全文获取类型
收费全文 | 9868篇 |
免费 | 1800篇 |
国内免费 | 1055篇 |
专业分类
化学 | 6893篇 |
晶体学 | 85篇 |
力学 | 591篇 |
综合类 | 52篇 |
数学 | 1086篇 |
物理学 | 4016篇 |
出版年
2024年 | 47篇 |
2023年 | 234篇 |
2022年 | 357篇 |
2021年 | 431篇 |
2020年 | 403篇 |
2019年 | 381篇 |
2018年 | 380篇 |
2017年 | 311篇 |
2016年 | 525篇 |
2015年 | 460篇 |
2014年 | 595篇 |
2013年 | 785篇 |
2012年 | 942篇 |
2011年 | 902篇 |
2010年 | 622篇 |
2009年 | 608篇 |
2008年 | 621篇 |
2007年 | 532篇 |
2006年 | 498篇 |
2005年 | 407篇 |
2004年 | 311篇 |
2003年 | 235篇 |
2002年 | 199篇 |
2001年 | 173篇 |
2000年 | 200篇 |
1999年 | 221篇 |
1998年 | 173篇 |
1997年 | 163篇 |
1996年 | 183篇 |
1995年 | 144篇 |
1994年 | 120篇 |
1993年 | 91篇 |
1992年 | 99篇 |
1991年 | 56篇 |
1990年 | 75篇 |
1989年 | 54篇 |
1988年 | 28篇 |
1987年 | 28篇 |
1986年 | 37篇 |
1985年 | 38篇 |
1984年 | 13篇 |
1983年 | 17篇 |
1982年 | 8篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1957年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Zhang De-Qing Wang Shao-Jun Sun Hong-Shan Wang Xiu-Li Cao Mao-Sheng 《Journal of Sol-Gel Science and Technology》2007,41(2):157-161
Lead zirconate titanate nanopowders Pb(Zr0.52Ti0.48)O3 (PZT) were prepared by modified sol-gel process in ethylene glycol system with zirconium nitrate as the zirconium source.
The research showed that it was critical to add lead acetate after the reaction of zirconium nitrate and tetrabutyl titanate
in ethylene glycol system for preparing PZT of exact titanium content. The reaction mechanisms of the sol synthesis, preparation
of xerogel and agglutinating process were characterized through using FT-IR, NMR, TG-FTIR, and GC-MS. The experiment proved
that ethylene glycol system did not rely on hydrolysis and condensation reactions in the process of the sol formation, but
on the formation of chain or network large molecules from complexation of ethylene glycol and all Ti and minor Pd, Zr. In
the preparation of xerogel, the complexation reaction was so completed that it formed large molecules network composed of
metal and dioxyethyl. Bulk weight loss happened before 350°C in the process of sintering xerogel to prepare PZT nanopowders.
Volatile matters and vapor phase decomposition resultants were primarily oxy-compounds including ethylene glycol, aldehyde-ketone
compounds, carbon dioxide and nitrate radical conversion matters. After 350°C, primary vapor phase decomposition resultants
were carbon dioxide and minor carbonyl compound. 相似文献
102.
我们利用Born-Mayer-Huggins相互作用势函数对(KF)N(N=108,256,500和864)团簇进行了分子动力学(MD)模拟。为了避免周期性边界条件对相变、成核和重结晶的干扰作用,对体系采用了自由边界。基于MD模拟结果,对团簇的熔化温度、熔化焓、扩散系数、成核速率、固液界面自由能、临界核大小等进行了计算和讨论。在对(KF)864双晶团簇的热退火模拟中,观察到了固态的重结晶和晶粒的生长。经典的成核理论成功地解释了(KF)864双晶团簇的重结晶MD模拟结果。 相似文献
103.
Zheng-Zhong Lin Rui-hu Wang Lei Han Rong Cao 《Journal of solid state chemistry》2004,177(7):2494-2498
The indium(III)-organic compound [In(HBtc)2(4,4′-bpy)](4,4′-Hbpy)(H2O)0.5 (Btc=1,3,5-benzenetricarboxylate, 4,4′-bpy=4,4′-bipyridine) has been synthesized under hydrothermal condition and characterized by IR, fluorescent spectroscopy, TGA and single-crystal X-ray diffraction analyses. The compound crystallizes in monoclinic, space group P2(1)/c, a=17.0884(2) Å, b=12.28390(10) Å, c=17.9456(4) Å, β=104.1920(10)°, V=3652.03(10) Å3, Z=4, R1=0.0572 and wR2=0.1116 [I>2σ(I)]. All the indium atoms in the compound are hepta-coordinated and link 1,3,5-benzenetricarboxylate forming a 2-D layer structure with rhombus grids. 相似文献
104.
