全文获取类型
收费全文 | 18778篇 |
免费 | 3544篇 |
国内免费 | 5307篇 |
专业分类
化学 | 7520篇 |
晶体学 | 529篇 |
力学 | 2399篇 |
综合类 | 780篇 |
数学 | 8648篇 |
物理学 | 7753篇 |
出版年
2024年 | 152篇 |
2023年 | 549篇 |
2022年 | 683篇 |
2021年 | 644篇 |
2020年 | 578篇 |
2019年 | 588篇 |
2018年 | 344篇 |
2017年 | 590篇 |
2016年 | 672篇 |
2015年 | 681篇 |
2014年 | 1299篇 |
2013年 | 1000篇 |
2012年 | 1155篇 |
2011年 | 1166篇 |
2010年 | 1098篇 |
2009年 | 1103篇 |
2008年 | 1443篇 |
2007年 | 1215篇 |
2006年 | 1255篇 |
2005年 | 1275篇 |
2004年 | 1129篇 |
2003年 | 1120篇 |
2002年 | 907篇 |
2001年 | 925篇 |
2000年 | 803篇 |
1999年 | 637篇 |
1998年 | 600篇 |
1997年 | 616篇 |
1996年 | 602篇 |
1995年 | 541篇 |
1994年 | 449篇 |
1993年 | 315篇 |
1992年 | 381篇 |
1991年 | 344篇 |
1990年 | 317篇 |
1989年 | 219篇 |
1988年 | 67篇 |
1987年 | 64篇 |
1986年 | 26篇 |
1985年 | 23篇 |
1984年 | 13篇 |
1983年 | 19篇 |
1982年 | 7篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1959年 | 8篇 |
1936年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
The relationship between the rheological properties of nylon-6,6 solutions and the morphology of their electrospun nanofibers was established. The viscosity of nylon-6,6 in formic acid(90%) was measured in the concentration range of 5 wt%-25 wt% using a programmable viscometer. Electrospinning of nylon-6,6 solutions was carried out under controlled parameters. The chemical structure, morphology and thermal properties of the obtained nanofibers were investigated using Fourier transform infrared spectroscopy(FTIR), scanning electron microscopy(SEM) and differential scanning calorimetry(DSC), respectively. Entanglement concentration(ce) was found to be 15 wt% and a power law relationship between specific viscosity and solution concentration was observed with exponents of 2.0 and 3.3 for semi-dilute unentangled(c ce) and semi-dilute entangled(c ce) regimes, respectively. The diameter and uniformity of the nanofibers were found to be dependent on the viscosity. Moreover, the average diameter of electrospun nanofibers was found to be dependent on zero shear rate viscosity and normalized concentration(c/ce) in a power law relationship with exponents of 0.298 and 0.816, respectively. For nylon-6,6 solutions, the entanglement concentration(ce = 15 wt%) provides the threshold viscosity required for the formation of a stable polymeric jet during electrospinning and producing uniform beadless fibers. For concentrations less than ce, beaded fibers with some irregularities are formed. DSC analysis showed an increase in crystallinity of all electrospun samples compared to original polymer. Furthermore, Based on FTIR spectroscopy, α phase is dominant in electrospun nanofibers and minor amount of β and γ phases is also available. 相似文献
992.
A new strategy was developed to fabricate superhydrophobic nylon 6 nanofibers, in which the blend solutions of poly(dimethylsiloxane)(PDMS) prepolymer and nylon 6 was spun using an innovative solution blowing process, and then the PDMS prepolymer contianning nanofibers were cured to obtain PDMS/nylon 6 nanofiber mats. Morphology, surface composition, non-wetting property and protective performance were investigated. The results showed that the addition of PDMS prepolymer improved the spinnability of the spinning solutions, and the PDMS/nylon 6 nanofibers had smooth surfaces and diameters from 100 nm to 350 nm. The presence of PDMS effectively enhanced the hydrophobicity of the nanofiber mats, showing water contact angles of 132° to 161° for PDMS contents of 1 wt% to 3 wt%. The PDMS/nylon 6 mats also possessed excellent protective and transport properties. The results indicated the potential application of the novel nanofiber mats in protective clothing. 相似文献
993.
A series of the copolymers of ethylene with 1-hexene(M1–M9) synthesized by metallocene catalyst Et[Ind]2ZrCl2/MAO was studied by differential scanning calorimetry and successive self-nucleation and annealing(SSA) thermal fractionation. The distribution of methylene sequence length(MSL) in the different copolymers was determined using the SSA method. The comonomer contents of samples M4 and M5 are 2.04 mol% and 2.78 mol%, respectively. Both M4 and M5 have low comonomer content and their MSL distribution profiles exhibit a monotonous increase trend with their MSL. The longest MSL of M5 is 167, and its corresponding molar percent is 43.95%, which is higher than that of M4. Moreover, the melting temperature(Tm) of M5 is also higher than that of M4. The comonomer contents of samples M7, M8, and M9 are 8.73 mol%, 14.18 mol% and 15.05 mol%, respectively. M7, M8, and M9 have high comonomer contents, and their MSL distribution profiles display unimodality. M7 has a lower peak value of 33 and a narrow MSL distribution, resulting in a Tm lower than that of M8 and M9. The MSL and its distribution are also key points that influence the melting behavior of copolymers. Sometimes, MSL and its distribution of copolymers have a greater impact on it than the total comonomer contents, which is different from traditional views. 相似文献
994.
