全文获取类型
收费全文 | 18108篇 |
免费 | 2332篇 |
国内免费 | 2874篇 |
专业分类
化学 | 14226篇 |
晶体学 | 282篇 |
力学 | 755篇 |
综合类 | 264篇 |
数学 | 1967篇 |
物理学 | 5820篇 |
出版年
2024年 | 32篇 |
2023年 | 170篇 |
2022年 | 414篇 |
2021年 | 459篇 |
2020年 | 484篇 |
2019年 | 543篇 |
2018年 | 454篇 |
2017年 | 416篇 |
2016年 | 708篇 |
2015年 | 773篇 |
2014年 | 928篇 |
2013年 | 1313篇 |
2012年 | 1468篇 |
2011年 | 1512篇 |
2010年 | 1161篇 |
2009年 | 1125篇 |
2008年 | 1384篇 |
2007年 | 1274篇 |
2006年 | 1161篇 |
2005年 | 973篇 |
2004年 | 915篇 |
2003年 | 827篇 |
2002年 | 925篇 |
2001年 | 742篇 |
2000年 | 570篇 |
1999年 | 441篇 |
1998年 | 277篇 |
1997年 | 285篇 |
1996年 | 235篇 |
1995年 | 209篇 |
1994年 | 185篇 |
1993年 | 174篇 |
1992年 | 158篇 |
1991年 | 113篇 |
1990年 | 104篇 |
1989年 | 87篇 |
1988年 | 50篇 |
1987年 | 52篇 |
1986年 | 41篇 |
1985年 | 36篇 |
1984年 | 26篇 |
1983年 | 18篇 |
1982年 | 12篇 |
1981年 | 14篇 |
1980年 | 10篇 |
1978年 | 11篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1974年 | 5篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
131.
采用固相反应法制备Ca0.88TiO3:0.12Eu3+(CTE)红色荧光粉,研究了CTE荧光粉的结构与发光性能。XRD结果表明,不同退火温度下的CTE荧光粉皆为钙钛矿结构,其晶粒尺寸随退火温度的升高而增大,1 300℃退火时晶相最佳,与SEM观察的结果相一致。CTE荧光粉的激发光谱由350~500 nm范围内的一系列窄带吸收峰组成,其中的最强峰位于398 nm 附近;发射谱主要包含595 nm和616 nm两个峰,属于Eu3+离子跃迁发光。616 nm发射峰明显强于595 nm发射峰,说明Eu3+是处在无反演对称中心的格位。CTE 荧光粉的发光随着退火温度的升高而增强,1 300℃时达到最大值,这可归因于其结晶状况的改善。另外,CTE荧光粉还具有色纯度高与热稳定性好等优点,这些将使CTE成为一种潜在的用于近紫外激发的白光LED红色荧光粉材料。 相似文献
132.
表面增强拉曼光谱技术是近年来快速发展的一种痕量特征标记性物质检测技术, 达到了分子识别水平, 具有很高的灵敏度, 能够精确地分析和了解物质结构和成分。人体唾液中含有大量体征相关的特异性蛋白质和分泌物质。唾液检测具有无创、获取方便、适合开展大规模普查等优点。表面增强拉曼光谱检测技术可以快速地对唾液进行检测, 二者的结合, 有可能成为一种癌症诊断的新方法。本文利用表面增强光谱技术对肝癌患者进行实验研究, 通过对肝癌患者唾液的采集、样品的处理、光谱的采集及光谱分析, 肝癌特异性标记物AFP(alpha fetoprotein)的检测及分析等过程得到肝癌相关特征信息, 表明肝癌患者与正常人的唾液样本分类良好, 灵敏度及特异性较好; 进一步验证了AFP抗原的分子组成, 且AFP抗原的拉曼强度基本随浓度的递增呈递减趋势; 对肝癌患者的唾液样本及AFP抗原的拉曼光谱进行比对分析, 提示利用表面增强拉曼光谱检测唾液有可能诊断早期癌症。 相似文献
133.
基于Boltzmann光谱法的焊接电弧温度场测量计算 总被引:1,自引:0,他引:1
电弧等离子体是非均匀等离子体,其内部进行着复杂的能量和质量输运过程,等离子体的温度测量具有重要意义。Boltzmann作图法测量温度较谱线绝对强度法、标准温度法等测温方法更为方便。基于Boltzmann作图法原理,对TIG电弧进行实时的空间扫描,分析了谱线的选取原则,测量计算出TIG焊电弧等离子体的温度场分布。 相似文献
134.
Spinel lithium titanate (Li4Ti5O12) as novel anode material for room-temperature sodium-ion battery 下载免费PDF全文
This is the first time that a novel anode material, spinel Li4Ti5O12 which is well known as a “zero-strain” anode material for lithium storage, has been introduced for sodium-ion battery. The Li4Ti5O12 shows an average Na storage voltage of about 1.0 V and a reversible capacity of about 145 mAh/g, thereby making it a promising anode for sodium-ion battery. Ex-situ X-ray diffraction (XRD) is used to investigate the structure change in the Na insertion/deinsertion process. Based on this, a possible Na storage mechanism is proposed. 相似文献
135.
