全文获取类型
收费全文 | 51307篇 |
免费 | 10170篇 |
国内免费 | 4150篇 |
专业分类
化学 | 51792篇 |
晶体学 | 580篇 |
力学 | 1166篇 |
综合类 | 271篇 |
数学 | 3836篇 |
物理学 | 7982篇 |
出版年
2024年 | 33篇 |
2023年 | 177篇 |
2022年 | 423篇 |
2021年 | 586篇 |
2020年 | 1617篇 |
2019年 | 2943篇 |
2018年 | 1381篇 |
2017年 | 1015篇 |
2016年 | 3862篇 |
2015年 | 4080篇 |
2014年 | 4141篇 |
2013年 | 5071篇 |
2012年 | 4151篇 |
2011年 | 3439篇 |
2010年 | 3786篇 |
2009年 | 3763篇 |
2008年 | 3495篇 |
2007年 | 2821篇 |
2006年 | 2443篇 |
2005年 | 2422篇 |
2004年 | 2184篇 |
2003年 | 2004篇 |
2002年 | 2824篇 |
2001年 | 2050篇 |
2000年 | 1794篇 |
1999年 | 803篇 |
1998年 | 320篇 |
1997年 | 312篇 |
1996年 | 248篇 |
1995年 | 215篇 |
1994年 | 191篇 |
1993年 | 179篇 |
1992年 | 164篇 |
1991年 | 121篇 |
1990年 | 115篇 |
1989年 | 91篇 |
1988年 | 54篇 |
1987年 | 56篇 |
1986年 | 45篇 |
1985年 | 39篇 |
1984年 | 30篇 |
1983年 | 21篇 |
1982年 | 14篇 |
1981年 | 19篇 |
1980年 | 10篇 |
1978年 | 11篇 |
1977年 | 10篇 |
1976年 | 9篇 |
1974年 | 10篇 |
1973年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
基于实验室对柴油车用V2O5-WO3/TiO2催化剂配方以及涂覆成型技术的大量研究,设计了一条产量为6000只/月的NH3选择性催化还原NOx (NH3-SCR)催化剂中试生产线,并对生产的催化剂产品进行了发动机台架测试. 结果表明,实验室制备的V2O5-WO3/TiO2粉体催化剂和生产线产品,在空速为50000 h-1和200-450 ℃条件下NOx转化率均可达80%以上;采用大尺寸堇青石载体涂覆后制备的V2O5-WO3/TiO2整体催化剂经实验室小样测试,在空速为10000-30000 h-1和250-450 ℃条件下NOx转化率也为80%以上. 发动机台架测试结果表明,该催化剂产品可使重型柴油机NOx排放达到国IV标准中欧洲稳态循环(ESC)和欧洲瞬态循环(ETC)排放限值的要求. 该生产线经适当调整后也可用于生产非钒基NH3-SCR整体催化剂,以满足未来钒基NH3-SCR催化剂更新换代的需求. 相似文献
992.
合成了2,2-二甲基-N-羟乙基-1,3-噁唑烷(OX),并以其为结构单元合成了2种多官能度的噁唑烷化合物——丙二酸二-2,2-二甲基-N-羟乙基-1,3-噁唑烷酯(OX1)和1,1,5,5-戊烷端四甲酸四-2,2-二甲基-N-羟乙基-1,3-噁唑烷酯(OX2).采用13C核磁共振谱、质谱、元素分析和傅里叶变换红外光谱对产物结构进行表征,并探索了产物用于单组份湿固化聚氨酯(SPU)体系的潜固化剂性能.研究了固化体系的表观性能、力学性能和热性能.结果表明:纯湿固化SPU发泡且膨胀,拉伸强度为3.2 MPa,断裂伸长率为364%;而SPU-OX2固化体系的拉伸强度达27.2 MPa,是纯SPU固化体系拉伸强度(3.2 MPa)的9倍;SPU-OX2固化体系断裂伸长率达457%,是纯SPU固化体系断裂伸长率(364%)的1.26倍.聚氨酯中加入噁唑烷OX1和OX2为潜固化剂,不仅较好改善了聚氨酯的表观性能,而且增加了聚氨酯预聚体的力学性能和热稳定性. 相似文献
993.
994.