采用化学水浴沉淀法再结合溶胶-凝胶和程序升温溶剂热两步法制备了一系列不同比例的CdS/TiO2。通过XRD、XPS、SEM、UV-Vis/DRS等手段对所合成纳米复合材料的组成、结构和形貌等进行了表征。结果表明,该系列CdS/TiO2为锐钛矿TiO2和六方相CdS的混合晶相,其在可见光有较强吸收,并且随着复合材料中TiO2比例的增加,BET值明显增大。另外,所合成CdS/TiO2呈圆球形,颗粒比较均匀。紫外光催化降解甲基橙的实验结果表明,摩尔比为1:4的CdS/TiO2的活性明显高于CdS、P25以及其它不 相似文献
995.
996.
997.
建立差示扫描量热法测定比沙可啶原料药样品纯度的方法。考察了炉体气氛、升温速率、称样量3个因素对差示扫描量热法测定结果的影响,确定差示扫描量热法最佳实验条件:升温速率为2.0 K/min,称样量为2.0~4.0 mg,炉内气体为静态空气。差示扫描量热法测定比沙可啶样品纯度为99.86%,测定结果的相对标准偏差为0.02%(n=6),差示扫描量热法测定比沙可啶样品纯度值与HPLC测定结果具有良好的一致性。该方法可以快速、准确地测定比沙可啶化学纯度,为比沙可啶纯度测定提供了一种新的分析方法。 相似文献
998.
研究了钴块中氢含量的测定方法。采用石英坩埚加载样品,热抽取法进行测定。不加任何助熔剂,分析功率为100%,匹配的分析时间为110 s,比较水平设定为0.1%。在选定的参数下,样品能在完全熔融状态下释放出全部氢。方法加标回收率为80%~119%,测定结果的相对标准偏差小于15%(n=6),检出限为0.5μg/g。热抽取法的精密度优于熔融法的精密度,满足微量气体分析的要求。 相似文献
999.
在极化连续模型框架下比较了线性响应与两种不同态特定方法计算的溶液中Alexa Fluor 350(AF350)分子激发能和光谱移动值的差异. AF350的第一激发态S0→S1电子跃迁属于π→π*跃迁, 主要对应于最高占据分子轨道(HOMO)到最低空轨道(LUMO)的跃迁. 该分子激发态偶极矩大于基态偶极矩, 激发态时溶质溶剂相互作用比基态时更强, 随着溶剂极性增大, 会发生光谱红移的现象. 与实验值相比, 线性响应和两种态特定方法均高估了激发能, 其中以IBSF(Improta-Barone-Scalmani-Frisch)方法得到的激发能最小, 矫正的基态反应场方法(cGSRF)得到的激发能最大. 对于光谱移动值, 3种方法与实验值相比都偏小, 线性响应方法(LR)计算出的误差最大, 而IBSF方法得到的结果与实验值最吻合, 是预测溶液中AF350分子激发能和光谱移动值最准确的方法. 对比了Marcus传统理论和基于约束平衡的非平衡溶剂化理论的结果, 发现后者得到的激发能和光谱移动值更接近于实验值. 相似文献
1000.
在苯乙烯单体的对位引入具有亲水性链段的乙二醇单元, 利用可逆加成断裂链转移聚合方法(RAFT), 可控合成了几种新的两亲性嵌段共聚物聚(4-乙烯基苄基乙二醇单甲醚)-b-聚丙烯酸(PMnEOS-b-PAA, n=1~3), 对其温敏和pH敏性质进行了初步研究. 同时, 研究了PMnEOS-b-PAA分别在亲水性环境下(四氢呋喃/水)和亲脂性环境下(四氢呋喃/甲苯)自组装体的形貌. 将聚对二乙二醇单甲醚苯乙烯-b-聚丙烯酸与聚苯乙烯-b-聚丙烯酸按质量比1:1[m(PMDEOS-b-PAA)/m(PS-b-PAA)=1:1]共混进行共组装, 在四氢呋喃/甲苯体积比为2∶1的混合溶剂中, 发现了一类新型具有均匀分布内部孔道且表面光滑的球形组装体. 进一步研究了该组装体的可重复性和组装机理, 为其后期应用于工业上的负载催化、 小分子识别与释放提供了一种新的策略. 相似文献