全文获取类型
收费全文 | 4143篇 |
免费 | 488篇 |
国内免费 | 578篇 |
专业分类
化学 | 3707篇 |
晶体学 | 27篇 |
力学 | 85篇 |
综合类 | 28篇 |
数学 | 296篇 |
物理学 | 1066篇 |
出版年
2024年 | 6篇 |
2023年 | 38篇 |
2022年 | 89篇 |
2021年 | 114篇 |
2020年 | 143篇 |
2019年 | 133篇 |
2018年 | 113篇 |
2017年 | 110篇 |
2016年 | 167篇 |
2015年 | 160篇 |
2014年 | 181篇 |
2013年 | 397篇 |
2012年 | 266篇 |
2011年 | 280篇 |
2010年 | 229篇 |
2009年 | 270篇 |
2008年 | 260篇 |
2007年 | 243篇 |
2006年 | 252篇 |
2005年 | 230篇 |
2004年 | 210篇 |
2003年 | 200篇 |
2002年 | 164篇 |
2001年 | 109篇 |
2000年 | 104篇 |
1999年 | 106篇 |
1998年 | 96篇 |
1997年 | 71篇 |
1996年 | 73篇 |
1995年 | 59篇 |
1994年 | 52篇 |
1993年 | 45篇 |
1992年 | 42篇 |
1991年 | 33篇 |
1990年 | 26篇 |
1989年 | 28篇 |
1988年 | 24篇 |
1987年 | 14篇 |
1986年 | 16篇 |
1985年 | 6篇 |
1984年 | 15篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有5209条查询结果,搜索用时 46 毫秒
991.
992.
In this study, we focused on the effect of ultrasound on ion exchange kinetics to obtain the Li-, Ca- and Ce-rich NaX zeolite. The results were compared to those obtained from the traditional batch exchange method under similar conditions. Contact time and initial cation concentration (fold equivalent excess) were studied. Ultrasound enhanced the replacement of Na+ ion with Li+, Ca2+ and Ce3+ ions in the extra-framework of zeolite up to 76%, 72% and 66%, respectively. The intraparticle diffusion is the rate limiting step in the ion exchange for both exchange methods. As compared to the traditional exchange method, the ultrasonic method applied in this study was found to be very effective on the exchange amount at equilibrium. 相似文献
993.
We have studied the magnetic and transport properties of Fe doped La0.65A0.35Mn0.95Fe0.05O3 (A = Ca, Sr, Pb, Ba) manganites. All the compositions show ferromagnetic/metal to paramagnetic/insulator transition (TC) except the Pb doped sample which is insulating and ferromagnetic (FM) in the entire temperature range. The magnetization and TC are decreased by decreasing the cation size on La site. The transition temperature and magnetic moment at 77 K is a maximum for Sr doped sample and is decreasing if we increase or decrease the cation size from Sr size. The maximum value of TC and magnetic moment for Sr based sample is most likely due to the closer ionic sizes of La and Sr as compared to the other dopants (Ca, Pb, and Ba). We observed a spin freezing type effect in the Pb doped sample below 120 K in resistivity, ac susceptibility and in magnetization. This suggests that the AFM interactions introduced by the Fe are most effective in the Pb doped composition leading to increased competition between the FM and AFM interactions. This FM and AFM interaction generates some degree of frustration leading to the appearance of spin glass like phase whose typical magnetic behavior is studied for small ion when the metallic like behavior is lost. 相似文献
994.
为了实时探测离子土固化剂(ISS)在固土过程中的阳离子交换量(CEC),采用激光击穿光谱技术(LIBS),对ISS处理后的土壤溶液中的K、Ca、Na、Mg、Al、Si这6种离子含量进行探测。分别对由不同配比(1∶100、1∶200和1∶300)的ISS处理过的高岭土、膨胀土、红粘土等5种标准土壤进行了研究,结果表明:同一土壤中的各种离子随ISS配比的变化趋势各不相同;不同土壤与ISS作用的效果各不相同;ISS的配比并非越高越好。这些工作为ISS固化机理的研究提供实验依据,而利用激光击穿光谱技术进行探测的方式为ISS固化机理的研究提供了新的思路。 相似文献
995.
