全文获取类型
收费全文 | 3508篇 |
免费 | 548篇 |
国内免费 | 511篇 |
专业分类
化学 | 2260篇 |
晶体学 | 78篇 |
力学 | 442篇 |
综合类 | 40篇 |
数学 | 278篇 |
物理学 | 1469篇 |
出版年
2024年 | 8篇 |
2023年 | 57篇 |
2022年 | 139篇 |
2021年 | 118篇 |
2020年 | 123篇 |
2019年 | 98篇 |
2018年 | 119篇 |
2017年 | 159篇 |
2016年 | 158篇 |
2015年 | 166篇 |
2014年 | 200篇 |
2013年 | 319篇 |
2012年 | 211篇 |
2011年 | 238篇 |
2010年 | 160篇 |
2009年 | 207篇 |
2008年 | 213篇 |
2007年 | 185篇 |
2006年 | 212篇 |
2005年 | 190篇 |
2004年 | 176篇 |
2003年 | 139篇 |
2002年 | 103篇 |
2001年 | 106篇 |
2000年 | 89篇 |
1999年 | 72篇 |
1998年 | 73篇 |
1997年 | 79篇 |
1996年 | 68篇 |
1995年 | 68篇 |
1994年 | 47篇 |
1993年 | 48篇 |
1992年 | 33篇 |
1991年 | 22篇 |
1990年 | 25篇 |
1989年 | 19篇 |
1988年 | 18篇 |
1987年 | 13篇 |
1986年 | 15篇 |
1985年 | 15篇 |
1984年 | 6篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1973年 | 2篇 |
1959年 | 1篇 |
1948年 | 1篇 |
排序方式: 共有4567条查询结果,搜索用时 15 毫秒
151.
建立基于光谱分析的轮台白杏叶片铁(Fe)、锰(Mn)元素浓度估算模型,为快速建立轮台白杏树体微量营养元素诊断体系提供技术途经。采用Unispec-SC光谱仪测定土壤肥力存在显著差异条件下轮台白杏果实不同生育时期叶片的光谱反射率,通过分析叶片中Fe和Mn元素浓度与Rλ和f′(Rλ)的相关性,筛选出光谱指示波段,并采用线性回归模型建立光谱估算模型以估算叶片Fe和Mn营养元素浓度。结果表明:轮台白杏果实不同生育时期叶片Fe元素的光谱敏感波段各不相同,果实坐果期和硬核期敏感波段分别为873和874nm,375和437nm。果实成熟期敏感波段为836和837nm而果实收后期敏感波段为325和1 054nm;轮台白杏果实四个生育时期Mn元素的光谱敏感波段分别为913和1 129nm,425和970nm,390和466nm,423和424nm;轮台白杏叶片Fe和Mn元素浓度均与光谱反射率一阶微分f′(Rλ)相关性最强,与之建立的线性光谱估算模型拟合度最高,且达到了显著或极显著水平。表明果实不同生育时期,轮台白杏叶片Fe和Mn元素的光谱指示波段不同,可根据双波段f′(Rλ)采用线性模型估算白杏叶片Fe和Mn元素浓度。 相似文献
152.
T. Szabová V. Hanušik A. Mitro Š. Bártha Š. Wirdzek 《Isotopes in environmental and health studies》2013,49(11-12):482-484
Für die Ermittlung der Transferkoeffizienten im System Boden–Pflanze wurden in drei Stellen der Tschechoslovakei Boden- und Pflanzenproben entnommen. Die niedrigsten Transferkoeffizienten für Getreide und für mehrjährige Futterpflanzen wurden in der Ostslovakei festgestellt. Die Werte der Transferkoeffizienten waren durch den Bodentyp, den Gesamt-Sorptionskapazität und dem Grad der Sorptionssättigung beeinfluβt. Soil and plant samples were sampled for the determination of soil to plant transfer coefficients in three various Czechoslovak sites. The lowesl transfer factors for cereals and fodder were determined in the site of Eastern Slovakia. Values of transfer factors depend on soil type, cation exchange capacity and base saturation of soils. 相似文献
153.
154.
实现均匀照度光伏聚光镜设计 总被引:2,自引:0,他引:2
为满足聚光光伏系统的聚光需求,解决传统点聚焦式聚光光伏系统中聚焦光斑不均匀、径长比过大和聚光比较小的缺点,在不增加二次匀光器件的前提下,设计了径长比小、聚焦光斑相对均匀、聚光比高的聚光光伏系统。根据几何光学柯勒照明原理、等光程原理和反射定律,通过数值求解等光程方程组获得聚光镜各个面型的轮廓曲线。利用TracePro软件对所设计的聚光系统进行光线追迹模拟,结果表明:在聚光比为725的情况下,聚焦光斑最大照度仅为太阳照度的2300倍,是点聚焦情况下的1/10左右,系统的径长比为0.3,接收角为0.72°。系统设计实现了结构紧凑,聚光性能高的设计目标,为高倍聚光光伏系统的小型化,简单化提供了一种有效的解决途径。 相似文献
155.
