全文获取类型
收费全文 | 29322篇 |
免费 | 4101篇 |
国内免费 | 2699篇 |
专业分类
化学 | 20241篇 |
晶体学 | 284篇 |
力学 | 1911篇 |
综合类 | 274篇 |
数学 | 3275篇 |
物理学 | 10137篇 |
出版年
2024年 | 74篇 |
2023年 | 568篇 |
2022年 | 754篇 |
2021年 | 891篇 |
2020年 | 1099篇 |
2019年 | 1055篇 |
2018年 | 874篇 |
2017年 | 777篇 |
2016年 | 1225篇 |
2015年 | 1171篇 |
2014年 | 1477篇 |
2013年 | 1984篇 |
2012年 | 2561篇 |
2011年 | 2657篇 |
2010年 | 1646篇 |
2009年 | 1632篇 |
2008年 | 1736篇 |
2007年 | 1659篇 |
2006年 | 1489篇 |
2005年 | 1228篇 |
2004年 | 941篇 |
2003年 | 789篇 |
2002年 | 752篇 |
2001年 | 561篇 |
2000年 | 558篇 |
1999年 | 651篇 |
1998年 | 578篇 |
1997年 | 574篇 |
1996年 | 659篇 |
1995年 | 506篇 |
1994年 | 464篇 |
1993年 | 388篇 |
1992年 | 371篇 |
1991年 | 322篇 |
1990年 | 255篇 |
1989年 | 202篇 |
1988年 | 157篇 |
1987年 | 136篇 |
1986年 | 127篇 |
1985年 | 118篇 |
1984年 | 88篇 |
1983年 | 61篇 |
1982年 | 41篇 |
1981年 | 36篇 |
1980年 | 18篇 |
1978年 | 19篇 |
1977年 | 23篇 |
1976年 | 20篇 |
1975年 | 28篇 |
1974年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
871.
A MORPHOLOGICAL STUDY OF POLY(DIVINYLBENZENE-co-ACRYLIC ACID) IN CROSSLINKING PRECIPITATION POLYMERIZATION 总被引:1,自引:0,他引:1
Feng Bai Xin-lin Yang Wen-qiang Huang 《高分子科学》2006,(2):163-171
Divinylbenzene-80 (DVB-80) and polar monomer acrylic acid (AA) having hydrogen bonding at a total monomer loading of 5 vol% were precipitated-copolymerized in a variety of organic solvents with 2,2'-azobis(isobutyronitrile) (AIBN) as initiator. The experiments were investigated from a two-dimensional matrix, i.e., the actual crosslinking degree of DVB varying from 0 to 80% and the solvent composition varying from 0 to 100% of toluene mixture with acetonitrile, when the mixture of acetonitrile and toluene was used as the reaction solvent. Under various reaction conditions, six distinct morphologies including soluble polymers, swellable microgels, coagulum, irregular microparticles, and nano-/micrometer microspheres were formed and the structures of these polymer architectures were described. A morphological map was utilized to discuss the effects of both crosslinking degree of DVB and composition of solvent on the transitions between morphology domains. The results demonstrated that the microspheres are formed by an internal contraction due to the marginal solvency of the continuous phase and the crosslinking of the polymer network through the covalent bonding from DVB as well as the interchain hydrogen-bonding between the carboxylic acid units. 相似文献
872.
873.
IntroductionThepreparativeapproachofaneffectivebilTldslliccatalystisalwaysasubjectofboortantsignificanceinheterogeneouscatalysis.InourrecentstUdies,wefoundthatthebAnetallitcarbonylclustercoCo3(CO)12favorablygivesthebAnetalliccoCo3clusterontheSiOZsubdueaft… 相似文献
874.