Hong Zhu Feng Wen Peng Li Xiang Liu Jianmei Cao Min Jiang Feng Ming Zhaoqing Chu 《Applied biochemistry and biotechnology》2014,172(6):3163-3175
Brachypodium distachyon has been proposed as a new model system for gramineous plants with a sequenced genome and an efficient transformation system. Many transgenic B. distachyon plants have been generated in recent years. To develop a reliable fast method for detecting transgenic B. distachyon and quantifying its transgene copy numbers, a species-specific reference gene is of great priority to be validated both in qualitative PCR and quantitative real-time PCR detection. In this study, we first proved that the BdFIM (B. distachyon fimbrin-like protein) gene is a suitable reference gene in qualitative PCR and quantitative real-time PCR for B. distachyon. Fourteen different B. distachyon varieties were tested by both qualitative and quantitative PCRs, and identical amplification products of BdFIM were obtained with all of them, while no amplification products were observed with samples from 14 other plant species, suggesting that BdFIM gene was specific to B. distachyon. The results of Southern blot analysis revealed that the BdFIM gene was low copy number in seven tested B. distachyon varieties. In conclusion, the BdFIM gene can be used as a reference gene, since it had species specificity, low heterogeneity, and low copy number among the tested B. distachyon varieties. Furthermore, the copy number of inserted sequences from transgenic B. distachyon obtained by real-time PCR methods and Southern blot confirmed that the BdFIM gene was an applicable reference gene in B. distachyon. 相似文献
995.
This contribution reports ethylene polymerization behavior of titanium complexes incorporating bis(phenoxyimine) ligands. Six phenoxy-imine Ti(IV) complexes {6-R1-2-[CH=N(2,6-difluoro-3,5-diR2-4-R3Ph)]C6H3O}2TiCl2(1: R1 = H, R2 = H, R3 = H; 2: R1 = H, R2 = H, R3 = 4-vinylphenyl; 3: R1 = CH3, R2 = H, R3 = H; 4: R1 = CH3, R2 = H, R3 = 4-vinylphenyl; 5: R1 = CH3, R2 = F, R3 = H; 6: R1 = CH3, R2 = F, R3 = 4-vinylphenyl) have been synthesized and evaluated for ethylene polymerization using dried MAO(simplified as DMAO) as cocatalyst. An obvious catalytic heterogeneity of Cat 2(Complex 2/DMAO) towards ethylene polymerization was observed, which was illustrated by decreased activity, multimodal molecular weight distribution and partially improved particle morphology comparing with Cat 1. Moreover, Cat 3 exhibits "living" characteristics in the process under certain conditions(25 °C, less than 20 min). Otherwise, the moderate to high ethylene polymerization activity of ca. 105-106 g PE/(mol Ti·h) and high molecular weight(Mw = 105-106) of polyethylene can be obtained by changing the skeleton structure of these complexes. 相似文献
996.
Jian Gao You-Yong Xu Hong-Mei Yang Hong Xu Feng Xue Sha Li Xiao-Hai Feng 《Applied biochemistry and biotechnology》2014,173(6):1419-1430
An inulinase-producing strain, Paenibacillus polymyxa ZJ-9, was isolated from natural sources to produce R,R-2,3-butanediol via one-step fermentation of raw inulin extracted from Jerusalem artichoke tubers. The inulinase gene from P. polymyxa ZJ-9 was cloned and overexpressed in Escherichia coli BL21 (DE3), and the purified recombinant inulinase was estimated to be approximately 56 kDa by both sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS–PAGE) and gel filtration chromatography. This result suggests that the active form of the inulinase is probably a monomer. Terminal hydrolysis fructose units from the inulin indicate that enzymes are exo-inulinase. The purified recombinant enzyme showed maximum activity at 25 °C and pH 6.0, which indicate its extreme suitability for industrial applications. Zn2+, Fe2+, and Mg2+ stimulated the activity of the purified enzyme, whereas Co2+, Cu2+, and Ni2+ inhibited enzyme activity. The K m and V max values for inulin hydrolysis were 1.72 mM and 21.69 μmol min?1 mg?1 protein, respectively. The same parameters toward sucrose were 41.09 mM and 78.7 μmol min?1 mg?1 protein, respectively. Considering its substrate specificity and other enzymatic characteristics, we believe that this inulinase gene from P. polymyxa ZJ-9 could be transformed into other special bacterial strains to allow inulin conversion to other biochemicals and bioenergy through one-step fermentation. 相似文献
997.
