全文获取类型
收费全文 | 7452篇 |
免费 | 1029篇 |
国内免费 | 758篇 |
专业分类
化学 | 2477篇 |
晶体学 | 111篇 |
力学 | 1039篇 |
综合类 | 191篇 |
数学 | 2541篇 |
物理学 | 2880篇 |
出版年
2024年 | 6篇 |
2023年 | 69篇 |
2022年 | 147篇 |
2021年 | 231篇 |
2020年 | 252篇 |
2019年 | 184篇 |
2018年 | 195篇 |
2017年 | 303篇 |
2016年 | 316篇 |
2015年 | 249篇 |
2014年 | 419篇 |
2013年 | 638篇 |
2012年 | 411篇 |
2011年 | 471篇 |
2010年 | 386篇 |
2009年 | 498篇 |
2008年 | 519篇 |
2007年 | 474篇 |
2006年 | 450篇 |
2005年 | 393篇 |
2004年 | 308篇 |
2003年 | 307篇 |
2002年 | 251篇 |
2001年 | 241篇 |
2000年 | 212篇 |
1999年 | 179篇 |
1998年 | 152篇 |
1997年 | 148篇 |
1996年 | 129篇 |
1995年 | 142篇 |
1994年 | 78篇 |
1993年 | 73篇 |
1992年 | 60篇 |
1991年 | 43篇 |
1990年 | 54篇 |
1989年 | 42篇 |
1988年 | 39篇 |
1987年 | 22篇 |
1986年 | 21篇 |
1985年 | 29篇 |
1984年 | 28篇 |
1983年 | 5篇 |
1982年 | 22篇 |
1981年 | 16篇 |
1979年 | 10篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有9239条查询结果,搜索用时 30 毫秒
91.
Stephan Wasielewski Eduard Rott Ralf Minke Heidrun Steinmetz 《Molecules (Basel, Switzerland)》2021,26(1)
Sludge water (SW) arising from the dewatering of anaerobic digested sludge causes high back loads of ammonium, leading to high stress (inhibition of the activity of microorganisms by an oversupply of nitrogen compounds (substrate inhibition)) for wastewater treatment plants (WWTP). On the other hand, ammonium is a valuable resource to substitute ammonia from the energy intensive Haber-Bosch process for fertilizer production. Within this work, it was investigated to what extent and under which conditions Carpathian clinoptilolite powder (CCP 20) can be used to remove ammonium from SW and to recover it. Two different SW, originating from municipal WWTPs were investigated (SW1: c0 = 967 mg/L NH4-N, municipal wastewater; SW2: c0 = 718–927 mg/L NH4-N, large industrial wastewater share). The highest loading was achieved at 307 K with 16.1 mg/g (SW1) and 15.3 mg/g (SW2) at 295 K. Kinetic studies with different specific dosages (0.05 gCLI/mgNH4-N), temperatures (283–307 K) and pre-loaded CCP 20 (0–11.4 mg/g) were conducted. At a higher temperature a higher load was achieved. Already after 30 min contact time, regardless of the sludge water, a high load up to 7.15 mg/g at 307 K was reached, achieving equilibrium after 120 min. Pre-loaded sorbent could be further loaded with ammonium when it was recontacted with the SW. 相似文献
92.
Muhammad Saad Khan Cornelius Borecho Bavoh Khor Siak Foo Azmi Mohd Shariff Zamzila Kassim Nurzatil Aqmar Bt Othman Bhajan Lal Iqbal Ahmed Mohammad Azizur Rahman Sina Rezaei Gomari 《Molecules (Basel, Switzerland)》2021,26(2)
This study evaluates the kinetic hydrate inhibition (KHI) performance of four quaternary ammonium hydroxides (QAH) on mixed CH4 + CO2 hydrate systems. The studied QAHs are; tetraethylammonium hydroxide (TEAOH), tetrabutylammonium hydroxide (TBAOH), tetramethylammonium hydroxide (TMAOH), and tetrapropylammonium hydroxide (TPrAOH). The test was performed in a high-pressure hydrate reactor at temperatures of 274.0 K and 277.0 K, and a concentration of 1 wt.% using the isochoric cooling method. The kinetics results suggest that all the QAHs potentially delayed mixed CH4 + CO2 hydrates formation due to their steric hindrance abilities. The presence of QAHs reduced hydrate formation risk than the conventional hydrate inhibitor, PVP, at higher subcooling conditions. The findings indicate that increasing QAHs alkyl chain lengths increase their kinetic hydrate inhibition efficacies due to better surface adsorption abilities. QAHs with longer chain lengths have lesser amounts of solute particles to prevent hydrate formation. The outcomes of this study contribute significantly to current efforts to control gas hydrate formation in offshore petroleum pipelines. 相似文献
93.
