全文获取类型
收费全文 | 1732篇 |
免费 | 78篇 |
国内免费 | 83篇 |
专业分类
化学 | 793篇 |
晶体学 | 63篇 |
力学 | 49篇 |
综合类 | 13篇 |
数学 | 24篇 |
物理学 | 467篇 |
综合类 | 484篇 |
出版年
2024年 | 1篇 |
2023年 | 13篇 |
2022年 | 42篇 |
2021年 | 26篇 |
2020年 | 39篇 |
2019年 | 36篇 |
2018年 | 43篇 |
2017年 | 47篇 |
2016年 | 63篇 |
2015年 | 44篇 |
2014年 | 84篇 |
2013年 | 126篇 |
2012年 | 71篇 |
2011年 | 113篇 |
2010年 | 73篇 |
2009年 | 121篇 |
2008年 | 106篇 |
2007年 | 137篇 |
2006年 | 122篇 |
2005年 | 80篇 |
2004年 | 78篇 |
2003年 | 77篇 |
2002年 | 54篇 |
2001年 | 42篇 |
2000年 | 47篇 |
1999年 | 27篇 |
1998年 | 22篇 |
1997年 | 34篇 |
1996年 | 18篇 |
1995年 | 20篇 |
1994年 | 18篇 |
1993年 | 12篇 |
1992年 | 13篇 |
1991年 | 13篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有1893条查询结果,搜索用时 15 毫秒
31.
Mojumdar S. C. Kozánková J. Chocholoušek J. Majling J. Fábryová D. 《Journal of Thermal Analysis and Calorimetry》2004,78(1):73-82
Fluoroapatite containing glass-ceramics were prepared from Li2O-CaO-CaF2-P2O5-SiO2 system. The glass was melted at 1480°C for 1 h. The object of observation was the preparing crystal phase of fluoroapatite
in amorphous glass matrix. The morphology of lithium disilicate glass-ceramics was studied by SEM. The crystal growth and
thermal properties of fluoroapatite were studied by X-ray diffraction and DTA. The more the content of P2O5, the more the presence of fluoroapatite particles. SEM investigation clearly indicated the phase separation and formation
of a primary crystalline phase of fluoroapatite in the studied glass-ceramics. DTA curves of the fluoroapatite samples exhibit
exothermic effects in the temperature range 337-694°C depending on the composition of the materials. The position of exothermic peak for lithium disilicate on DTA curves moves with increasing specific surfacetowards lower temperatures which points on its preferential surface crystallization. As far as physical qualities are concerned,
mainly color and gloss, the best qualities of all observed materials belong to glass-ceramics with 10% P2O5.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
32.
《Electroanalysis》2003,15(19):1567-1570
Application of thin colloidal Au multilayers on glass substrates as optically transparent electrodes is reported herein. Evaluation of the electrode by scanning electron microscopy and as an optically transparent thin layer electrode (OTTLE) illustrate its use for spectroelectrochemical applications. A spectroelectrochemical Nernst plot acquired for the oxidation of o‐tolidine yielded a value of E°=0.605 V (vs. Ag/AgCl) and n=1.88. Both of these values are in excellent agreement with previously published values obtained using a traditional Au minigrid OTTLE. The colloidal Au multilayer electrode is expected to offer many advantages over the minigrid OTTLE in specific applications. These advantages include direct control over microscopic surface topography, large electrochemical surface area, applicability to many different electrode geometries, and increased microscopic surface roughness for SERS and SPR applications. 相似文献
33.
《Surface and interface analysis : SIA》2004,36(12):1523-1532
The effects of three non‐chromate‐based deoxidizers, namely NaBrO3/HNO3, (NH4)4Ce(SO4)4/H2SO4 and Fe(III)/HF/HNO3, on the Al alloy 7475‐T7651 were investigated. Several analytical methods were employed, including SEM, AES, transmission electron microscopy (TEM), electron energy‐loss spectroscopy (EELS) and glow discharge optical emission spectrometry (GDOES), to study the effects on the surface of this alloy after each treatment compared with the as‐received and alkaline‐cleaned alloy surfaces. The untreated alloy was found to have a thick oxide of 200–320 nm, consisting mainly of MgO. Alkaline cleaning results in an etching effect that thins the oxide and also deposits a thin silicate layer on the surface. In the case of the deoxidizers, there is significant removal of the native oxide of the alloy by the NaBrO3/HNO3 deoxidizer. There is also evidence of intermetallic attack on the alloy. The (NH4)4Ce(SO4)4/H2SO4 deoxidizer, which is a low‐etch‐rate deoxidizer, resulted in a slight thinning of the oxide. However, the effect was not significantly greater than with alkaline cleaning alone. The most effective deoxidizer in reducing the oxide thickness of the alloy is Fe(III)/HF/HNO3, in which the etch rate was sufficiently high to remove completely the native oxide. In this case, equilibrium between oxide removal and the formation of new oxides on the alloy surface was achieved. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
34.
刘家熙 《贵州师范大学学报(自然科学版)》1998,16(1):23-26
本文利用扫描电子显微镜辅以光学显微镜的方法,详细观察并描述了北京蹄盖蕨科蹄盖蕨属5种即东北蹄盖蕨(Athyriumbrevifrons),河北蹄盖蕨(A.hebeiense),中华蹄盖蕨(A.sinense),多齿蹄盖蕨(A.multidentatum),华北蹄盖蕨(A.pachyphlebium)孢子的形态结构,讨论了各种间的差异,为蹄盖蕨科系统分类提供孢粉学方面的依据。 相似文献
35.
