全文获取类型
收费全文 | 15352篇 |
免费 | 1767篇 |
国内免费 | 1578篇 |
专业分类
化学 | 7238篇 |
晶体学 | 154篇 |
力学 | 467篇 |
综合类 | 113篇 |
数学 | 1122篇 |
物理学 | 3708篇 |
综合类 | 5895篇 |
出版年
2024年 | 61篇 |
2023年 | 228篇 |
2022年 | 434篇 |
2021年 | 472篇 |
2020年 | 446篇 |
2019年 | 502篇 |
2018年 | 380篇 |
2017年 | 380篇 |
2016年 | 535篇 |
2015年 | 650篇 |
2014年 | 852篇 |
2013年 | 1030篇 |
2012年 | 1204篇 |
2011年 | 1227篇 |
2010年 | 1018篇 |
2009年 | 1039篇 |
2008年 | 1109篇 |
2007年 | 1022篇 |
2006年 | 859篇 |
2005年 | 730篇 |
2004年 | 590篇 |
2003年 | 441篇 |
2002年 | 423篇 |
2001年 | 366篇 |
2000年 | 290篇 |
1999年 | 380篇 |
1998年 | 304篇 |
1997年 | 246篇 |
1996年 | 245篇 |
1995年 | 220篇 |
1994年 | 191篇 |
1993年 | 149篇 |
1992年 | 147篇 |
1991年 | 127篇 |
1990年 | 102篇 |
1989年 | 83篇 |
1988年 | 60篇 |
1987年 | 53篇 |
1986年 | 39篇 |
1985年 | 29篇 |
1984年 | 9篇 |
1983年 | 9篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1959年 | 1篇 |
1957年 | 2篇 |
1936年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Uncoupled metabolism stimulated by chemical uncoupler and oxic-settling-anaerobic combined process to reduce excess sludge production 总被引:1,自引:0,他引:1
The effects of three uncoupled metabolic systems (conventional activated sludge process with the addition of 3,3′,4′,5-tetrachlorosalicylanilide
[TCS], oxic-settling-anaerobic [OSA] process modified by insertion of a sludge-holding tank in the sludge return line, and
TCS and OSA combined process) on reducing excess sludge production were studied. Compared with the control conventional activated
sludge process, the most effective system was the combined process, which could reduce excess sludge production by 46.90%.
The 180-d operation results confirmed that TCS is an effective chemical uncoupler in reducing the sludge yield but that it
had an adverse effect on substrate removal capability, effluent nitrogen concentration, and sludge settleability. The OSA
process decreased excess sludge production by only 26% but had less adverse effect on effluent quality and could improve sludge
settleability. The effluent total phosphorous concentration of the three systems was slightly lower than of the control unit.
Microbial populations were monitored by both microscopic and molecular biologic analysis method (polymerase chain reaction
[PCR]-denaturing gradient gel electrophoresis [DGGE]). The presence of TCS caused metazoans to disappear and decreased the
number and activity of protozoa. PCR amplification of 16S rRNA and sequent DGGE analysis found a shift in the diversity of
the predominant species. The results imply that OSA combined with the chemical uncoupler process may effectively reduce excess
sludge yield and not affect process performance significantly. 相似文献
62.
63.
用结晶动力学方法对不同摩尔比的 Mg O/B2 O3在摩尔分数为 1 8% Mg Cl2 -H2 O中的过饱和溶液在2 0℃的结晶动力学过程进行了研究 .n(Mg O) /n(B2 O3) =1 /1和 1 /1 .5时分别结晶析出 2 Mg O· 2 B2 O3·Mg Cl2 · 1 4H2 O和 2 Mg O· 3 B2 O3· 1 5 H2 O(多水硼镁石 ) .n(Mg O) /n(B2 O3) =1 /2时依次结晶析出 Mg O·3 B2 O3· 7.5 H2 O,Mg O· 2 B2 O3· 9H2 O和 2 Mg O· 3 B2 O3· 1 5 H2 O(多水硼镁石 ) 3种固相 .n(Mg O) /n(B2 O3)=1 /3时结晶析出不同结晶水的六硼酸镁 Mg O· 3 B2 O3· 7H2 O和 Mg O· 3 B2 O3· 7.5 H2 O.析出固相采用X射线粉末衍射、红外光谱和热分析进行物相鉴定 .拟合并给出结晶动力学方程 ,同时对水合镁硼酸盐的结晶反应机理进行了探讨 相似文献
64.
65.
Wei ZHANG Ni SONG Zhong Zhen LI Ying Xia LI 《中国化学快报》2006,17(3):285-288
Obyanamide 1 is a cyclic depsipeptide that was recently isolated from the marine cyanobacterium Lyngbya confervoides by Moore and co-workers1. This compound displayed a potent inhibitory effect on both KB and LoVo cells in vitro. Several structural analog… 相似文献
66.
