations. The magnetic susceptibilities show Curie-Weiss behaviour. At temperatures below 50 K there are obviously antiferromagnetic interactions. 相似文献
137.
138.
Ursula Ehrmann Lante Carbognani Claudio Ceccarelli 《Journal of separation science》1992,15(7):467-471
Two methods have been developed for the determination of trace levels of acetonitrile and propionitrile in light naphthas. One involves clean-up and preconcentration by medium pressure liquid chromatography (MPLC) in which nitriles are concentrated ten-fold while the complex hydrocarbon matris is removed. The concentrated solution is then analyzed by high resolution GC with flame ionization detection. The second method involves direct injection of samples into a capillary GC equipped with chemiluminescence nitrogen detection. The results and repeatability obtained from both methods are comparable. Preconcentration enables identification by GC-MS and simultaneous analysis of other polar species, with no need for selective detectors. Direct analysis is, on the other hand, much less time-consuming and requires less sample. 相似文献
139.
140.
The thermodynamics of the conversion of aqueous glucose to fructose has been investigated using both heat conduction microcalorimetry
and high pressure liquid chromatography (HPLC). The reaction was carried out in both aqueous Tris/HCl buffer and in aqueous
phosphate buffer in the pH range 7–8 using the enzyme glucose isomerase and the cofactors CoCl2 and MgSO4. The temperature range over which this reaction was investigated was 298.15–358.15 K. We have found that the enthalpy of
reaction is independent of pH over the range investigated. A combined analysis of both the HPLC and microcalorimetric data
leads to the following results at 298 15 K:ΔG° = 349 ± 53 J mol-1, ΔH° = 2.78 ± 0.20 kJ mol-1, and ΔC
p
°
= 76 ± 30 J mol-1 K-1. The stated uncertainties are based upon an analysis of both the random and systematic errors inherent in the measurements.
Comparisons are made with literature data. The percent conversion of glucose to fructose has been calculated for the temperature
range 300–373.15 K. 相似文献