全文获取类型
收费全文 | 22189篇 |
免费 | 4105篇 |
国内免费 | 4126篇 |
专业分类
化学 | 15961篇 |
晶体学 | 426篇 |
力学 | 1610篇 |
综合类 | 302篇 |
数学 | 2697篇 |
物理学 | 9424篇 |
出版年
2024年 | 46篇 |
2023年 | 400篇 |
2022年 | 703篇 |
2021年 | 770篇 |
2020年 | 906篇 |
2019年 | 823篇 |
2018年 | 757篇 |
2017年 | 740篇 |
2016年 | 971篇 |
2015年 | 1030篇 |
2014年 | 1282篇 |
2013年 | 1615篇 |
2012年 | 1960篇 |
2011年 | 2070篇 |
2010年 | 1500篇 |
2009年 | 1448篇 |
2008年 | 1642篇 |
2007年 | 1467篇 |
2006年 | 1439篇 |
2005年 | 1287篇 |
2004年 | 976篇 |
2003年 | 743篇 |
2002年 | 760篇 |
2001年 | 703篇 |
2000年 | 590篇 |
1999年 | 560篇 |
1998年 | 381篇 |
1997年 | 343篇 |
1996年 | 384篇 |
1995年 | 327篇 |
1994年 | 306篇 |
1993年 | 244篇 |
1992年 | 223篇 |
1991年 | 188篇 |
1990年 | 153篇 |
1989年 | 149篇 |
1988年 | 133篇 |
1987年 | 86篇 |
1986年 | 76篇 |
1985年 | 59篇 |
1984年 | 45篇 |
1983年 | 38篇 |
1982年 | 21篇 |
1981年 | 35篇 |
1980年 | 11篇 |
1979年 | 5篇 |
1965年 | 3篇 |
1959年 | 2篇 |
1957年 | 5篇 |
1936年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 734 毫秒
291.
头发锌含量测定的应用价值评价 总被引:1,自引:0,他引:1
以114例2-5岁儿童为对象,同时取其末梢血和头发,测定锌含量。以全血锌值为标准,评价头锌测定的应用价值。结果显示,头发锌测定的灵敏度可达86.1%,特异度为52.4%,符合率为73.7%。 相似文献
292.
An efficient Pd‐catalyzed three‐component coupling reaction of allyl carboxylates, norbornenes and diboronates is described, which allows efficient assembly of C(sp3)—C(sp3) and C(sp3)—B bonds in a single process. Moreover, this approach shows advantages of good chemo‐ and regioselectivity, as well as good substrates suitability. 相似文献
293.
Density of supercritical CO_2-tetrahydrofuran binary mixture and the partial molar volume of the cosolvent 总被引:1,自引:0,他引:1
The densities of supercritical CO2-tetrahydrofuran (cosolvent) binary mixture weremeasured at temperatures in range of 308.15 to 323.15 K and at pressure up to 16.5 MPa.The concentrations of tetrahydrofuran were from 0 to 0.57 mol/L.The partial molar volume of tetrahydrofuran was calculated based on the relationship between the density of the mixture and the concentration of the cosolvent.It is observed that the partial molar volume of the cosolvent is negative and the absolute value decreases with increasing pressure and the concentration of the cosolvent. 相似文献
294.
Xiangpeng Wang Yunxiang Zheng Chunxiao Zhang Lina Zong 《International Journal of Polymer Analysis and Characterization》2020,25(5):300-314
Abstract A novel hydrolysis-resistant superabsorbent composite was prepared via the solution polymerization based on acrylic acid (AA) and sodium bentonite (SBT) as monomers, tetraallylammonium bromine (TAAB) as crosslinker and ammonium persulfate (APS) as initiator. The mechanism of polymerization and the structure of the superabsorbent polymer (SAP) were studied by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (X-ray), and scanning electron microscopy (SEM). The reaction conditions such as different mass ratios of APS to AA, TAAB to AA, SBT to AA, neutralization degree of AA were optimized by orthogonal experiment, and the influence of each reaction condition on the capacity of water absorption at 150?°C was investigated via single-factor controlled experiment. The hydrolysis resistance and swelling kinetics of the SAP were studied in different solutions at 150?°C. Compared to traditional SAPs, the SAP synthesized with TAAB as crosslinker performed a more excellent hydrolysis resistance and water absorbency capacity at high temperatures. The water absorbency in distilled water or 0.1?mol L?1 NaCl solution could reach 392.6 and 145.2?g g?1at 150?°C, respectively. The SAP maintaining high swelling capacity in the pH range of 5–9 indicated its wide application values in the acidic or alkaline environment at high temperature. In addition, the SAP exhibited good reusability which could still retain about 73% of its initial water absorbency after reswelling six times at 150?°C. 相似文献
295.