Rate Coefficients and Kinetic Isotope Effects of Cl+XCl→XCl+Cl (X=H,D, Mu) Reactions from Ring Polymer Molecular Dynamics 下载免费PDF全文
The ring-polymer molecular dynamics (RPMD) was used to calculate the thermal rate coefficients and kinetic isotope effects of the heavy-light-heavy abstract reaction Cl+XCl\begin{document}$ \rightarrow $\end{document} XCl+Cl (X = H, D, Mu). For the Cl+HCl reaction, the excellent agreement between the RPMD and experimental values provides a strong proof for the accuracy of the RPMD theory. And the RPMD results are also consistent with results from other theoretical methods including improved-canonical-variational-theory and quantum dynamics. The most novel finding is that there is a double peak in Cl+MuCl reaction near the transition state, leaving a free energy well. It comes from the mode softening of the reaction system at the peak of the potential energy surface. Such an explicit free energy well suggests strongly there is an observable resonance. And for the Cl+DCl reaction, the RPMD rate coefficient again gives very accurate results compared with experimental values. The only exception is at the temperature of 312.5 K, results from RPMD and all other theoretical methods are close to each other but slightly lower than the experimental value, which indicates experimental or potential energy surface deficiency. 相似文献
94.
本文选取了子宫内膜癌患者108例作为观察组,另选取子宫内膜良性病变患者105例作为对照组,均行DCE-MRI检查后发现,观察组容量转移常数(K trans)、速率常数(K ep)、血管外细胞外间隙容积比值(V e)高于对照组(P<0.05);K trans、K ep、V e值联合诊断子宫内膜癌的曲线下面积(AUC)为0.841;K trans、K ep、V e值与临床分期、肌层浸润程度呈正相关,与分化程度呈负相关(P<0.05)。本研究结果提示DCE-MRI定量参数对子宫内膜癌具有一定鉴别诊断价值,有助于进一步区分子宫内膜癌患者肌层浸润程度、临床分期、分化程度。 相似文献
95.
This work duly investigates the recovery of Pd (II) chlorocomplexes from industrial wastewater. Chemical structures and thermodynamic stabilities of the complex formed are evaluated via density functional theory (DFT). By applying synergistic solutions of thiourea mixed with hydrochloric acid (HCl), the stripping reaction of Pd (II) in the loaded Aliquat 336 occurs and Pd (II) chlorocomplexes coordinated thiourea ligands are formed, thus 80.19% of Pd (II) chlorocomplexes can be recovered. The aim of this study is to gain a better understanding of the stripping mechanisms and the structure of the complexes formed via the synergistic system. Such an understanding is still limited since little research has been conducted in this field. Owing to their molecular geometry, ligand coordination and donor groups play a vital role in the reactivity of palladium (II) complex. Quantum models have been developed to evaluate the chemical structure and thermodynamics stability of ((NH2)2CS·PdCl2) namely: (i) DFT with B3LYP/6-31g(d,p) and MP2/6-31g(d,p) basis set, (ii) MP2 with cc-pVTZ basis set and (iii) CCSD(T)/cc-pVTZ. Results demonstrate that the highest geometric stability exhibited is the structure of Pd-S bonding with 180° Cl-Pd-Cl. The distance (r) and angle (a) of the selected geometrical parameters for (NH2)2CS·PdCl2 complex are reported. Additionally, FTIR and UV–vis spectroscopies have been conducted to analyze the sampling solutions. Further, the calculated vibrational frequencies and experimental spectroscopic results show good agreement with the optimized geometry. 相似文献
96.
