全文获取类型
收费全文 | 14270篇 |
免费 | 2405篇 |
国内免费 | 1599篇 |
专业分类
化学 | 9910篇 |
晶体学 | 129篇 |
力学 | 943篇 |
综合类 | 103篇 |
数学 | 1764篇 |
物理学 | 5425篇 |
出版年
2024年 | 49篇 |
2023年 | 297篇 |
2022年 | 526篇 |
2021年 | 554篇 |
2020年 | 586篇 |
2019年 | 522篇 |
2018年 | 501篇 |
2017年 | 444篇 |
2016年 | 639篇 |
2015年 | 694篇 |
2014年 | 786篇 |
2013年 | 1071篇 |
2012年 | 1225篇 |
2011年 | 1260篇 |
2010年 | 865篇 |
2009年 | 802篇 |
2008年 | 895篇 |
2007年 | 827篇 |
2006年 | 741篇 |
2005年 | 612篇 |
2004年 | 453篇 |
2003年 | 380篇 |
2002年 | 338篇 |
2001年 | 304篇 |
2000年 | 249篇 |
1999年 | 314篇 |
1998年 | 281篇 |
1997年 | 209篇 |
1996年 | 234篇 |
1995年 | 228篇 |
1994年 | 191篇 |
1993年 | 168篇 |
1992年 | 137篇 |
1991年 | 143篇 |
1990年 | 146篇 |
1989年 | 112篇 |
1988年 | 94篇 |
1987年 | 78篇 |
1986年 | 60篇 |
1985年 | 64篇 |
1984年 | 28篇 |
1983年 | 34篇 |
1982年 | 29篇 |
1981年 | 21篇 |
1980年 | 13篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1977年 | 10篇 |
1974年 | 6篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
We examine the ferroelectric-relaxor behavior of (Ba0.65Sr0.35)(Zr0.35Ti0.65)O3 (BSZT) ceramics in the temperature range from 80 to 380 K. A broad dielectric maximum, which shifts to higher temperature with increasing frequency, signifies the relaxor-type behavior of these ceramics. The value of the relaxation parameter γ∼2 estimated from the linear fit of the modified Curie-Weiss law, indicates the relaxor nature of the BSZT ceramics. The dielectric relaxation rate follows the Vogel-Fulcher relation with TVF=107 K, Ea=0.121 eV, and ν0=6.83×1014 Hz, further supports such relaxor nature. The slim P-E hysteresis loop and ‘butterfly’ shape dc bias field dependence of permittivity at T>Tm (Tm, the temperature of permittivity maximum) clearly signifies the occurrence of nanopolar clusters, which is the typical characteristic of ferroelectric relaxor. At 300 K and 10 kHz, the dielectric constant and loss tan δ are ∼1100 and 0.0015, respectively. The high tunability (∼25%) and figure of merit (∼130) at room temperature show that the BSZT ceramics could be a promising candidate for tunable capacitor applications. 相似文献
52.
完全Rees矩阵半群的分解及性质 总被引:1,自引:0,他引:1
用等价关系Q^~出了完全Rees矩阵半群的一种分解.而且得到了它的每个Q^~一类的表示. 相似文献
53.
54.
For any two points p and q in the Euclidean plane, define LUNpq = { v | v ∈ R2, dpv < dpq and dqv < dpq}, where duv is the Euclidean distance between two points u and v . Given a set of points V in the plane, let LUNpq(V) = V ∩ LUNpq. Toussaint defined the relative neighborhood graph of V, denoted by RNG(V) or simply RNG, to be the undirected graph with vertices V such that for each pair p,q ∈ V, (p,q) is an edge of RNG(V) if and only if LUNpq (V) = ?. The relative neighborhood graph has several applications in pattern recognition that have been studied by Toussaint. We shall generalize the idea of RNG to define the k-relative neighborhood graph of V, denoted by kRNG(V) or simply kRNG, to be the undirected graph with vertices V such that for each pair p,q ∈ V, (p,q) is an edge of kRNG(V) if and only if | LUNpq(V) | < k, for some fixed positive number k. It can be shown that the number of edges of a kRNG is less than O(kn). Also, a kRNG can be constructed in O(kn2) time. Let Ec = {epq| p ∈ V and q ∈ V}. Then Gc = (V,Ec) is a complete graph. For any subset F of Ec, define the maximum distance of F as maxepq∈Fdpq. A Euclidean bottleneck Hamiltonian cycle is a Hamiltonian cycle in graph Gc whose maximum distance is the minimum among all Hamiltonian cycles in graph Gc. We shall prove that there exists a Euclidean bottleneck Hamiltonian cycle which is a subgraph of 20RNG(V). Hence, 20RNGs are Hamiltonian. 相似文献
55.
Keith Vaughan Ronald J. LaFrance York Tang Donald L. Hooper 《Journal of heterocyclic chemistry》1991,28(7):1709-1713
3-Alkyl-4-arylazomethylene-3,4-dihydro-1,2,3-benzotriazines 5a and 5b undergo facile 1,4-addition of methanol or ethanol to afford stable crystalline hydrazones, 6 and 8 which have been characterised by spectroscopic analysis. In particular, the nmr spectrum of 8 shows novel features due to the diastereotopic nature of the CH2 protons in the two O-ethyl groups. Compound 5b shows a property unique to the compounds in this series; refluxing this arylazomethylenetriazine in ethanol affords a rearranged product, characterised as the 1-aryl-2-cinnolinylhydrazine ( 9 ). The formation of 9 is rationalised as a ring opening-ring closure process analogous to the Dimroth rearrangement. The cinnoline 9 displays some novel chemistry arising from the facility of the arylhydrazino substituent to react with acid to give a fragmentation product, 3-methylindazole ( 7 ). 相似文献
56.
57.
本文用多因子方差分析模型和多重比较分析了服装穿着时的主观热感觉与服装品种、环境条件、穿着者的活动状态与籍贯之间的关系。通过主成分回归建立了用生理指标预测主观热感觉的模型,从而得出一种定量地评价服装属性的方法。 相似文献
58.
本文描述了用膜吸收法测量激光等离子体辐射温度空间分布的原理和方法给出了柱形缝靶轴向辐射温度随空间位置变化的特征,对测量结果进行了分析讨论。 相似文献
59.
超软X射线流气式正比计数管 总被引:2,自引:0,他引:2
介绍了一种用于测量183~933eV超软X射线的圆柱形、侧窗式、流气式正比计数管,工作气体是0.11MPa的P-10气体或氦气-丙烷混合气体。计数管内径为φ25mm,直径为φ0.3mm的入射窗是由厚度80~90μgcm~2聚乙烯甲醛制成的。该计数管的特点:(1)薄窗,对软X射线透过率高。(2)流气式,工作寿命长。(3)能量分辨率好。(4)计数率高(1×10~(14)个/s)。(5)可测能区宽(0.183~10keV)。(6)可以方便更换窗膜材料、厚度及窗口直径。近几年来该计数管已经为高强度低能X光源提供较好监测。 相似文献
60.