The sintering and the structure of clinkers, modified by the introduction of different ionic forms of sulfur and phosphorus
into the raw mix, were examined. One reference synthetic mixture and 25 modified mixtures were prepared by mixing the reference
sample with 0.5, 1.0, 1.5, 2.0 and 2.5%w/w of chemical grade CaSO4, CaS, Ca3(PO4)2, CaHPO4 and Ca(H2PO4)2. Free lime content in all samples was measured. The sintering reactions in samples were recorded by means of differential
thermal analysis. The texture of the clinkers was examined using a scanning electron microscope and EDX. It is concluded that,
despite of their relatively low doping concentration in the raw mix, P and S affect considerably its reactivity and the texture
of the clinker. The various ionic forms of the same element (SO4
2-, S2- for S, PO4
3-, HPO4
2- and H2PO4
- for P) exhibit a different and unequivocal effect on the reactivity of the synthetic raw mix and on the texture of the corresponding
clinkers. S (in both forms) and P (added as HPO4
2-) are mainly dissolved in the melt and they have a positive effect on the burnability of the raw mix. P (added as PO4
3- or H2PO4-) is preferentially accumulated in belite. In this case further stabilization of β-C2S occurs and the binding of the free lime is hindered.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
36.
Synthesis of potassium bismuth titanate ferroelectric by heating of mixtures prepared using oxide precursors, i.e. Bi2O3, TiO2 and K2CO3 was investigated. DTA, TG, XRD and SEM methods were used to study the formation of intermediate compounds and the final product.
Usage of associated homogenization and grinding of precursors mixture permits to decrease the temperature of formation of
K0.5Bi0.5TiO3.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
37.
Cengiz Soykan Ramazan Coskun Ali Delibas Ertan Sahin 《高分子科学》2007,(5):491-500
2-Acrylamido-2-methy1-1-propanesulfonic acid(AMPS),and maleic acid(MA)copolymerized with different feed ratios using N,N-dimethylformamide as a solvent and benzoyl peroxide(Bz_2O_2)as an initiator at 70℃.Structure and composition of copolymers for a wide range of monomer feed were determined by elemental analysis(content of N for AMPS-units).Monomer reactivity ratios for AMPS(M_1)-MA(M_2)pair were determined by the application of conventional linearization methods such as Fineman-Ross(F-R),Kelen-Tüd(?)s(KT)and Extended Kelen-Tüds(EKT)and a nonlinear error invariable model method using a computer program RREVM.The characterizations were done by Fourier transform infrared spectroscopy(FTIR),differential scanning calorimetry(DSC)thermal gravimetry analysis(TGA),and and X-ray diffraction.The antimicrobial effects of polymers were also tested on various bacteria,and yeast. 相似文献
38.
Mg-based hydrogen storage alloys Mg0.9Ti0.1-xZrxNi (x=0.02, 0.04, 0.06) were successfully prepared by means of mechanical alloying (MA). The effects of Zr addition on the discharge capacity and the cycle performance of the Mg-based electrodes were also studied. It was found that the discharge capacities were improved with addition of a small amount of Zr and the cycle performances of the alloy were stabilized with the addition of Ti. The effects of surface modification or coating on the properties of Mg0.9Ti0.06Zr0.04Ni were also studied. The results indicated that coating with graphite improved both the discharge capacity and cycle life of the amorphous Mg0.9Ti0.06Zr0.04Ni electrode. 相似文献
39.
合成了NTAAM,NTMAAM,NPMAAM功能单体,发现这类功能单体与铈离子组成的体系能引发丙烯酰胺聚合,带有这类功能单体的聚合物和共聚物铸成的膜,能用铈离子引发丙烯酰胺接枝共聚合。这可从反应后的膜比基膜有较大的吸水率、与水有较小的接触角,也可从扫描电子显微镜观察到表面上有凸起图象的高分子链,以及从膜表面的X射线光电子能谱所证实。同时研究了共聚物膜的组成、反应条件对接枝共聚合的影响。 相似文献
40.
Lauro J.Q. Maia Valmor R. Mastelaro Antonio C. Hernandes 《Journal of solid state chemistry》2007,180(2):611-618
A new sol-gel route was applied to obtain Y0.9Er0.1Al3(BO3)4 crystalline powders and amorphous thin films by using Al(acac)3, B(OPri)3, Y(NO3)3·6H2O, and Er(NO3)3·5H2O as starting materials dissolved in propionic acid and ethyl alcohol mixtures. Our study shows that propionic acid acts as good chelant agent for yttrium and erbium ions while ethyl alcohol allows to dissolve Al(acac)3. This process makes the resulting sols very stable to obtain homogeneous gels and transparent amorphous thin films. In addition, the propionic acid prevents the sol precipitation, making easy porous- and crack-free thin film depositions. Chemical reactions involved in the complexation were discussed. As-prepared powders and films are amorphous and present a good thermal stability due to their high glass transition (746 °C) and crystallization temperatures (830 °C). This new sol-gel route showed to be adequate to obtain dense and crack-free thin films free of organic and hydroxyl groups that can be considered as promising materials to be used in integrated optical systems. 相似文献