全文获取类型
收费全文 | 36731篇 |
免费 | 3110篇 |
国内免费 | 2536篇 |
专业分类
化学 | 25091篇 |
晶体学 | 445篇 |
力学 | 1538篇 |
综合类 | 235篇 |
数学 | 4129篇 |
物理学 | 10939篇 |
出版年
2023年 | 295篇 |
2022年 | 360篇 |
2021年 | 908篇 |
2020年 | 853篇 |
2019年 | 923篇 |
2018年 | 829篇 |
2017年 | 804篇 |
2016年 | 1296篇 |
2015年 | 1250篇 |
2014年 | 1686篇 |
2013年 | 2448篇 |
2012年 | 2806篇 |
2011年 | 3173篇 |
2010年 | 2219篇 |
2009年 | 2116篇 |
2008年 | 2726篇 |
2007年 | 2426篇 |
2006年 | 2427篇 |
2005年 | 2075篇 |
2004年 | 1768篇 |
2003年 | 1451篇 |
2002年 | 1525篇 |
2001年 | 1026篇 |
2000年 | 882篇 |
1999年 | 615篇 |
1998年 | 402篇 |
1997年 | 318篇 |
1996年 | 347篇 |
1995年 | 261篇 |
1994年 | 264篇 |
1993年 | 255篇 |
1992年 | 201篇 |
1991年 | 166篇 |
1990年 | 148篇 |
1989年 | 135篇 |
1988年 | 97篇 |
1987年 | 90篇 |
1986年 | 62篇 |
1985年 | 89篇 |
1984年 | 56篇 |
1983年 | 45篇 |
1982年 | 65篇 |
1981年 | 61篇 |
1980年 | 38篇 |
1979年 | 49篇 |
1978年 | 56篇 |
1977年 | 36篇 |
1976年 | 44篇 |
1975年 | 38篇 |
1974年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
利用红外特征峰波数偏移值与单体含量间的非线性定量关系,BP人工神经网络-FTIR法在较宽的含量范围(10%-90%之间)之内,准确测定了苯乙烯-丙烯酸丁酯共聚物中丙烯酸丁酯单体含量,回收率在97.3%-101.8%之间. 相似文献
52.
53.
Homo-apioneplanocin A was efficiently synthesized via stereoselective hydroxymethylation, regio- and chemoselective hydroboration, and chemoselective oxidation as key steps from d-ribose. 相似文献
54.
Vinyl and diene derivatives of thiolactomycin have been prepared via Horner-Wadsworth-Emmons olefination from protected 5-formyl-3,5-dimethylthiotetronic acid. Several 4-position protecting groups and a variety of phosphonates were evaluated, with MOM protection and β-ketophosphonates yielding the highest ratio of the desired product to deformylated product. 相似文献
55.
Fiber-optic reflex sensor for in-line production measurement 总被引:1,自引:0,他引:1
The paper describes experiments concerning distance measurement with fiber-optic sensor and improvement of the measurement principle. The sensor probe was well-designed with a small structure and multi-function. The novel optical fiber probe arrangement as well as the possibilities of use for complex measurement problems are explained using the measurement of an internal screw thread as an example. The experimental results show that the measurement uncertainty of the thread minor diameter can reach ±10 μm, and the stability of the measurement system is better than 0.07%. 相似文献
56.
The fuzzification of convex sets in median algebras is considered, and some of their properties are investigated. A characterization of finite valued fuzzy convex set is given. 相似文献
57.
Let
d−1{(x1,…,xd)
d:x21+···+x2d=1} be the unit sphere of the d-dimensional Euclidean space
d. For r>0, we denote by Brp (1p∞) the class of functions f on
d−1 representable in the formwhere dσ(y) denotes the usual Lebesgue measure on
d−1,
and Pλk(t) is the ultraspherical polynomial.For 1p,q∞, the Kolmogorov N-width of Brp in Lq(
d−1) is given bythe left-most infimum being taken over all N-dimensional subspaces XN of Lq(
d−1).The main result in this paper is that for r2(d−1)2,where ANBN means that there exists a positive constant C, independent of N, such that C−1ANBNCAN.This extends the well-known Kashin theorem on the asymptotic order of the Kolmogorov widths of the Sobolev class of the periodic functions. 相似文献
58.
Yu-Soo Lee Hyun-Ju Chung Jong-Han Joung Eung-Jo Kim Hee-Je Kim 《Optics & Laser Technology》2004,36(1):461-61
We describe the pulse forming of pulsed CO2 laser using multi-pulse superposition technique. Various pulse shapes, high duty cycle pulse forming network (PFN) are constructed by time sequence. This study shows a technology that makes it possible to make various long pulse shapes by activating SCRs of three PFN modules consecutively at a desirable delay time with the aid of a PIC one-chip microprocessor. The power supply for this experiment consists of three PFN modules. Each PFN module uses a capacitor, a pulse forming inductor, a SCR, a high voltage pulse transformer, and a bridge rectifier on each transformer secondary. The PFN modules operate at low voltage by driving the primary of HV pulse transformer. The secondary of the transformer has a full-wave rectifier, which passes the pulse energy to the load in a continuous sequence.We investigated various long pulse shapes as different trigger time intervals of SCRs among three PFN modules. As a result, we could obtain laser beam with various pulse shapes and durations from about 250 to 1000 μs. 相似文献
59.
A (w,r) cover‐free family is a family of subsets of a finite set such that no intersection of w members of the family is covered by a union of r others. A (w,r) superimposed code is the incidence matrix of such a family. Such a family also arises in cryptography as the concept of key distribution pattern. In the present paper, we give some new results on superimposed codes. First we construct superimposed codes from super‐simple designs which give us results better than superimposed codes constructed by other known methods. Next we prove the uniqueness of the (1,2) superimposed code of size 9 × 12, the (2,2) superimposed code of size 14 × 8, and the (2,3) superimposed code of size 30 × 10. Finally, we improve numerical values of upper bounds for the asymptotic rate of some (w,r) superimposed codes. © 2004 Wiley Periodicals, Inc. 相似文献
60.