以丙烯酸(AA)、丙烯酰胺(AM)、2-丙烯酰胺基-2-甲基丙磺酸(AMPS)为原料,蒙脱土(MMT)为改性剂,过硫酸钾和无水亚硫酸氢钠为氧化还原引发体系,六亚甲基四胺(C6H12N4)为交联剂,采用水溶液聚合法合成了一种新型高吸水聚合物凝胶型屏蔽暂堵剂(MTC 1),其结构经IR表征。在最优合成条件(AM/AMPS/AA=11/7/1, m/m/m,引发剂加量0.6 wt%,于45 ℃反应2.5 h)下,MTC-1的最大吸水倍率为1006倍。岩心流动实验结果表明:MTC-1的堵水性能较高,封堵效率达到83.72%。 相似文献
296.
近些年来,无铅低熔点填料日渐引起人们的兴趣,且将得到广泛的应用。Cu-Zn-Sn合金正是无铅低熔点填料的一种基本合金。该体系相图虽有一些报道,但他们的研究只涉及富铜区(>50Wt%)的液相限和等温截面(500℃)。我们颇感兴趣的富Sn区的液相限,尚未见文献报道,因此测定该体系富Sn区的液相限是必要的。关于Cu-Zn-Sn体系的三个相关二元体系,已有文献报道,也较成熟。对于Cu-Zn体系,富Zn区600℃以下存在两个转熔反应,其转熔点分别为Q(~88Wt%Zn,598℃),W(97.5-98.5Wt%Zn,424℃);Cu-Sn体系富Sn区有一个转熔反应,其转熔点 相似文献
297.
298.
缓慢炭化部分氧化对制备煤质活性炭的影响 总被引:5,自引:0,他引:5
研究了炭化升温速度、炭化低温区引入空气部分氧化对活性炭制备过程中炭化阶段、炭化物结构、活性炭性能等的影响。结果表明,炭化时低温部分氧化可提高炭化物得率,使炭化物微晶的d002值升高和Lc值减小;而较慢的炭化升温速度有利于制备优质活性炭。缓慢炭化、部分氧化可以在一定程度上控制炭化路径,使炭化向生成取向性差、难石墨化、各向同性、呒定形炭多的炭化物的方向进行;并讨论了它们控制炭化的作用机理。以此为指导, 相似文献
299.
将取代色满酮(1)与芳肼反应生成的腙与HNCO发生[3+2]环加成反应,加成产物(2)经氧化得到偕偶氮异氰酸酯(3).化合物3在HBF4的催化下发生环化-重排反应,得到新颖的三环系2-氧代-1,2,4-三唑并[3,2-d][1,5]苯并氧氮杂化合物5a~5g. 相似文献
300.
Addition of a small amount of polar solvent (i.e., modifier) to CO2 in packed column supercritical fluid chromatography (SFC) has shown major improvements in both polar analyte solubility and interaction of the polar analyte with the stationary phase. Recently, the addition of an ionic component (i.e., additive) to the primary modifier by one of us has been shown to extend even further the application of SFC to polar analytes. In this work, the effect of various ionic additives on the elution of ionic compounds, such as sodium 4-dodecylbenzene sulfonate and sodium 4-octylbenene sulfonate, has been studied. The additives were lithium acetate, ammonium acetate, tetramethylammonium acetate, tetrabutylammonium acetate, and ammonium chloride dissolved in methanol. Three stationary phases with different degrees of deactivation were considered: conventional cyanopropyl, deltabond cyanopropyl, and bare silica. The effect of additive concentration and additive functionality on analyte retention was investigated. Sodium 4-dodecylbenzene sulfonate was successfully eluted using all the additives with good peak shape under isocratic/isobaric/isothermal conditions. Different additives, however, yielded different retention times and in some cases different peak shapes. 相似文献