全文获取类型
收费全文 | 4854篇 |
免费 | 659篇 |
国内免费 | 1646篇 |
专业分类
化学 | 5034篇 |
晶体学 | 406篇 |
力学 | 185篇 |
综合类 | 59篇 |
数学 | 117篇 |
物理学 | 1358篇 |
出版年
2024年 | 11篇 |
2023年 | 84篇 |
2022年 | 246篇 |
2021年 | 264篇 |
2020年 | 249篇 |
2019年 | 157篇 |
2018年 | 153篇 |
2017年 | 216篇 |
2016年 | 242篇 |
2015年 | 213篇 |
2014年 | 256篇 |
2013年 | 502篇 |
2012年 | 348篇 |
2011年 | 418篇 |
2010年 | 331篇 |
2009年 | 354篇 |
2008年 | 381篇 |
2007年 | 418篇 |
2006年 | 395篇 |
2005年 | 352篇 |
2004年 | 281篇 |
2003年 | 252篇 |
2002年 | 215篇 |
2001年 | 137篇 |
2000年 | 128篇 |
1999年 | 88篇 |
1998年 | 89篇 |
1997年 | 70篇 |
1996年 | 57篇 |
1995年 | 42篇 |
1994年 | 51篇 |
1993年 | 31篇 |
1992年 | 31篇 |
1991年 | 19篇 |
1990年 | 15篇 |
1989年 | 4篇 |
1988年 | 10篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 9篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1959年 | 1篇 |
排序方式: 共有7159条查询结果,搜索用时 125 毫秒
111.
贵金属对钴基催化剂上肉桂醛选择加氢反应的影响 总被引:5,自引:0,他引:5
研究了贵金属改性的Co/γ-Al2O3催化剂上的肉桂醛选择加氢反应.结果表明,通过Pt,Pd和Ru贵金属改性,提高了催化剂的活性,但只有Pt改性的催化剂具有较高的选择性.w(Pt)<0.5%时,催化剂的活性随着Pt含量的增加呈直线升高,当w(Pt)=0.5%时,催化剂活性可提高近5倍,但催化剂的选择性变化很小.XRD结果表明,催化剂经还原后,γ-Al2O3上的钴主要为α-Co0.TPR结果表明,Pt的加入提高了Co3O4的还原性能,且Pt含量越高,Co3O4的还原温度越低.XPS结果表明,Pt改性的催化剂样品,其Co3O4大部分被还原为Co0.由于Pt与Co具有协同效应,故Pt改性的Co/γ-Al2O3催化剂既具有较高的活性,又具有很高的选择性. 相似文献
112.
A method of ultrasonic treatment (UST) was first used to modify the structure and electrochemical performance of nickel hydroxide for the active material of nickel series alkaline batteries. The experimental results showed that UST was an effective method to improve the electrochemical performance of β-Ni(OH)2 such as specific discharge capacity, discharge potential, electrochemical reversibility and oxygen evolution over-potential. The results of electrochemical impedance spectroscopy, powder X-ray diffraction and particle size distribution indicated that the improvement of the performance of β-Ni(OH)2 through UST was attributed to the reduction of the charge-transfer resistance (Rt) and the diffusion impedance (Zw), which resulted from the decrease of the crystallite and particle size and the increase of interlayer spacing. Diffusion coefficient of proton DH of ultrasonic treated β-Ni(OH)2 gained by CV tests was 1.13 × 10^-11 cm^2/s, and the average discharge specific capacity of ultrasonic treated β-Ni(OH)2 electrode was 301 mAh/g. 相似文献
113.
114.
Ben Salah M Vilminot S André G Richard-Plouet M Bourée-Vigneron F Mhiri T Kurmoo M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(8):2048-2057
[Mn(3)(OH)(2)(SO(4))(2)(H(2)O)(2)] and its deuterated analogue were synthesized by a hydrothermal technique and characterized by differential thermal analysis, thermogravimetric analysis, and IR spectroscopy. Its nuclear structure, determined by single-crystal X-ray analysis and Rietveld analysis of neutron powder-diffraction data, consists of a 3D network of chains of edge-sharing Mn(1)O(6), running along the c axis, connected by the apices of Mn(2)O(6) and SO(4) units. It is isostructural to the nickel analogue. Determination of the magnetic structure and measurements of magnetization and heat capacity indicate the coexistence of both magnetic long-range ordering (LRO) and short-range ordering (SRO) below a Néel temperature of 26 K, while the SRO is retained at higher temperatures. The moments of the two independent Mn atoms lie in the bc plane, and that of Mn(1) rotates continuously by 54 degrees towards the c axis on decreasing the temperature from 25 to 1.4 K. While the SRO may be associated with frustration of the moments within a Mn(3) trimer, the LRO is achieved by antiparallel alignment of the four symmetry-related trimers within the magnetic unit cell. A spin-flop field, measured by dc and ac magnetization on a SQUID, is observed at 15 kOe. 相似文献
115.