运用分子动力学方法对比模拟研究了碳化硅的体熔化、表面熔化和晶体生长过程.分别采用MEAM 势和Tersoff势两种势函数描述碳化硅.结果表明:体熔化时,两种势函数描述的SiC的原子平均能量、 Lindemann指数和结构有序参数与温度的变化关系相似,但MEAM势对应的体熔点(4250 K)比Tersoff势(4750 K) 的要高.表面熔化时,两种势函数描述的SiC在相同的过热度下熔化速度相近;而在相同的温度条件下,MEAM 作用的SiC表面熔化速度更快.这是由于MEAM势SiC的热力学熔点(3338 K)低于Tersoff势SiC的热力学熔点 (3430 K)的缘故.两种势函数作用的SiC在晶体生长方面差异很大.MEAM势SiC的晶体生长速度与过冷度有关, 过冷度约为400 K时晶体生长速度最快.但Tersoff势SiC晶体却在过冷度为0—1000 K的范围内均不能生长. 综合考虑,MEAM势比Tersoff势能更好地描述碳化硅的熔化和凝固行为. 相似文献
136.
Xuewen Wu Dalian Ding Haiyan Jiang Xiaowei Xing Suping Huang Hong Liu Zhedong Chen Hong Sun 《Journal of nanoparticle research》2012,14(1):708-13
Hydroxyapatite nanoparticles (nHAT) are known to have excellent biocompatibility, and have attracted increasing attention
as new candidates of non-viral vectors for gene therapy. In our previous studies, nHAT carrying a therapeutic gene and a reporter
gene were successfully transfected into the spiral ganglion neurons in the inner ear of guinea pigs in vivo as well as in
the cultured cell lines, although the transfection efficiencies were never higher than 30%. In this study, the surface modification
of nHAT with polyethylenimine (PEI) was made (PEI–nHAT, diameter = 73.09 ± 27.32 nm) and a recombinant plasmid carrying enhanced
green fluorescent protein (EGFP) gene and neurotrophin-3 (NT-3) gene was constructed as pEGFPC2–NT3. The PEI modified nHAT
and the recombinant plasmid was then connected to form the nHAT-based vector–gene complex (PEI–nHAT–pEGFPC2–NT3). This complex
was then placed onto the intact round window membranes of the chinchillas for inner ear transfection. Auditory brainstem response
(ABR) was tested to evaluate auditory function. Green fluorescence of EGFP was observed using confocal microscopy 48 h after
administering vector–gene complexes. There was no significant threshold shift in tone burst-evoked ABR at any tested frequency.
Abundant, condensed green fluorescence was found in dark cells on both sides of the crista and around the macula of the utricle.
Scattered EGFP signals were also detected in vestibular hair cells, some Schwann cells in the cochlear spiral ganglion region,
some outer pillar cells in the organ of Corti, and a few cells in the stria vascularis. The density of green fluorescence-marked
cells was obviously higher in the vestibular dark cell area than in other areas of the inner ear, suggesting that vestibular
dark cells may have the ability to actively engulf the nHAT-based vector–gene complexes. Considering the high transfection
efficiency in the vestibular system, PEI–nHAT may be a potential vector for gene therapy of inner ear diseases, especially
vestibular disorders, and deserves further study. 相似文献
137.
138.
139.
针对跟踪目标尺度变化问题,提出了基于灰度对数似然图像分割的快速主动轮廓跟踪算法。改进的主动轮廓跟踪算法将根据以目标与背景的颜色差异而建立的对数似然图对图像进行阈值分割和数学形态学处理,再将Kalman滤波器结合到主动轮廓跟踪算法进行目标跟踪。改进的主动轮廓跟踪算法对目标分割准确,轮廓特征显著,跟踪效果稳定,算法能很好地适应跟踪目标尺度变化。通过Kalman滤波器对目标位置点的预测减少了主动轮廓跟踪算法收敛的迭代次数,使算法的运算效率提高了33%左右。 相似文献
140.
Due to its enormously high flexibility laser forming has been gaining importance in recent years. This rapidness and flexibility demand very precise controlling strategies especially when simulating the process of large plates and challenging the limited computation power of the current workstation. A simple, robust and accurate modeling method of laser forming has been demonstrated to solve this problem. The simplified model is meshed by multi-layered shell element, simulated with a more real scanning method and fewer parameters. The intelligent meshing strategies have reduced the number of elements dramatically. Thus the simulation efficiency has been improved significantly. By comparing the simulation results under the simplified model with the results under the traditional model for laser forming, the applicability of proposed method has been proven. The method of these simplified models is also suitable to simulate complex finite element models, which take much time to simulate. It would throw some light on the thermal mechanically coupled-field simulation of large sheet. 相似文献