《印度化学会志》2021,98(10):100171
The higher wear resistance of Ni based nano composite coatings makes them potential replacement in protecting the substrate materials. The role of surface roughness of the coating along with wear parameters on the specific wear rate, pin temperature, and COF are addressed in the present study. The use of hard nano Al2O3 particles found significant role in increasing the resistance to wear for Ni matrix coatings on Al6061 material. The resistance to dislocation offered by these nano Al2O3 particles and smear out of debris with plastic deformation indicated abrasive and adhesive nature of wear mechanism in combination. The optimization of wear parameters are carried out by surface response method based grey relation analysis. The normal load applied onto the pin has significant influence on the specific wear rate and temperature rise in the pin. The surface roughness of the coating has also found instrumental in the higher pin temperature and friction coefficient. 相似文献
97.
Moment analysis method using partial filling CE was developed for the kinetic study on solute permeation at the interface of spherical molecular aggregates. Moment equations for partial filling CE were developed by classifying CE systems into five categories according to the migration velocities of solute and molecular aggregate. The method was applied to the study on the dissolution of electrically neutral solutes into SDS micelles. Elution peaks were measured by partial filling CE while changing the concentration of SDS and the filling ratio of SDS micellar zone to the capillary (ϕM). Partition equilibrium constants (Kp) and rate constants of interfacial solute permeation of SDS micelles (kin and kout) were determined from the first absolute and second central moments of the elution peaks by using the moment equations. Their values were comparable irrespective of ϕM and were almost the same as those previously measured by complete filling CE. The positive correlation of Kp with the hydrophobicity of the solutes was explained in terms of the change in kin and kout. It was demonstrated that the moment analysis method using partial filling CE is effective for studying solute permeation kinetics at the interface of spherical molecular aggregates. 相似文献
98.
Convergence Rate of Solutions to a Hyperbolic Equation with $p(x)$-Laplacian Operator and Non-Autonomous Damping 下载免费PDF全文
This paper concerns the convergence rate of solutions to a hyperbolic
equation with $p(x)$-Laplacian operator and non-autonomous damping. We apply the Faedo-Galerkin method to establish the existence of global solutions,
and then use some ideas from the study of second order dynamical system to
get the strong convergence relationship between the global solutions and the
steady solution. Some differential inequality arguments and a new Lyapunov
functional are proved to show the explicit convergence rate of the trajectories. 相似文献
99.
二氧化碳(CO2)是植物光合作用的原材料,也是一种温室气体,其过量地排放会影响动植物的生态环境。在碳达峰、碳中和的背景下,研制高灵敏度的CO2检测装置具有重要意义。为了监测大气环境中CO2含量的变化,设计了一种长光程共振式CO2气体光声传感器,并以此搭建了光声检测装置。以中心波长为2 004 nm的分布式反馈激光器(DFB)作为激发光源,激光射入由漫反射材料制成的球型吸收腔,在腔内多次反射以增加气体的吸收路径。吸收腔外部被两个高热传导率的铝制半球包裹,降低由池体吸收光能后产生的热噪声。吸收腔上耦合一根声学管,当其工作在一阶纵向共振模态时,光声信号被放大,在管子末端达到极大值。为了进一步增大光声信号,通过饱和加湿样品的方式来加快CO2气体的弛豫速率,加湿后的样品产生的光声信号是干燥样品的2.1倍左右。使用一系列浓度的湿润CO2样品标定光声检测装置,结果表明,光声信号与浓度之间呈现良好的线性关系。在此基础上,通过对标准气体的检测实验,验证了装置的准确性与稳定性... 相似文献
100.
油气混相过程的界面传质特性对气驱提高原油采收率技术非常重要。本文针对吉林某油田的实际油组分,采用分子动力学模拟研究了气驱油过程,分析了不同气体和驱替压力下油气两相的状态变化以及界面特性,获得不同驱替气体的最小混相压力(MMP)。结果表明,随着驱替气体压力的升高,气相的密度逐渐增大,油相膨胀密度降低,气相与油相的混合程度增强,油气两相界面厚度增加,界面张力随之减小。同时发现,驱替相中二氧化碳浓度越高,在同等气体压力下,油气界面更厚,油气混合程度更高。纯CO2驱油得到的MMP远远小于纯N2驱油,当这两种气体摩尔比为1 : 1混合时MMP介于两种纯气体之间,说明要达到同样的驱油效果二氧化碳需要的压力更小。最后,本文从分子微观作用力角度解释了驱替气体不同时影响油气混相程度的机制,通过分子平均作用势曲线发现油相分子对CO2的吸引力要大于N2分子,因此CO2分子更容易与油相混合,驱替效果更明显。 相似文献