采用共沉淀法制备不同物质的量比Mn掺杂的铁基载氧体(Mn-Fe2O3),并进行XRD、BET和TEM表征。开展不同温度下Mn-Fe2O3与CO的化学链燃烧实验,研究载氧体的反应特性,确定较优的掺杂量和反应温度。结果表明,适量的Mn掺杂有助于改善铁基载氧体的反应活性,Fe∶Mn物质的量为50∶1时燃烧反应转化率最高。多循环化学链燃烧实验证实了载氧体稳定性较好。不同升温速率(30、40、50℃/min)下反应动力学分析表明,Mn-Fe2O3与CO的化学链燃烧还原反应均属于随机成核和随后生长的Avrami-Erofeev方程模型,并依据模型分别计算出了该模型的活化能和频率因子。 相似文献
156.
157.
Experiment on relationship between the magnetic gradient of low-carbon steel and its stress 总被引:1,自引:0,他引:1
Xingliang Jian Xingchao Jian Guoyong Deng 《Journal of magnetism and magnetic materials》2009,321(21):3600-3606
In geomagnetic field, a series of tensile experiments on the low-carbon steel sticks were carried out. A special homemade detector was used to measure the magnetic gradient on the material surface. The results showed that the relationship between the magnetic gradient and the stress varied with different conditions of measurement. There was no obvious correlation between the magnetic gradient and the tensile stress if the sample remained on the material test machine. If the sample was taken off from the machine, the measured magnetic gradient was linear with the prior maximum stress. In Nanjing, PR China, a place of 32°N latitude, the slope of the linear relationship was about 67 (uT/m)/MPa. This offered a new method of non-destructive stress testing by measuring the magnetic gradient on the ferromagnetic component surface. The prior maximum applied stress of the sample could be tested by measuring the present surface magnetic gradient. Actually this phenomenon was the metal magnetic memory (MMM). The magnetic gradient near the stress concentration zone of the sample, the necking point, was much larger than other area. Thus, the hidden damage in the ferromagnetic component could be detected early by measuring the magnetic gradient distribution on its surface. In addition, the magnetic memory signal gradually weakened as the sample was taken off and laid aside. Therefore, it was effective for a given period of time to detect the stress or stress concentration based on the MMM testing. 相似文献
158.
An analysis is performed to study the free convection of a dusty‐gas flow along a semi‐infinite isothermal vertical cylinder. The governing equations of the flow problem are transformed into non‐dimensional form and the resulting nonlinear, coupled parabolic partial differential equations have been solved numerically using an implicit finite difference scheme of Crank–Nicholson type. The flow variables such as gas–velocity, dust‐particle velocity and temperature, shearing stress and heat transfer coefficients are calculated numerically for various parameters occurring in the problem. It is observed that due to the presence of dust particles, the gas velocity is found to decrease. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
159.
Liyong Chen Mingzu Zhang Zhuang Liu Zixu Gu Yingfeng Tu Peihong Ni 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(11):862-871
A biocompatible complex has been prepared as gene carrier via electrostatic interaction, which is composed of a polycation, that is, poly[(dimethylamino)ethyl methacrylate] end-capped with cholesterol moiety (Chol-PDMAEMA30), along with a polyanion named poly(aspartic acid)-grafted-poly(ethylene glycol) (PASP-g-PEG). The complexes have less cytotoxicity compared to the case of alone Chol-PDMAEMA30 or branched polyethylenimine (PEI) system. In the present study, biocompatible complexes have been prepared as gene carrier via electrostatic interaction, which is composed of a polycation, that is, poly[(dimethylamino)ethyl methacrylate] end-capped with cholesterol moiety (Chol-PDMAEMA30), along with a polyanion named poly(aspartic acid)-grafted-poly(ethylene glycol) (PASP-g-PEG). We first synthesized polysuccinimide (PSI) via condensation polymerization of aspartic acid, and then used PEG-NH2 to react with the partial pentacyclic rings of PSI to yield a kind of graft copolymer polysuccinimide-grafted-poly(ethylene glycol) (PSI-g-PEG). After hydrolysis of the residual succinimide units, a new biodegradable and biocompatible graft copolymer PASP-g-PEG was prepared successfully. Chol-PDMAEMA30 was synthesized via oxyanion-initiated polymerization, as reported in our previous literature. We investigated the interactions between every pair among calf thymus DNA, Chol-PDMAEMA30, and PASP-g-PEG by agarose gel retardation assay. The results indicate that the prepared complexes could completely bind DNA and may become more stable during systemic circulation. The complexes have less cytotoxicity compared to the case of alone Chol-PDMAEMA30 or branched polyethylenimine (PEI) system. Furthermore, the physicochemical properties of the complexes were also investigated by zeta potential, transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements. These biodegradable and biocompatible polymeric carriers have potential applications in gene delivery. 相似文献
160.
《高分子科学杂志,A辑:纯化学与应用化学》2013,50(4):351-363
ABSTRACT The influence of the presence of an inorganic carrier in the polymerization systems was investigated in this paper. Ten polymerization systems that contained different inorganic carriers such as Al2O3, SiO2 were studied. The ten polymerization systems are dibutyltin Maleate (DBTM) and allyl thiourea (AT), DBTM and stearic acid vinyl ester (SAVE), SAVE and maleic anhydride (MH), SAVE and acrylamide (AM), itaconic acid (IA) and AM, SAVE and IA, AM and MH, IA, sodium acrylate (AANa), AM. It was found that the activity of monomer can be changed in the presence of a carrier and furthermore, the different species of carrier influence the activity of same monomer differently in the polymerization. 相似文献