Dunhua Hong Taisong Pan Dayu Feng Zhenlong Huang Feiyi Liao XiaoMei Li Yin Zhang Min Gao 《Journal of Sol-Gel Science and Technology》2014,71(3):464-469
(LaBa)Co2O5+δ (LBCO) thin films were successfully fabricated on Si (001) substrates by polymer-assisted deposition method. Microstructures were examined by X-ray diffraction technique, which confirmed the films were a single phase, pseudo cubic structure. The electrical transport properties of the films were investigated by the temperature dependence of films resistance, which suggested that LBCO films have typical semiconductor properties. After circle tests, the oxygen–hydrogen response rate did not show obvious variation in the specific temperature environment of 580 °C. This demonstrates that the LBCO thin films have a superior stability in both oxygen and hydrogen (6 % H2, 94 % N2) environment. Simultaneously, the drastic changes of films resistance (from ~106 to ~102 Ω) with the switch of redox (O2–H2) environment within such a short time (~2.2 s) indicated that LBCO films have an excellent oxygen sensitive property and extraordinary fast surface exchange rate. 相似文献
998.
建立了离子交换色谱-直接电导检测法分离测定3种同系物季铵盐离子液体阳离子(四甲基铵、四乙基铵和四丙基铵阳离子)的方法。采用磺酸型阳离子交换色谱柱,以乙二胺-柠檬酸-乙腈为淋洗液,考察了淋洗液种类、浓度及色谱柱温度对3种阳离子保留的影响。并根据测定对象不同,调整乙二胺浓度及乙腈含量以改善分离效果。淋洗液中增加乙腈含量,可明显缩短四丙基铵阳离子的保留时间,并改善其色谱峰形。季铵阳离子同系物的保留符合碳数规律。优化的色谱条件为:流速1.0 mL/min,色谱柱温度40℃;以0.02 mmol/L乙二胺-0.12 mmol/L柠檬酸(pH 4.0)为淋洗液分离测定四甲基铵;以0.2 mmol/L乙二胺-0.4 mmol/L柠檬酸-1%乙腈(pH 4.0)为淋洗液分离测定四乙基铵和四丙基铵。所测阳离子的检出限(S/N=3)分别为0.015,0.22,1.88 mg/L,相对标准偏差(n=5)均小于2.3%。将方法应用于表面活性剂和实验室合成的离子液体的分析,加标回收率为99%~104%。本方法简单、准确、可靠,具有良好的实用性。 相似文献
999.
The pollution characteristics of ambient fine particulate matter(PM2.5) containing polycyclic aromatic hydrocarbons(PAHs) and nitrated PAHs(NPAHs) in samples collected during a typical winter time period in Taiyuan of China were investigated.The obtained results revealed that the mean mass concentrations of PM2.5,SPAHs(sum of 16 PAHs) and SNPAHs(sum of 3 NPAHs) on PM2.5were161.4 mg/m3,119.8 ng/m3and 0.446 ng/m3,respectively.Diagnostic ratios of PAHs and NPAHs implied that coal consumption might be the main source of the PM2.5pollution.The measured PM2.5mass concentrations,BaP equivalent toxicity(28.632 ng/m3) and individual carcinogenicity index(3.14 10 5) were much higher than those of the recommended safety standards. 相似文献
1000.
F. Khoerunnisa D. Minami T. Fujimori S. Y. Hong Y. C. Choi H. Sakamoto M. Endo K. Kaneko 《Adsorption》2014,20(2-3):301-309
We tuned the electronic properties of single wall carbon nanotube (SWCNT) with intercalation of naphthalene derivatives (NDs) having different electron donor or acceptor property in the SWCNT bundles. Characterization of the adsorbed SWCNT with Raman spectroscopy and electrical conductivity measurement clearly indicate the charge transfer interaction of ND molecules with SWCNT. Also X-ray diffraction supports the intercalation of ND molecules in the interstitial spaces and groove sites of SWCNT bundle. Intercalation of ND molecules enhances remarkably the CO2 adsorptivity, which can be ascribed to the key importance of the interaction of the quadrupole moment of CO2 with the local electrical field on the SWCNT induced by the charge transfer interaction. 相似文献