全文获取类型
收费全文 | 33228篇 |
免费 | 3916篇 |
国内免费 | 3148篇 |
专业分类
化学 | 10305篇 |
晶体学 | 435篇 |
力学 | 1102篇 |
综合类 | 279篇 |
数学 | 2615篇 |
物理学 | 8639篇 |
无线电 | 16917篇 |
出版年
2024年 | 124篇 |
2023年 | 377篇 |
2022年 | 813篇 |
2021年 | 1081篇 |
2020年 | 1148篇 |
2019年 | 970篇 |
2018年 | 905篇 |
2017年 | 1246篇 |
2016年 | 1402篇 |
2015年 | 1501篇 |
2014年 | 2218篇 |
2013年 | 2844篇 |
2012年 | 2301篇 |
2011年 | 2279篇 |
2010年 | 1539篇 |
2009年 | 1762篇 |
2008年 | 1997篇 |
2007年 | 2154篇 |
2006年 | 1898篇 |
2005年 | 1636篇 |
2004年 | 1507篇 |
2003年 | 1272篇 |
2002年 | 1077篇 |
2001年 | 939篇 |
2000年 | 846篇 |
1999年 | 702篇 |
1998年 | 548篇 |
1997年 | 504篇 |
1996年 | 433篇 |
1995年 | 417篇 |
1994年 | 344篇 |
1993年 | 319篇 |
1992年 | 258篇 |
1991年 | 162篇 |
1990年 | 114篇 |
1989年 | 96篇 |
1988年 | 135篇 |
1987年 | 69篇 |
1986年 | 46篇 |
1985年 | 57篇 |
1984年 | 55篇 |
1983年 | 30篇 |
1982年 | 35篇 |
1981年 | 30篇 |
1980年 | 21篇 |
1979年 | 18篇 |
1978年 | 13篇 |
1977年 | 8篇 |
1976年 | 14篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
921.
热脱附/冷阱聚焦/气相色谱/质谱法卷烟香气分析 总被引:1,自引:0,他引:1
利用热脱附装置(TDS)对烟丝粉末进行快速升温,模拟卷烟燃吸过程中蒸馏区的部分反应,用冷阱收集所脱附的物质,以GC/MS对进行定性分析.成功的分离出了有机酸、饱和烃、芳香烃、烯烃以及含氮、氧杂原子的化合物等系列超过100种有机化合物,并且还发现了典型的存在于环境中的化学污染物——邻苯二甲酸二丁酯. 相似文献
922.
通过分子设计,将带有-OH端基的钌(Ⅱ)联吡啶配合物[Ru(bpy)2L]PF6{L=希夫碱4-[(N甲-基-N羟-乙基)氨基]苯甲醛缩肼基硫代甲酸甲酯}与甲苯-2,4异-氰酸酯、丙烯酸羟乙酯发生共聚合反应,合成了新的侧链型聚氨酯高分子钌聚合物(4)。4的结构经红外光谱和差热分析表征。 相似文献
923.
采用差示扫描量热法(DSC)、X射线衍射(XRD)和透射电镜(TEM)研究了Y对Nd-Fe-Al-Ni非晶合金热稳定性和晶化行为的影响。结果表明,淬火态的Nd60Fe20Al10Ni10-xYx(x=0,2)合金基本为非晶组织同时还含有少量的淬态相,Y的加入抑制了淬态相的析出。加入Y后,非晶合金的晶化开始温度和晶化峰值温度都向高温方向移动,证明其热稳定性提高。Y的加入改变了合金的晶化方式和最终晶化产物,使非晶基体中析出的晶化相更加弥散圆整细小。并且Y具有在化学上钝化氧杂质的作用,从而抑制了氧的有害作用。利用Kissinger方程获得了Nd60Fe20Al10Ni8Y2非晶合金的晶化开始和晶化峰值激活能分别为1.21和1.16 eV。 相似文献
924.
Al2O3/SnO2 co-nanoparticles were prepared with a modified sol-gel technique followed by a thermal treatment process. With these co-nanoparticles the grafted collagen-Al2O3/SnO2 nanocomposites were obtained using a supersonic dispersion method. X-ray diffraction, FT-IR analysis, transmission electron microscopy, TGA/DTA and infrared emissivity test were performed to characterize the resulting nanoparticles and nanocomposites, respectively. The Al2O3/SnO2 co-nanoparticles showed a narrow distribution of size between 20-40 nm and could be uniformly absorbed on the tri-helix scaffolds of the grafted collagen without any aggregation. The nanocomposites possessed better thermal stability and substantially lower infrared emissivity than the grafted collagen and Al2O3/SnO2 co-nanoparticles with an increase of degradation temperature from 39 to 210 ℃ and a decrease of infrared emissivity from 0.850 of the grafted collagen and 0.708 of the Al2O3/SnO2 co-nanoparticles to 0.424, which provided a potential application of the nanocomposites to areas such as photoelectronics. 相似文献
925.