Determination of Organophosphorus Pesticides in Soil by MMSPD-GC-NPD and Confirmation by GC-MS 总被引:1,自引:0,他引:1
A novel method, modified matrix solid-phase dispersion (MMSPD), has been developed for quantitative analysis of organophosphorus
pesticide residues in soil. It was based on matrix solid-phase dispersion (MSPD) and continuous liquid-solid extraction (continuous
LSE), using Florisil as sorbent and dichloromethane as the recycling solvent. Two soils with different texture and physicochemical
properties were studied to validate the method. The effect of residence time of pesticides in soil was also studied. MMSPD
was compared with MSPD and continuous LSE respectively. Determination was carried out by gas chromatography with nitrogen-phosphorus
detection (GC-NPD). The method gave recoveries ranging from 72–105% with relative standard deviations (RSDs) lower than 15%
for the pesticides studied. The limits of detection (LODs) ranged from 0.1 to 0.6 ng g−1. Two pesticide residues have been detected in real soil samples from Fujian, China, using MMSPD. The pesticides were confirmed
by gas chromatography-mass spectrometry (GC-MS) in a selected-ion monitoring (SIM) mode.
Revised: 4 and 9 April 2006 相似文献
67.
Yongsong Luo Xiaohong Xia Yonggang Zhang Jialin Li Zhijie Jia 《Journal of solid state chemistry》2007,180(6):1928-1933
Luminescence of the short multiwalled carbon nanotubes (MWNTs) conjugated with carboxylic acid groups has been studied. The results show that the carboxyl-functionalized short MWNTs could emit luminescence and the emission peak appears at 500 nm with a corresponding optimal excitation wavelength centering at 310 nm. When the short MWNTs are filtered through 0.15 μm polytetrafluoroethylene (PTFE) membrane, the ultrashort MWNTs are obtained from the filtrate. An interesting feature for the ultrashort MWNTs is that the emission intensity is strengthened and the peak is slightly blue shifted to 460 nm. This result indicates that the luminescence properties of MWNTs are strongly affected by the tube length. After chemical oxidization cutting, defects and carboxylic acid groups at the tube end and/or sidewall can be produced; the more shorten of MWNTs, the better dispersion and carboxylic passivation of the nanotubes, and the more intense luminescence emissions. The broad emissions are logically attributed to the trapping of excitation energy by defect sites in the carboxyl-functionalized nanotube structure. 相似文献
68.
In the present paper, we describe a microfluidics-based sensing system for nonelectroactive anions under negative separation electric field by mounting a single carbon fiber disk working electrode (WE) in the end part of a poly(dimethylsiloxane) microchannel. In contrast to work in a positive separation electric field described in our previous paper (Anal. Chem. 2004, 76, 6902-6907), here the electrochemical reduction reaction at the WE is not coupled with the separation high-voltage (HV) system, whereas the electrochemical oxidation reaction at the WE is coupled with the separation HV system. The electroactive indicator is the carbon fiber WE itself but not dissolved oxygen. This provides a convenient and sensitive means for the determination of nonelectroactive anions by amperometry. The influences of separation voltage, detection potential, and the distance between the WE and the separation channel outlet on the response of the detector have been investigated. The present detection mode is successfully used to electrochemically detect F-, Cl-, SO4(2-), CH3COO-, H2PO4-. Based on the preliminary results, a detection limit of 2 microM and a dynamic range up to three orders of magnitude for Cl- could be achieved. 相似文献
69.
A simple and sensitive fluorimetric method for the determination of ascorbic acid (AA) is described. The method is based on the condensation reaction between AA and o-phenylenediamine (OPDA) in the absence of the oxidant. The fluorescence intensity is measured at excitation and emission wavelengths of 360 and 430 nm, respectively. Under optimum condition, a linear relationship is obtained between the fluorescence intensity and the concentration of AA in the range of 0.05-40 μg ml−1. The detection limit is 0.006 μg ml−1, which is obviously lower than that of other fluorimetric methods reported. 相似文献
70.
Raman spectroscopic analysis of supersaturated aqueous solution of MgO.B2O3-32%MgCl2-H2O during acidification and dilution 总被引:1,自引:0,他引:1
Zhihong L Bo G Shuni L Mancheng H Shuping X 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2004,60(13):3125-3128
Raman spectra of supersaturated aqueous solution of MgO.B2O3-32%MgCl2-H2O during acidification/alkalization and dilution have been studied. The assignments of the recorded Raman shift are given. The main existing forms of polyborate anions and their interaction in borate aqueous solution have been proposed through spectroscopic analysis. The experimental results indicate that the higher concentration of cation are beneficial not only to the dissolution of boric acid but also to the polymerization of polyborate anions. The existing forms and interaction among them also depend on the concentration of boron and the pH value in solution. 相似文献