Cao Ning-Jun Xu Qin Chen Chee-Shan Gong C. S. Chen L. F. 《Applied biochemistry and biotechnology》1994,(1):521-530
Cellulose gel with < 10% of crystallinity was prepared by treatment of microcrystalline cellulose, Avicel, with zinc chloride
solution at a ratio of zinc chloride to cellulose from 1.5 to 18 (w/w). The presence of zinc ions in the cellulose gels enhanced
the rate of hydrolysis and glucose yield. The evidence obtained from X-ray diffraction, iodine absorption experiments; and
Nuclear Magnetic Resonance spectra analysis suggested the presence of zinc-cellulose complex after Avicel was treated with
zinc chloride. Zinc-cellulose complex was more susceptible to hydrolysis than amorphous cellulose. Under the experimental
condition, cellulose gels with zinc ions were hyrolyzed to glucose with 95% theoretical yield and a concentration of 14% (w/v)
by cellulases within 20 h. The same gel was hydrolyzed by acid to glucose with 91.5% yield and a concentration of 13.4% (w/v). 相似文献
105.
Xylan is the major component of hemicellulose, which consists of up to one-third of the lignocellulosic biomass. When the
zinc chloride solution was used as a pretreatment agent to facilitate cellulose hydrolysis, hemicellulose was hydrolyzed during
the pretreatment stage. In this study, xylan was used as a model to study the hydrolysis of hemicellulose in zinc chloride
solution. The degradation of xylose that is released from xylan was reduced by the formation of zinc-xylose complex. The xylose
yield was >90% (w/w) at 70°C. The yield and rate of hydrolysis were a function of temperature and the concentration of zinc
chloride. The ratio of zinc chloride can be decreased from 9 to 1.3 (w/w). At this ratio, 76% of xylose yield was obtained.
When wheat straw was pretreated with a concentrated zinc chloride solution, the hemicellulose hydrolysate contained only xylose
and trace amounts of arabinose and oligosaccharides. With this approach, the hemicellulose hydrolysate can be separated from
cellulose residue, which would be hydrolyzed subsequently to glucose by acid or enzymes to produce glucose. This production
scheme provided a method to produce glucose and xylose in different streams, which can be fermented in separated fermenters. 相似文献
106.
107.
108.
[reaction: see text] Two readily prepared catalysts have been developed for the hydroamination of alkynes by 1,1-disubstituted hydrazines. The catalyses are facile with terminal alkynes and some internal alkynes. If the hydrazine bears an aryl group, Fischer cyclization can occur in a one-pot procedure. In addition, reactions with acetylene to produce a plethora of hydrazones are described. Catalytic reactions involving acetylene and substituted hydrazines are complete in less than 2 h at room temperature and 1 atm of pressure. 相似文献
109.
Reactions of the substituted 2‐formyl chromones with aroylhydrazines gave corresponding 2‐(aroylhydrazonomethylidyne) chromones. Then 2‐(3′‐acetyl‐5′‐aryl‐2′,3′‐dihydro‐1′,3′,4′‐oxadiazol‐2′‐yl) chromones were prepared by these 2‐(aroylhydrazonomethylidyne) chromones under refluxing with Ac2O. All target compounds were characterized through elemental analysis and IR, 1H NMR, MS. 相似文献
110.
In this work, the enzymatic degradation of poly(butylene succinate-co-butylene terephthalate) (PBST) copolyesters was studied using the lipase from Pseudomonas (Lipase PS®). The biodegradation behavior was found to strongly depend on the overall impacts of several important factors as the BT comonomer structure and molar content, thermal characteristics, morphology, the enzyme-substrate, and so forth. Further, the biodegraded residual film samples were allowed to be analyzed by means of gel permeation chromatography (GPC), proton nuclear magnetic resonance (1H NMR), differential scanning calorimeter (DSC), small angle X-ray scattering (SAXS), and scanning electron microscope (SEM). On the experimental evidences, an exo-type mechanism of enzymatic chain hydrolysis preferentially occurring in the amorphous region was suggested for the PBST film samples. 相似文献