全文获取类型
收费全文 | 2883篇 |
免费 | 159篇 |
国内免费 | 551篇 |
专业分类
化学 | 3311篇 |
晶体学 | 31篇 |
力学 | 13篇 |
综合类 | 21篇 |
数学 | 1篇 |
物理学 | 216篇 |
出版年
2024年 | 1篇 |
2023年 | 17篇 |
2022年 | 53篇 |
2021年 | 41篇 |
2020年 | 58篇 |
2019年 | 67篇 |
2018年 | 61篇 |
2017年 | 66篇 |
2016年 | 91篇 |
2015年 | 93篇 |
2014年 | 82篇 |
2013年 | 324篇 |
2012年 | 150篇 |
2011年 | 154篇 |
2010年 | 141篇 |
2009年 | 146篇 |
2008年 | 143篇 |
2007年 | 207篇 |
2006年 | 174篇 |
2005年 | 168篇 |
2004年 | 183篇 |
2003年 | 151篇 |
2002年 | 120篇 |
2001年 | 119篇 |
2000年 | 117篇 |
1999年 | 107篇 |
1998年 | 77篇 |
1997年 | 67篇 |
1996年 | 80篇 |
1995年 | 46篇 |
1994年 | 56篇 |
1993年 | 62篇 |
1992年 | 42篇 |
1991年 | 30篇 |
1990年 | 15篇 |
1989年 | 10篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 11篇 |
1985年 | 3篇 |
1984年 | 10篇 |
1983年 | 3篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有3593条查询结果,搜索用时 343 毫秒
881.
The solvation parameter model is used to characterize the retention properties of five open-tubular column stationary phases (ZB-5 ms, DB-5 ms, DB-XLB, DB-17 ms, and DB-35 ms) based on silarylene-siloxane copolymer chemistries at five equally spaced temperatures over the range 60-140 degrees C. System constant differences and regression models for varied compounds are used to establish the selectivity equivalence of the silarylene-siloxane copolymer stationary phases and to compare their separation characteristics with poly(dimethyldiphenylsiloxane) stationary phases containing a nominally similar concentration of phenyl groups. These studies demonstrate that ZB-5 ms and DB-5 ms are selectivity equivalent. DB-XLB is significantly more dipolar and polarizable than DB-5 ms. In general terms, the silarylenesiloxane copolymer stationary phases are slightly less cohesive and more dipolar and polarizable with similar hydrogen-bond basicity to the poly(dimethyldiphenylsiloxane) stationary phases they were designed to replace. None of the silarylenesiloxane copolymer or poly(dimethyldiphenylsiloxane) stationary phases are hydrogen-bond acidic. Selectivity differences between the two types of stationary phase are temperature dependent and tend to be smaller at higher temperatures within the temperature range studied. Consequently, selectivity differences cannot be globalized without reference to the temperature for the comparison. 相似文献
882.
An efficient synthesis of triptophenolide methyl ether 4 from the readily available abietic acid 3 in nine steps is described and successfully applied to the synthesis of (−)-triptolide 1. The route is of characteristic of low cost, high yield and easy operation. In addition, every reaction in this route has been successfully scaled-up to a 100 g substrate level without loss of yield. 相似文献
883.
The study presents a new analytical method for speciation analysis in fractionation of aluminium fluoride complexes and free Al3+ in soil samples. Aluminium speciation was studied in model solutions and soil extract samples by means of high performance ion chromatography (HPIC) with UV-VIS detection using post-column reaction with tiron for the separation and detection of aluminium fluoride complex and Al3+ forms during one analysis. The paper presents particular stages of the chromatographic process optimization involving selecting the appropriate eluent strength, type of elution or concentration and quantity of derivatization reagent. HPIC was performed on a bifunctional analytical column Dionex IonPac CS5A. The use of gradient elution and the eluents A: 1 M NH4Cl and B: water acidified to pH of eluent phase, enabled full separation of fluoride aluminium forms as AlF2+, AlF30, AlF4− (first signal), AlF2+ (second signal) and form Al3+ in a single analytical procedure. The proposed new method HPIC-UVVIS was applied successfully in the quantitative and qualitative analysis of soil samples. 相似文献
884.
超临界二氧化碳中无水相涂料的合成与表征 总被引:1,自引:0,他引:1
为了减小传统纸张涂布中涂料水分对涂布能耗、涂布质量以及对涂布原纸质量的影响,研究了以超临界二氧化碳为反应介质,制备聚甲基丙烯酸甲酯(PMMA)和碳酸钙颜料混合的粉末涂料粒子.通过FTIR、GPC-十八角度激光光散射联用技术对PMMA组成结构进行了表征,考察了反应体系中引发剂浓度、单体浓度、稳定剂浓度、反应温度和反应时间对聚合反应的转化率和聚合产物的分子量的影响.实验表明,当反应条件为反应压力10MPa,反应温度75℃,反应时间8h,单体浓度0.10g/mL,引发剂浓度0.10×10-2g/mL,稳定剂浓度0.06×10-2g/mL时,其聚合反应的转化率较高,同时PMMA的分子量适中,分子量分布窄.SEM观察到混合涂料粒子颗粒均匀,表明颜料在粉末涂料体系中分散性良好. 相似文献
885.
