全文获取类型
收费全文 | 3400篇 |
免费 | 341篇 |
国内免费 | 281篇 |
专业分类
化学 | 2555篇 |
晶体学 | 55篇 |
力学 | 151篇 |
综合类 | 15篇 |
数学 | 317篇 |
物理学 | 929篇 |
出版年
2023年 | 38篇 |
2022年 | 69篇 |
2021年 | 107篇 |
2020年 | 99篇 |
2019年 | 119篇 |
2018年 | 95篇 |
2017年 | 78篇 |
2016年 | 142篇 |
2015年 | 166篇 |
2014年 | 182篇 |
2013年 | 244篇 |
2012年 | 296篇 |
2011年 | 273篇 |
2010年 | 190篇 |
2009年 | 195篇 |
2008年 | 231篇 |
2007年 | 188篇 |
2006年 | 167篇 |
2005年 | 161篇 |
2004年 | 112篇 |
2003年 | 106篇 |
2002年 | 107篇 |
2001年 | 65篇 |
2000年 | 48篇 |
1999年 | 47篇 |
1998年 | 38篇 |
1997年 | 26篇 |
1996年 | 32篇 |
1995年 | 32篇 |
1994年 | 30篇 |
1993年 | 16篇 |
1992年 | 20篇 |
1991年 | 21篇 |
1990年 | 17篇 |
1989年 | 12篇 |
1987年 | 12篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 8篇 |
1982年 | 5篇 |
1980年 | 25篇 |
1978年 | 23篇 |
1977年 | 48篇 |
1976年 | 18篇 |
1975年 | 18篇 |
1974年 | 11篇 |
1973年 | 6篇 |
1970年 | 5篇 |
1968年 | 5篇 |
1943年 | 10篇 |
排序方式: 共有4022条查询结果,搜索用时 0 毫秒
51.
AlxGa1−xAs LPE growth was studied within the temperature range of 930–900°C with Al concentrations in solutions from 0.04 to 2.4 at.%. AlAs concentration in layers has been shown to grow with the cooling rate increase of solution. Interface and volume nucleation parameter dependence of Ki and Kv and formation time tf on Al concentration in Ga solution have been found. Addition of Al to Ga solution increases critical values of As supersaturation (supercooling) and, as a result, increase in thickness of AlxGa1−xAs layers compared with GaAs layers have been determined in spite of As concentration lowering in Ga solution. 相似文献
52.
Cellulase could not be selectively collected from fermentation broth by simple foam fractionation, because of the presence
of other more surface-active compounds. A new approach of affinity foam fractionation was investigated for improvement. A
hardwood hydrolysate (containing cellulose oligomers, substrates to cellulase) and two substrate analogs, i.e., carboxymethyl
cellulose (CMC) and xylan hydrolysate, were added before the foaming process. The substrates and substrate analogs were indeed
found to bind the cellulase selectively and form more hydrophobic complexes that partition more readily onto bubble surfaces.
In this study, the effects of the type and concentration of substrate/analog as well as the presence of cells at different
growth stages were examined. The foam fractionation properties evaluated included foaming speed, foam stability, foamate volume,
and enrichment of filter paper unit (FPU) and individual cellulase components (i.e., endoglucanases, exoglucanases, and β-glucosidases).
Depending on the broth and substrate/analog employed, the foamate FPU could be more than fourfold higher than the starting
broth FPU. Addition of substrate/analog also deterred the enrichment of other extracellular proteins, resulting in the desired
cellulase purification in the foamate. The value of E/P (enzyme activity-FPU/g/L of proteins) in the foamate reached as high
as 18, from a lactose-based fermentation broth with original E/P of 5.6. Among cellulase components, exoglucanases were enriched
the most and β-glucosidases the least. The study with CMC of different molecular weights (MW) and degrees of substitution
(DS) indicated that the CMC with low DS and high MW performed better in cellulase foam fractionation. 相似文献
53.
