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71.
Radial imaging techniques, such as projection-reconstruction (PR), are used in magnetic resonance imaging (MRI) for dynamic imaging, angiography, and short-T2 imaging. They are less sensitive to flow and motion artifacts, and support fast imaging with short echo times. However, aliasing and streaking artifacts are two main sources which degrade radial imaging quality. For a given fixed number of k-space projections, data distributions along radial and angular directions will influence the level of aliasing and streaking artifacts. Conventional radial k-space sampling trajectory introduces an aliasing artifact at the first principal ring of point spread function (PSF). In this paper, a shaking projection (SP) k-space sampling trajectory was proposed to reduce aliasing artifacts in MR images. SP sampling trajectory shifts the projection alternately along the k-space center, which separates k-space data in the azimuthal direction. Simulations based on conventional and SP sampling trajectories were compared with the same number projections. A significant reduction of aliasing artifacts was observed using the SP sampling trajectory. These two trajectories were also compared with different sampling frequencies. ASP trajectory has the same aliasing character when using half sampling frequency (or half data) for reconstruction. SNR comparisons with different white noise levels show that these two trajectories have the same SNR character. In conclusion, the SP trajectory can reduce the aliasing artifact without decreasing SNR and also provide a way for undersampling recon- struction. Furthermore, this method can be applied to three-dimensional (3D) hybrid or spherical radial k-space sampling for a more efficient reduction of aliasing artifacts. 相似文献
72.
Structural stability and electrical properties of AIB2-type MnB2 under high pressureStructural stability and electrical properties of AIB2-type MnB2 under high pressureStructural stability and electrical properties of AIB2-type MnB2 under high pressureStructural stability and electrical properties of AIB2-type MnB2 under high pressure 下载免费PDF全文
The structural stability and electrical properties of A1B2-type MnB2 were studied based on high pressure angle- dispersive x-ray diffraction, in situ electrical resistivity measured in a diamond anvil cell (DAC) and first-principles calcu- lations under high pressure. The x-ray diffraction results show that the structure of A1B2-type MnB2 remains stable up to 42.6 GPa. From the equation of state of MnB2, we obtained a bulk modulus value of 169.9~3.7 GPa with a fixed pressure derivative of 4, which indicates that A1B2-type MnB2 is a hard and incompressible material. The electrical resistance un- dergoes a transition at about 19.3 GPa, which can be explained by a transition of manganese 3d electrons from localization to delocalization under high pressure. 相似文献
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2006年9月下旬绍兴市优质课在上虞举行,课题是《方程的根与函数的零点》笔者有幸听了6位老师的精彩上课,颇有收获,多数教师在教学中体现了先进正确的教学理念:“以学生为本”、“问题引导学习”、“教与学重心前移”、“教会学生数学地思维”、“改进教学方式”、“促进学生主动,探究学习”等等.但由于准备时间仓促,每节课中总会或多或少有点遗憾,下面结合教学实例谈一下自己对几个问题粗浅的认识. 相似文献
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矩函数及其在旋成面叶栅气动反命题及杂交命题中的应用 总被引:2,自引:0,他引:2
本文首先对旋转的任意旋成面上的理想可压缩流动引入一新通用函数——矩函数,并论证它的主要性质,接着将它推广到粘性气体的情况。然后,提出一个基于矩函数的旋成面叶栅气动反命题及A型杂交命题的通用解法。研究表明,矩函数对求解杂交命题(反命题是其特例)和跨声速流动具有明显的优越性。 相似文献
79.
Feasibility of similarity coefficient map for improving morphological evaluation of T2* weighted MRI for renal cancer 下载免费PDF全文
The purpose of this paper is to investigate the feasibility of using a similarity coefficient map(SCM) in improving the morphological evaluation of T2* weighted(T2*W) magnatic resonance imaging(MRI) for renal cancer.Simulation studies and in vivo 12-echo T2*W experiments for renal cancers were performed for this purpose.The results of the first simulation study suggest that an SCM can reveal small structures which are hard to distinguish from the background tissue in T2*W images and the corresponding T2* map.The capability of improving the morphological evaluation is likely due to the improvement in the signal-to-noise ratio(SNR) and the carrier-to-noise ratio(CNR) by using the SCM technique.Compared with T2* W images,an SCM can improve the SNR by a factor ranging from 1.87 to 2.47.Compared with T2* maps,an SCM can improve the SNR by a factor ranging from 3.85 to 33.31.Compared with T2*W images,an SCM can improve the CNR by a factor ranging from 2.09 to 2.43.Compared with T2* maps,an SCM can improve the CNR by a factor ranging from 1.94 to 8.14.For a given noise level,the improvements of the SNR and the CNR depend mainly on the original SNRs and CNRs in T2*W images,respectively.In vivo experiments confirmed the results of the first simulation study.The results of the second simulation study suggest that more echoes are used to generate the SCM,and higher SNRs and CNRs can be achieved in SCMs.In conclusion,an SCM can provide improved morphological evaluation of T2*W MR images for renal cancer by unveiling fine structures which are ambiguous or invisible in the corresponding T2*W MR images and T2* maps.Furthermore,in practical applications,for a fixed total sampling time,one should increase the number of echoes as much as possible to achieve SCMs with better SNRs and CNRs. 相似文献
80.
以油溶性过氧化羟基异丙苯(CHP)为氧化剂,Al2O3为催化剂,引入超声作用,以正辛烷为模拟油品对油中二苯并噻吩的氧化进行了研究。考察了反应温度、反应时间、催化剂用量、氧硫比、超声功率对二苯并噻吩(DBT)降解率的影响并进行了正交实验,结果表明,各因素影响程度大小依次为:反应温度>催化剂用量>超声功率>反应时间>氧硫比,在反应温度为70℃,反应时间为45 min,氧硫比为5:1,催化剂用量为0.3 g,超声功率为60 W的最佳氧化条件下,DBT的降解率达到了88.0%。 相似文献