The telechelic α,ω‐alkyne‐poly(methyl methacrylate) (alkyne‐PMMA‐alkyne) was synthesized by single electron transfer radical coupling (SETRC) reaction of α‐alkyne, ω‐bromine‐poly(methyl methacrylate) (alkyne‐ PMMA‐Br). The propargyl 2‐bomoisobutyrate (PgBiB) was first prepared to initiate atom transfer radical polymerization (ATRP) of methyl methacrylate at 45°C using CuCl/1,1,4,7,10,10‐hexamethyl triethylenetetramine (HMTETA) as homogeneous catalytic system. Then the SETRC reaction was conducted at room temperature in the presence of nascent Cu(0) and N,N,N′,N′ ′,N′ ′‐pentamethyldiethyllenetriamine (PMDETA). The precursor alkyne‐PMMA‐Br and coupled product alkyne‐PMMA‐alkyne were characterized by GPC and 1H NMR in detail. 相似文献
886.
887.
麻疯油转酯化产物的高效液相色谱分析 总被引:2,自引:0,他引:2
建立了高效液相色谱法同时测定麻疯油转酯化产物中4种主要脂肪酸甲酯的分析方法。样品经膜处理后用丙酮溶解,采用Hypersil ODS(C18)色谱柱进行分离。以乙腈为流动相进行等度洗脱,内标法定量,同时对色谱条件进行优化。结果表明,在优化的色谱条件下4种脂肪酸甲酯在10 min内得到良好的分离,标准曲线的线性相关系数均达到0.999以上,平均回收率为96%~98%,重现性相对标准偏差为7.2%~10.2%,重复性相对标准偏差为0.31%~2.02%。与气相色谱法相比,该方法具有较高的灵敏度,可用于麻疯油转酯化产物中脂肪酸甲酯含量的测定,为麻疯油转酯化反应制备生物柴油的定性定量分析提供了参考依据。 相似文献
888.
建立了血液中可卡因(cocaine, COC)及其代谢物爱冈宁甲基酯(ecgonine methyl ester, EME)的气相色谱-质谱(CG-MS)和气相色谱-氢火焰离子化检测(GC-FID)方法。该方法采用微波萃取提取血液中的COC和EME,优化并确定了最佳提取条件: 以氯仿-异丙醇(体积比为9:1)混合溶液为提取溶剂,用0.05 mol/L Na2CO3-NaHCO3缓冲溶液调节样品溶液的pH至10.0,在40 ℃下微波萃取6 min;采用GC-MS对萃取液中的COC和EME进行定性,采用GC-FID进行定量检测。COC和EME的平均回收率分别为79.91%~99.85%,相对标准偏差(RSD)均小于3.10%,检出限(S/N=3)分别为60 mg/L和40 mg/L。该方法无需衍生化,快速、准确、灵敏,可同时检测血液中的COC和EME。 相似文献
889.
Thierry Badel Emmanuel Beyou Veronique Bounor‐Legaré Philippe Chaumont Jean Jacques Flat Alain Michel 《Journal of polymer science. Part A, Polymer chemistry》2007,45(22):5215-5226
The grafting of poly(methylmethacrylate) onto poly(ethylene‐co‐1‐octene) by in situ radical polymerization of methyl methacrylate is a process where the side reactions are difficult to characterize. To increase the understanding of both the nature and the extent of such reactions, products resulting from the same chemical system, where polymer is replaced by squalane and/or pentadecane, are analyzed. The influence of the temperature, the nature of peroxides (used as radicals generators) and the monomer concentration are investigated toward the chain length of the grafts. The resulting grafted PMMA and PMMA homopolymer are qualitatively analyzed by MALDI‐TOF spectroscopy and size exclusion chromatography. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5215–5226, 2007 相似文献
890.
A. Yu. Gladky V. K. Ermolaev V. N. Parmon 《Reaction Kinetics and Catalysis Letters》1999,67(1):183-189
Methyl radicals have been detected by direct mass-spectrometric detection of radicals in pyrolysis of propane under stationary
conditions at 500 Pa in a flow quartz reactor. The process is shown to be homogeneously initiated, the surface of quartz not
affecting the propane dissociation under the conditions employed. 相似文献