Tae Woo Kim Eun‐Jin Oh Ah‐Young Jee Seung Tae Lim Dae Hoon Park Minyung Lee Sang‐Hoon Hyun Prof. Jin‐Ho Choy Prof. Seong‐Ju Hwang Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(41):10752-10761
A colloidal suspension of exfoliated, layered cobalt oxide nanosheets has been synthesized through the intercalation of quaternary tetramethylammonium ions into protonated lithium cobalt oxide. According to atomic force microscopy, exfoliated nanosheets of layered cobalt oxide show a plateau‐like height profile with nanometer‐level height, underscoring the formation of unilamellar 2D nanosheets. The exfoliation of layered cobalt oxide was cross‐confirmed by X‐ray diffraction, UV/Vis spectroscopy, and transmission electron microscopy. The maintenance of the hexagonal in‐plane structure of the cobalt oxide lattice after the exfoliation process was evidenced by selected‐area electron diffraction and Co K‐edge X‐ray absorption near‐edge structure analysis. The zeta‐potential measurements clearly demonstrated the negative surface charge of cobalt oxide nanosheets. Adopting the nanosheets of layered cobalt oxide as a precursor, we were able to prepare the monodisperse CoO nanocrystals with a particle size of ≈10 nm as well as the heterolayered film composed of cobalt oxide monolayer and polycation. 相似文献
54.
Three basic cases of LPE growth with and without nucleation were examined. The expressions for layer thickness and growth efficiency calculations were developed. Criteria for LPE growth classification were obtained. It was shown the nucleation takes place at critical values of supersaturation of the solution. The values of critical supersaturation may be calculated from growth efficiency experimental data for LPE growth. 相似文献
55.
依据SMM-601型超导量子干涉显微镜检测部分构建计算模型,采用电磁场有限元方法分析了残余应力引起的非磁性金属材料铍电导率变化对超导量子干涉仪(SQUID)检测信号的影响,并讨论了该方法实际应用于非磁性金属材料残余应力检测的条件和可能性。 相似文献
56.
In this paper, we report two crystals Tm:YLF laser pumped by a fiber coupled diode laser with total power of 180 W at 792
nm. When the incident power reached up to 117.0 and 37.3 W of the emission wave-length at 1.91 μm was achieved. Through linear
fit, the corresponding slope efficiency of 38.5%, optical-to-optical conversion efficiency of 31.9% and the beam quality factor
of M
2 ∼ 1.1 were obtained, indicating nearly diffraction limited beam propagation. To our best knowledge, this is the best beam
quality among the reported paper results under the similar experiments. 相似文献
57.
The scanning two-photon fluorescence microscope produces optically sectioned images from the focal plane. It is sometimes desirable to acquire images from other planes of the specimen that are inclined with respect to the focal plane. In this Letter, we discuss the issues concerned with acquiring such images together with the effects of the inclination angle on image resolution and sectioning strength. To obtain images from oblique planes at high speed, a two-photon system was built wherein a novel optical system is used to provide aberration-free scanning. 相似文献
58.
An efficient 2 μm room temperature Ho:YVO(4) laser resonantly pumped by a 1.94 μm Tm:YAP laser is demonstrated, for the first time to our knowledge, in this Letter. Up to 8.58 W of laser output at 2053 nm is obtained with the optical-to-optical conversion efficiency as much as 41.2%. The output beam had quality of M2 factor with Mx(2) of 3.58 and My(2) of 1.76 at 8 W output level. In addition, we also obtained 4.18 W laser output at 2066 nm and 7.04 W at 2040 nm from the Ho:YVO(4) laser with conversion efficiency of 27.8% and 33.8%, respectively. 相似文献
59.
60.
Ju. I. Manin 《Publications Mathématiques de L'IHéS》1966,30(1):99-113
Résumé Letk be a perfect field of arbitrary characteristic. The main object of this paper is to establish some new objects associated
with algebraic surfaces F defined overk which are invariants for birational transformations defined overk. There are two main applications. The first is that if K is any extension ofk of degree 2, then there are infinitely many birationally inequivalent rational surfaces defined overk which all become birationally equivalent to the plane over K. The second application is to a partial classification of the
del Pezzo surfaces for birational equivalence overk. For our purposes a del Pezzo surface defined overk is a nonsingular rational surface with a very ample anticanonical system, so the nonsingular cubic surfaces are a special
care. As we use the language of schemes, we have to prove some classical results in the new framework, notably some results
of Enriques [7] on the classification of rational surfaces. In the last section we produce evidence for the conjecture that
if the fieldk is quasialgebraically closed (in the sense of Lang [11]), then a rational surface defined overk always has a point on it defined overk.
We shall now describe the contents of our paper in more detail.
相似文献