全文获取类型
收费全文 | 52944篇 |
免费 | 9916篇 |
国内免费 | 9563篇 |
专业分类
系统科学 | 1712篇 |
丛书文集 | 689篇 |
教育与普及 | 389篇 |
理论与方法论 | 131篇 |
现状及发展 | 428篇 |
研究方法 | 68篇 |
综合类 | 69003篇 |
自然研究 | 3篇 |
出版年
2024年 | 181篇 |
2023年 | 420篇 |
2022年 | 552篇 |
2021年 | 770篇 |
2020年 | 1258篇 |
2019年 | 3648篇 |
2018年 | 3784篇 |
2017年 | 4009篇 |
2016年 | 4178篇 |
2015年 | 4218篇 |
2014年 | 4486篇 |
2013年 | 3854篇 |
2012年 | 3426篇 |
2011年 | 3310篇 |
2010年 | 3396篇 |
2009年 | 2968篇 |
2008年 | 2454篇 |
2007年 | 2517篇 |
2006年 | 1810篇 |
2005年 | 1846篇 |
2004年 | 2119篇 |
2003年 | 1647篇 |
2002年 | 1522篇 |
2001年 | 1083篇 |
2000年 | 1019篇 |
1999年 | 1348篇 |
1998年 | 1216篇 |
1997年 | 1302篇 |
1996年 | 985篇 |
1995年 | 868篇 |
1994年 | 874篇 |
1993年 | 929篇 |
1992年 | 881篇 |
1991年 | 858篇 |
1990年 | 645篇 |
1989年 | 505篇 |
1988年 | 536篇 |
1987年 | 311篇 |
1986年 | 299篇 |
1985年 | 119篇 |
1984年 | 41篇 |
1983年 | 20篇 |
1982年 | 16篇 |
1977年 | 23篇 |
1976年 | 6篇 |
1972年 | 66篇 |
1970年 | 6篇 |
1969年 | 10篇 |
1968年 | 12篇 |
1967年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
在Gleeble-1500热模拟机上,采用等温压缩试验,研究了一种含Ti和Al的新型钴基耐热合金在850~1150℃温度范围的压缩变形行为.实验结果表明:该合金具有良好的抗高温流变性能,在850℃及应变速率0·0021~2·1s-1范围时其峰值流变应力可以达到360~475MPa.合金的流变行为可用Zener-Hollomon参数来描述. 相似文献
942.
Weighted geometric evaluation approach based on Projection pursuit (PP) model is presented in this paper to optimize the choice of schemes. By using PP model, the multi-dimension evaluation index values of schemes can be synthesized into projection value with one dimension. The scheme with a bigger projection value is much better, so the schemes sample can be an optimized choice according to the projection value of each scheme. The modeling of PP based on accelerating genetic algorithm can predigest the realized process of projection pursuit technique, can overcome the shortcomings of large computation amount and the difficulty of computer programming in traditional projection pursuit methods, and can give a new method for application of projection pursuit technique to optimize choice of schemes by using weighted geometric evaluation. The analysis of an applied sample shows that applying PP model driven directly by samples data to optimize choice of schemes is both simple and feasible, that its projection values are relatively decentralized and profit decision-making, that its applicability and maneuverability are high. It can avoid the shortcoming of subjective weighing method, and its results are scientific and objective. 相似文献
943.
A Project Risk Ranking Approach Based on Set Pair Analysis 总被引:1,自引:0,他引:1
Set Pair Analysis (SPA) is a new methodology to describe and process system uncertainty. It is different from stochastic or fuzzy methods in reasoning and operation, and it has been applied in many areas recently. In this paper, the application of SPA in risk ranking is presented, which includes review of risk ranking, introduction of Connecting Degree (CD) that is a key role in SPA., Arithmetic and Tendency Grade (TG) of CDs, and a risk ranking approach proposed. Finally a case analysis is presented to illustrate the reasonability of this approach. It is found that this approach is very convenient to operate, while the ranking result is more comprehensible. 相似文献
944.
朊圆酵母尿酸酶的基本特性研究 总被引:1,自引:0,他引:1
研究了尿酸酶分离纯化条件和pH、温度、底物浓度、一些有机和无机试剂对尿酸酶酶促反应速度的影响.研究结果表明,用Triton X-100及巯基乙醇处理细胞悬液(pH 8.5),再用超声波处理破碎细胞效果最佳.硫酸铵沉淀尿酸酶的最佳浓度为75%.用Phenyl-Sepharose CL-4B疏水层析和Xanthine-agarose亲和层析两步纯化步骤可获得理想纯度的酶制剂.在一定的条件下,尿酸酶的最适pH和酸碱稳定pH范围分别为8.5和5.0~12.最适温度为37℃;热稳定性较好,在50℃处理30 min时 相似文献
945.