Simutaneous analysis of seven commonly used herbicides, 2,4-D methyl ester, 2,4-D methyl ester, 2,4,5-T methyl ester, silvex methyl ester, ramrod, CIPC and DEF, by gas-liquid chromatography with an electron capture detector was attempted. Two packed columns (which are generally used for the analysis of chlorinated pesticides) and two chemically bonded fused silica capillary columns were used for the analysis. When the packed columns were used, ramrod always interfered with the analysis of 2,4-D methyl ester and/or silvex methyl ester. Complete separation with reasonable analysis time can be achieved using one of the chemically bonded fused silica capillary columns. 相似文献
875.
Tsai FT Chiou SJ Tsai MC Tsai ML Huang HW Chiang MH Liaw WF 《Inorganic chemistry》2005,44(16):5872-5881
Dinitrosyl iron complex [(-SC(7)H(4)SN)(2)Fe(NO)(2)](-) (1) was prepared by reaction of [S(5)Fe(NO)(2)](-) and bis(2-benzothiozolyl) disulfide. In synthesis of the analogous dinitrosyl iron compounds (DNICs), the stronger electron-donating thiolates [RS](-) (R = C(6)H(4)-o-NHCOCH(3), C(4)H(3)S, C(6)H(4)NH(2), Ph), compared to [-SC(7)H(4)SN](-) of complex 1, trigger thiolate-ligand substitution to yield [(-SC(6)H(4)-o-NHCOCH(3))(2)Fe(NO)(2)](-) (2), [(-SC(4)H(3)S)(2)Fe(NO)(2)](-) (3), and [(SPh)(2)Fe(NO)(2)](-) (4), respectively. At 298 K, complexes 2 and 3 exhibit a well-resolved five-line EPR signal at g = 2.038 and 2.027, respectively, the characteristic g value of DNICs. The magnetic susceptibility fit indicates that the resonance hybrid of {Fe(+)((*)NO)(2)}(9) and {Fe(-)((+)NO)(2)}(9) in 2 is dynamic by temperature. The IR nu(NO) stretching frequencies (ranging from (1766, 1716) to (1737, 1693) cm(-)(1) (THF)) of complexes 1-4 signal the entire window of possible electronic configurations for such stable and isolable {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-). The NO-releasing ability of {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) is finely tuned by the coordinated thiolate ligands. The less electron-donating thiolate ligands coordinated to {Fe(NO)(2)}(9) motif act as better NO-donor DNICs in the presence of NO-trapping agent [Fe(S,S-C(6)H(4))(2)](2)(2-). Interconversion between {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) and {Fe(NO)(2)}(10) [(Ph(3)P)(2)Fe(NO)(2)] was verified in the reaction of (a) [(RS)(2)Fe(NO)(2)](-), 10 equiv of PPh(3) and sodium-biphenyl, and (b) 2 equiv of thiol, [RS](-), and [(Ph(3)P)(2)Fe(NO)(2)], respectively. The biomimetic reaction cycle, transformation between {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) and {Fe(NO)(2)}(9) [(R'S)(2)Fe(NO)(2)](-), reversible interconversion of {Fe(NO)(2)}(9) and {Fe(NO)(2)}(10) DNICs, and degradation/reassembly of [2Fe-2S] clusters may decipher and predict the biological cycle of interconversion of {Fe(NO)(2)}(9) DNICs, {Fe(NO)(2)}(10) DNICs, and the [Fe-S] clusters in proteins. 相似文献
876.
Yi‐Jun Huang Guo‐Rong Qi Yu‐Hua Wang 《Journal of polymer science. Part A, Polymer chemistry》2002,40(8):1142-1150
A double metal‐cyanide catalyst based on Zn3[Co(CN)6]2 was prepared. This catalyst is very effective for the ring‐opening polymerization of propylene oxide. Polyether polyols of moderate molecular weight having low unsaturation (<0.015 meq/g) can be prepared under mild conditions. The molecular weight of polymer is entirely controlled by a reacted monomer‐to‐initiator ratio. The polymers prepared with stepwise addition of monomer exhibit a narrower molecular weight distribution as compared with those prepared with one‐step addition of monomer. Various compounds containing active hydrogen, except basic compounds and low‐carbon carboxylic acid, may be used as initiators. The reaction rate increases with increasing catalyst amount and decreases with rising initiator concentration. Polymerization involves a rapid exchange reaction between the active species and the dormant species. It was also proven that, to a certain extent, the chain termination of this catalytic system is reversible or temporary. 13C NMR analysis showed that the polymer has a random distribution of the configurational sequences and head‐to‐tail regiosequence. It is assumed that the polymerization proceeds via a cationic coordination mechanism. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1142–1150, 2002 相似文献
877.