Danielle Giron S. Monnier M. Mutz P. Piechon T. Buser F. Stowasser K. Schulze M. Bellus 《Journal of Thermal Analysis and Calorimetry》2007,89(3):729-743
Adequate very sensitive quantification methods are needed for the development and are also now required for the monitoring
of undesirable solid form(s) as routine tests. The pre-requisite for quantitation are selectivity, sensitivity and most important
the purity of standards and their proper storage, what is a challenge for metastable forms.
Several analytical techniques are available such as X-ray diffraction, spectroscopy, thermal analysis and microcalorimetry.
The different steps of the validation of the analytical methods and problems to be solved are discussed. Examples illustrate
the different techniques and compare their possible advantages and limits. The relative standard deviation of measurements
should allow for checking the homogenization procedure of mixtures for calibration. The validation should be carried out following
ICH guidelines for validation of analytical methods. Comparison of different techniques in adequate concentration range add
confidence in the analytical results. 相似文献
926.
Lidia Jasińska A. Balas J. T. Haponiuk G. Nowaczyk S. Jurga 《Journal of Thermal Analysis and Calorimetry》2007,88(2):419-423
New cross-linked poly(esterurethanes) (PEU) based on unsaturated olygo(alkyleneester)diol
(OAE), 4,4’-diphenylmethane diisocyanate (MDI) and styrene or methyl
methacrylate as curing monomers were prepared. The synthesis of PEU was performed
in two steps. In the first step OAE was obtained from adipic acid, maleic
anhydride and ethylene glycol. In the second step a prepolymer was obtained
in a reaction of OAE with different amounts of 4,4’-diphenylmethane
diisocyanate followed by crosslinking using previously mentioned curing monomers.
The influence of structure of the poly(esterurethanes) on thermal and dynamic
mechanical properties is studied. Thermogravimetric analysis shows that cross-linked
poly(esterurethanes) demonstrate high thermal stability. Moreover the dynamic
mechanical thermal analysis shows that the presence of styrene cross-linking
chains in polymers lead to the phase separation in cross-linked poly(esterurethanes). 相似文献
927.
Glutamic acid (H2glu) and its lithium, sodium
and ammonium monosalts were submitted to thermal analysis using thermogravimetry
(TG) and differential thermal analysis (DTA). The main goal of these studies
was to compare the relative thermal stability and to evaluate the effect of
the counter ion in the thermal decomposition pathways. Salts were obtained
by direct neutralization of the purified acid with LiOH, NaOH or NH4OH
and were characterized by elemental analysis (C, H and N) and IR spectroscopy.
Decomposition occurred after conversion to the pyroglutamic acid or the respective
pyroglutamates and ammonium salt loosing NH3 being
converted to H2glu before decomposition. 相似文献
928.
In this research, differential scanning calorimetry (DSC) and gas chromatography is used to determine the wax content of fourteen
crude oils of different sources. Different empirical equations were applied to compare the wax content of crude oils. For
the fourteen crude oil samples with the wax content ranging from 7.5 to 43.8 mass%, it was observed that the results of empirical
equations were in good agreement with those determined by DSC and GC. Accordingly, a correlation between ASTM pour point and
the temperature at which 2 mass% of wax has precipitated out from crude oil is developed. 相似文献
929.
S. Mentus Dijana Jelić Veselinka Grudić 《Journal of Thermal Analysis and Calorimetry》2007,90(2):393-397
Thermal decomposition of lanthanum nitrate to lanthanum oxide was carried out by both temperature programmed heating (TPH)
and citrate-gel combustion. The temperature programmed heating was carried out under flow of oxidizing (air), neutral (nitrogen)
and reducing (25 vol.% hydrogen+argone mixture) gases, and the processes were controlled by simultaneous thermogravimetry
and differential thermal analysis. It was shown that hydrogen atmosphere helps to reduce temperatures of all decomposition
steps. The results of TPH were utilized to check the nature of residues in the products of lanthanum nitrate-to-oxide conversion
performed via citrate-gel combustion technique. 相似文献
930.
Structure, thermal elongation coefficient, and conductivity of Y1 ? x Ca x Cr1 ? y Me y O3 (Me = Mg, Cu) in air are studied at 100–1000°C. Electrochemical activity of electrodes made of most conducting compositions Y1 ? x Ca x Cr1 ? y Me y O3 (Me = Mg, Cu), contacting solid electrolyte 0.9ZrO2 + 0.1Y2O3, is studied over a wide range of polarizations, in air, at 700–900°C. 相似文献