Nilgün YenerCengiz Biçer Mü?erref Önal Yüksel Sar?kaya 《Applied Surface Science》2012,258(7):2534-2539
To distinguish the ion exchanged and physically adsorbed methylene blue cations (MB+) on ionic surfaces, acid activated bentonite samples were used as porous adsorbents. A natural calcium bentonite (CaB) sample from Enez/Edirne, Turkey, was acid activated at 90 °C for 16 h with various HCl/CaB ratios. The irreversible exchange and physical adsorption of MB+ cations on the ionic solids have simultaneously occurred. The ion exchanged (mex) and physically adsorbed (mad) MB+ contents were obtained as the values of sorption capacity at c = 0 and the increase to a plateaus of adsorption isotherms, respectively. The mad value was taken to be monolayer adsorption capacity. Cation exchange capacity (CEC) and specific surface area (SMB) for each sample were calculated from the mex and mad values, respectively. Also, the BET specific surface areas (SBET) and pore size distribution were determined from low temperature nitrogen adsorption/desorption data. A linear correlation between the SMB and SBET values was found. 相似文献
996.
质子交换膜中的传质分析 总被引:1,自引:0,他引:1
质子交换膜燃料电池是最有应用前景的汽车动力替代系统。质子交换膜中的传质是质子交换膜燃料电池性能的控制因素之一。论文从宏观和微观角度分析了质子交换膜中的质子和水分的传递机理,分析了操作参数对质子在质子交换膜中传递的影响。研究发现:外载荷对质子和水分在质子交换膜中的传递有很大影响;(H5O2)+是水合质子的主要结构形式;通过(H5O2)+中氢氧键不断形成与断裂,电荷在质子交换膜中得以传递。研究结果对理解质子交换膜中的传质机理及其推广应用具有积极意义。 相似文献
997.
Titanate nanotubes (TiNTs) were prepared by alkaline hydrothermal processing. The TiNTs are thermodynamically unstable and easily transformed to the titania phase by heat or acid treatment. These phase transformations are affected by the preparation conditions. In this study, we investigated the effects of using the washing process to modify the sodium content of the TiNTs. After an alkaline hydrothermal process was used to prepare the TiNTs, the resulting suspensions were washed with weak acid solution and distilled water until the pH value of the wash solution reached approximately 1 or 7, and the products were identified as H‐TiNTs or Na‐TiNTs, respectively. The characteristics and photocatalytic activities of the H‐TiNTs and Na‐TiNTs were compared for various calcination temperatures. The H‐TiNTs were transformed completely to anatase‐type TiO2 by dehydration during calcination, while the crystallinity of the Na‐TiNTs increased with calcination temperature. However, the photocatalytic H2 production rates on calcined H‐TiNTs were much higher than on Na‐TiNTs, which could be attributed to the crystalline anatase phase. 相似文献
998.
Some charmonium-like resonances such as X(3872) can be interpreted as possible D(*)D(*) molecular states. Within the quark model, we study the structure of such molecular states and the similar B(*)D(*) molecular states by taking into account the light meson exchange (π, η, ρ, ω and σ) between two light quarks from different mesons. 相似文献
999.
FePt/Fe perpendicular exchange-coupled bilayers with different Fe thicknesses were prepared to study the exchange coupling effect and the magnetization switching mechanism. An Fe thickness of 3 nm was found to be the critical point where the coercivity reduction became saturated and had the largest thermal stability gain factor of 2.25. This thickness was close to the exchange length between the magnetically hard and soft layers. Within the exchange length the soft phase strongly coupled to the hard phase and the magnetization of the bilayer processed single switching; beyond the exchange length reversible magnetization increased with the Fe thickness and exchange spring effect was found. Our simulation results also revealed that the exchange length was the critical Fe thickness for effective coercivity reduction and for maintaining high remanence. 相似文献
1000.
《Journal of Coordination Chemistry》2012,65(1-2):55-65
Abstract Potentiometric titration data obtained at 50° for aqueous solutions containing aquochromium(III) ions and adenine were analyzed to yield values for the acid dissociation constants of adenine and the formation constants of 1:1 and 1:2 mononuclear complexes. There is also evidence for a 2:2 dimer for which elemental analysis data suggest a structure of the form (HL=neutral adenine): [(H2 O)2 (HL)Cr-μ(OH)3 -Cr(HL)(H2 O)2]3+. The kinetic studies indicate that the 1:1 complex is formed in excess adenine with a half-time of about two hours at pH=4.4 and 60°. The proposed mechanism involves an outer-sphere associative intermediate of appreciable stability (KA ~ 200) followed by rate-determining substitution of adenine for ligand water to produce the 1:1 complex. A much slower subsequent reaction yields the dimer as the final stable product. The formation of the 1:1 complex is an equilibration and both forward and reverse rate constants were determined. The pH dependence data indicate that the reactive species in the system are HL, Cr(H2 O) 5OH2+, and Cr(H2 O)4 (OH)2 +. 相似文献