The YbFe2O4structure type consists of triangular layers of lanthanide oxygen octahedra stacked with triangular double layers of transition metal oxygen triangular bipyramids. The crystallographic structures determined by neutron diffraction powder profile analysis at 300 and 11 K for new members of this structural family are reported. The compounds are found to be magnetically frustrated, by both lattice geometry and disorder. The magnetic properties of YbCuGaO4, LuCuGaO4, LuZnFeO4, LuCoGaO4, and LuCuFeO4reveal the effects of total spin, spin mixing, and interaction between spins on different sublattices on the magnetic frustration. The magnetism is increasingly frustrated as the spin on the magnetic ions is decreased. 相似文献
878.
Ibuprofen and ketoprofen are chemically similar non‐steroidal anti‐inflammatory drugs widely used in the treatment of arthritis. Using a molecular imprinting technique, a simple and rapid method was developed for the simultaneous separation and determination of ibuprofen and ketoprofen. Molecular imprinting introduces artificial binding sites into a synthetic polymer matrix, allowing it to exhibit selective rebinding of template molecules. Imprinted polymers can be regarded as an HPLC stationary phase, important for pharmaceutical analysis. Most molecularly imprinted polymers (MIPs) are synthesized by free radical polymerization of functional monomers, resulting in an excess of crosslinking monomers. In this study, MIPs have been prepared with a ibuprofen template, which can form intramolecular hydrogen bonds. Methacrylic acid (MAA) and ethyleneglycol dimethacrylate (EGDMA) were used as the functional monomer and cross‐linker, respectively. Bulk polymerization was carried out at 4 °C under UV radiation. The resulting MIP was ground into 25?44 μm particles, which were slurry‐packed into analytical columns. Template molecules were removed by methanol‐acetic acid (9:1, v/v). We evaluated the template binding performance of the MIP using HPLC, with ultraviolet (UV) detection at 234 nm. Chromatographic resolution of ibuprofen and ketoprofen on the MIPs were appraised using buffer/acetonitrile (45/55, v/v) as the mobile phase. Results show that the MIPs prepared using ibuprofen as the template had a significant molecular imprinting effect. The method was successfully applied to the separation and analysis of ibuprofen and ketoprofen in pharmaceuticals. 相似文献
879.
呋喃丹的极谱法测定研究及应用 总被引:6,自引:0,他引:6
用循环伏安法(CV)、线性扫描伏安法(LSV)对呋喃丹的极谱伏安行为及其测定进行了研究。在0.2mol/L NH3-NH4Cl底液中(pH 8.8),对其进行扫描,发现于-1.21V(vs.SCE)处产生一灵敏的还原峰,其峰电流与呋喃丹的浓度在5.0×10-6-1.0×10-mol/L范围内呈良好的线性关系(r=0.9995)。对5.0×10-5mol/L呋喃丹溶液进行6次平行实验,RSD为0.2%。实验结果表明,呋喃丹在极谱电极上有吸附性质。同时对电极反应机理进行了研究。该方法对微生物降解呋喃丹进行监测取得满意的结果,96 h内呋喃丹的降解率为98%。 相似文献
880.
meso—四(对—羟基苯基)卟啉与脱氧核糖核酸的作用及其分析应用 总被引:6,自引:6,他引:6
报道了非离子型卟啉meso-四卟啉与脱氧核糖核酸的作用。在PH4.9-5.4范围内,DNA能与聚合态的THP作用,产生不规则的电子吸收光谱。但如果体系中含有30%(V/V)乙醇,DNA与THPP作用产生459.0nm和700.0nm两个新峰。 相似文献