Tamas Benko Agnes Szanyi Peter Mizsey Zsolt Fonyo 《Central European Journal of Chemistry》2006,4(1):92-110
The sustainable development and consumption need more efficient use of natural resources. As a consequence, the use of industrial
solvents demands their recovery instead of end-of-pipe treatment. It is not always clear, however, which treatment alternative
should be applied. Based on an industrial case study, the environmental and economic evaluation and comparison of the treatment
alternatives of a non-ideal solvent mixture containing azeotropes is investigated for determining the preferable option. For
the recovery of the industrial solvent mixture, two different separation alternatives are evaluated: a less effective alternative
and a novel design based on hybrid separation tools. An end-of-pipe treatment alternative, incineration, is also considered
and the split of the solvent mixtures between recovery and incineration is investigated. The environmental evaluation of the
alternatives is carried out using ‘Eco-indicator 99 life-cycle impact assessment methodology’. Economic investigation is also
accomplished. The economic features clearly favour the total recovery, however, the environmental evaluation detects that
if a recovery process of low efficiency is applied, its environmental burden can be similar or even higher than that of the
incineration. This motivates engineers to design more effective recovery processes and reconsider the evaluation of process
alternatives at environmental decision making. 相似文献
116.
微量元素锌在治疗Bell麻痹中的作用 总被引:1,自引:0,他引:1
陈贤雄 《广东微量元素科学》2002,9(3):31-35
为探讨急性早期应用补锌和综合疗法治疗Bell麻痹 ,将 1 5 8例Bell麻痹患者 ,随机分成两组 :(1 )补锌组 (补锌 +综合疗法 ) ;(2 )对照组 (单纯综合疗法 )。对其进行了观察对比治疗 ,同时检测了所有患者治疗前后自身对照发锌值 ,还对所有患者进行了定位诊断、面神经电反应测试、并对面肌功能进行评分。结果表明 ,对定位诊断其病变在D段、面肌功能评分在 1 1分以下、面神经无电反应患者 ,即使给予早期综合治疗 ,效果仍较差 ,而早期给予补锌加综合治疗效果优于对照组 ,其它段内患者治疗次数和治愈天数也均优于对照组。可见微量元素锌在发挥免疫应答、免疫调控中的重要作用 相似文献
117.
Inverse gas chromatography (IGC) was used to determine the dispersive component of the free energy as well as the acid-base properties of cellulose fibre surfaces, before and after modification by corona treatment. It was found that the corona treatment increases both the dispersive contribution to surface energy and its acidic character, whereas only a slight increase in its basicity was observed. It was also found that some chemical degradation of the surface occurs at high corona currents. The extent of modification of the surface properties, as revealed by IGC, was correlated to the surface chemical composition deduced from XPS analysis as well as with the electrical conductance and the pH of the water suspensions of the cellulose fibres. 相似文献
118.
Polymer enhanced ultrafiltration (PEUF) is a newly developed method for the removal of heavy metals from aqueous solutions. This method was applied for the removal of mercury and cadmium with the presence of polyethyleneimine (PEI) as a water soluble polymer. After ultrafiltration experiments for metal–polymer mixtures, two separate streams, namely, retentate and permeate, former of which contains mainly metal–polymer complex and free polymer molecules while latter of which mainly contains free metal ions, were obtained. At the end of PEUF experiments, performance of operation was determined by concentration analyses which was achieved by atomic absorption spectroscopy (AAS) applied in a different way for permeate and retentate streams considering the effect of presence of polymer. For mercury analysis, cold vapor AAS was applied. It was observed that the presence of PEI did not affect the atomic absorption signal when 10% HCl was added to the sample solutions. For calcium and cadmium, flame AAS was used. It was observed that change in PEI concentration results in change in measured concentration of calcium and cadmium. Therefore, two new approaches were developed for accurate measurement of concentrations of calcium and cadmium. It was also observed that presence of other metals did not affect the accuracy of the measurement of a particular metal in the concentration range studied. 相似文献
119.
笼形聚偕胺肟树脂的研究—酚处理对树脂吸附贵金属性能的影响 总被引:5,自引:0,他引:5
研究了笼形聚偕胺肟树脂(CAO)和酸式(ACAO)及碱式(BCAO)树脂对于苯酚、对-氨基酚、对-硝基酚、对-甲氨基酚和对-苯二酚的吸附和吸留作用,测定了酚处理树脂的中和当量。它反映了笼形树脂吸留外来物的特性。酚处理树脂有效地吸附Ag 、AuCl4-、PtCl62-、PdCl42-和IrCl62-等贵金属离子,吸附方式似乎与溶液的pH值有关。SEM观察表明,Ag 、Au-Cl4-和IrCl62-离子在吸附过程中被还原为零价态,在树脂表面生成形态各异的结晶。 相似文献
120.
1 INTRODUCTION Polyoxometalates (POMs) have been attracting extensive interest of many chemists in solid-state materials chemistry owning to the various structures and great potential applications in catalysis, medi- cine, photo-electricity and magnetism[1~9]. Over the past decades, lots of novel POMs have been pre- pared by hydrothermal synthesis, such as discrete structure[10], one-dimensional chain-like structure[11, 12] and higher dimensional architecture[13~18]. However, in these… 相似文献