WANG Jian ZHU Jinmao LIN Qinqin LI Xiaojuan TENG Nianjun LI Zhensheng LI Bin ZHANG Aimin LIN Jinxing 《科学通报(英文版)》2006,51(7):815-823
Stem is an important organ consisting of storage, transportation and mechanical tissues and is closely related to yield. There is an irregular cavity in the ce- real crop stems and the lower the location, the shorter and thicker the walls of the internode… 相似文献
946.
TONG Jinggui LI Shengrong LI Xiannhui LI Qiuli FANG Nianqiao CHU Fengyou CHEN Fukun 《科学通报(英文版)》2006,51(17):2141-2145
Information on palaeoocean and palaeoclimate can be recorded in foraminifera, and formation ages of the fo- raminifera are nevertheless essential for such study of pa- laeoenvironment. The growth rate of seamount ferroman- ganese crusts is normally very l… 相似文献
947.
CHEN Zhen YANG Juekuan ZHUANG Ping CHEN Minhua ZHU Jian CHEN Yunfei 《科学通报(英文版)》2006,51(23):2931-2936
The thermal conductivities of InGaAs/ InGaAsP superlattices with different period lengths were measured from 100 to 320 K using 3ω method. In this temperature range, the thermal conductivities were found to decrease with an increase in temperature. For the period length-dependant thermal conductivity, the minimum value does exist at a certain period length, which demonstrates that at a short period length, superlattice thermal conductivity increases with a decrease in the period length. When the period is longer than a certain period length, the interface thermal resistance dominates in phonon transport. The experimental and theoretical results confirmed the previous predictions from the lattice dynamics analysis, i.e. with the increase in period length, the dominant mechanisms of phonon transport in superlattices will shift from wave mode to particle mode. This is crucial for the cutoff of the phonons and lays a sound foundation for the design of superlattice structures. 相似文献
948.
Genetic analysis and molecular mapping of a presenescing leaf gene psll in rice (Oryza sativa L.) 总被引:6,自引:0,他引:6
WANG dun WU Shujun ZHOU Yong ZHOU Lihui XU Jiefen HU Jing FANG Yunxia GU Minghong LIANG Guohua 《科学通报(英文版)》2006,51(24):2986-2992
A rice psl1 (presenescing leaf) mutant was obtained from a japonica variety Zhonghua 11 via radiation of ^60Co-γ in M2 generation. Every leaf of the mutant began to wither after it reached the biggest length, while the leaves of the wild variety could keep green for 25--35 d. In this study, genetic analysis and gene mapping were carried out for the mutant identified. The SSR marker analysis showed that the mutant was controlled by a single recessive gene (psl1) located on chromosome 2. Fine mapping of the psl1 locus was conducted with 34 new STS markers developed around psl1 anchored region based on the sequence diversity between Nipponbare and 93-11. The psl1 was further mapped between two STS markers, STS2-19 and STS2-26, with genetic distances of 0.43 and 0.11 cM, respectively, while cosegregated with STS2-25. A BAC contig was found to span the psl1 locus, the region being delimited to 48 kb. This result was very useful for cloning of the psl1 gene. 相似文献
949.
FANG Yudong ZHANG Yongfeng LIN Lin LIAO Guangxuan HUANG Xin CONG Beihua 《科学通报(英文版)》2006,51(24):3053-3058
Interaction of water mist with cooking oil fires is studied experimentally and theoretically. A LDV/APV system is used to measure the velocity and diameter of water mist at different pressures in the experiments,and the effect of water mist velocity and diameter on fire extinguishment efficiency is investi-gated. The experimental results show that water mist has excellent surface cooling effect; it can control and extinguish cooking oil fires quickly without re-ignition. The critical temperature (Tfo) is calculated by energy balance equation,and the fire plume momentum is calculated and compared with that of water mist in order to determine the critical velocity (νwy) of fire extinguishment. This paper provides references for cooking oil fires extinguishment with water mist. 相似文献
950.
朱尧辰 《国外科技新书评介》2006,(11):1-1
本书是关于科学计算方法的导引性教材,讲述应用计算机解决各种数学问题所使用的数值方法。书中除了包含必要的理论性材料外,均使用MATLAB程序,并通过大量来自应用领域(如工程、电子、生物、化工等)的典型问题和数值例子及习题给出基本方法。 相似文献