全文获取类型
收费全文 | 84861篇 |
免费 | 1481篇 |
国内免费 | 1698篇 |
专业分类
系统科学 | 2326篇 |
丛书文集 | 1299篇 |
教育与普及 | 1045篇 |
理论与方法论 | 632篇 |
现状及发展 | 32967篇 |
研究方法 | 816篇 |
综合类 | 46766篇 |
自然研究 | 2189篇 |
出版年
2015年 | 940篇 |
2014年 | 1280篇 |
2013年 | 1631篇 |
2012年 | 1480篇 |
2011年 | 3617篇 |
2010年 | 1523篇 |
2009年 | 1689篇 |
2008年 | 2066篇 |
2007年 | 2157篇 |
2006年 | 1964篇 |
2005年 | 2198篇 |
2004年 | 2883篇 |
2003年 | 2443篇 |
2002年 | 2138篇 |
2001年 | 2166篇 |
2000年 | 1845篇 |
1999年 | 2533篇 |
1998年 | 2043篇 |
1997年 | 2234篇 |
1996年 | 2014篇 |
1995年 | 1619篇 |
1994年 | 1802篇 |
1993年 | 1667篇 |
1992年 | 1767篇 |
1991年 | 1541篇 |
1990年 | 1562篇 |
1989年 | 1336篇 |
1988年 | 1190篇 |
1987年 | 1034篇 |
1986年 | 939篇 |
1985年 | 973篇 |
1984年 | 792篇 |
1982年 | 802篇 |
1981年 | 850篇 |
1980年 | 929篇 |
1979年 | 1450篇 |
1978年 | 1307篇 |
1977年 | 1326篇 |
1976年 | 1126篇 |
1975年 | 1104篇 |
1974年 | 1071篇 |
1973年 | 1259篇 |
1972年 | 1308篇 |
1971年 | 1347篇 |
1970年 | 1444篇 |
1969年 | 1324篇 |
1968年 | 1237篇 |
1967年 | 1153篇 |
1966年 | 946篇 |
1965年 | 770篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
1 Results Pyrimidinones (PMs) are a class of important heterocycles which have been well documented throughout the literature due to their biological importance. They exhibit a wide range of pharmaceutical and therapeutic properties[1].A rapid and efficient one-pot method for the synthesis of 4,6-diarylpyrimidin-2(1H)- ones and related heterocycles is described.The condensation of acetophenone derivatives,aldehydes and urea in the presence of sulfamic acid was employed to synthesize a variety of pyrimid... 相似文献
52.
某库岸滑坡稳定分析及治理措施研究 总被引:1,自引:0,他引:1
从地质条件出发,详细分析了滑坡的成因,以大型的岩土工程数值分析软件(ADINA)为研究手段,建立有限元计算模型,用强度折减的方法分析了滑坡体在初始工况、蓄水工况、暴雨工况、水位骤降工况和地震工况下的稳定情况,根据滑坡的失稳情况,在综合考虑的基础上,提出了削坡减载和地表排水相结合的治理措施,计算结果表明,治理措施是可行的,研究结果对设计和施工具有较大的参考价值. 相似文献
53.
Kang J.Y. Xu F.C. 《中国科学基金(英文版)》2004,(2):57-58,64
1 Introduction
Nanoscale science and technology have attracted greatattention since the novel properties become dominant for well-known materials as their sizes reduced to some critical dimension. These properties frequently originate in lattice distortions, structure transformations etc[1]. The variations of size and structure ofnanocrystals lead to the change of their electronic structures. Very recently, nanoscale materials with different electronic structures were utilized in fabricating nanoscale junctions[2].
…… 相似文献
54.
汤燕瑜 《玉林师范学院学报》2004,25(2):111-114
高师院校肩负着培养基础教育师资的重任。衡量师资素质的重要标准之一,是教师职业素质与技能掌握、运用的程度。因而职业技能教育是高师教育中一个重要组成部分。本文通过对高师院校英语职业技能教育的反思,以及对基础教育发展需求分析,提出从可持续发展、教育的内在价值与创新教育的三个层面探讨高师英语职业技能教育改革。 相似文献
55.
In econometrics, as a rule, the same data set is used to select the model and, conditional on the selected model, to forecast. However, one typically reports the properties of the (conditional) forecast, ignoring the fact that its properties are affected by the model selection (pretesting). This is wrong, and in this paper we show that the error can be substantial. We obtain explicit expressions for this error. To illustrate the theory we consider a regression approach to stock market forecasting, and show that the standard predictions ignoring pretesting are much less robust than naive econometrics might suggest. We also propose a forecast procedure based on the ‘neutral Laplace estimator’, which leads to an improvement over standard model selection procedures. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
56.
育种目标性状间的同一关系分析方法研究 总被引:5,自引:0,他引:5
借助集对分析理论,提出了一种新的育种目标性状间的关系分析方法,阐明了其基本原理与分析步骤,并结合小麦育种实例,介绍了这种分析方法的具体应用。在此基础上,讨论了这种分析方法的优越性及尚待改进之处。表3,参9。 相似文献
57.
A fast and accurate method based on FTIR is developed for determining the lignin content and its characteristic bands in alkali solutions. The work consisted of two main parts: 1) establishing a predictive model and 2) determination of the accuracy of the model. The model was derived by applying the partial least squares algorithm for multivariate analyses to a set of spectrum data obtained from selected IR absorption regions in the range of 1600 - 900 cm-1. The model was tested with spent liquors obtained from commercial pulp mills.The accuracy of the predictive model is extremely high suitable for commercial applications,particularly for monitoring the dynamic response of modern kraft pulping processes. 相似文献
58.
This paper assesses the information content of two survey indicators for consumption developments in the near future for eight European countries in the period 1985–1998. Empirical work on this topic typically focuses on consumer confidence, the perceptions of buyers of consumption goods. This paper examines whether perceptions of sellers of consumption goods, measured by retail trade surveys, may also improve short‐term monitoring of consumption. We find that both consumer confidence and retailer confidence embody valuable information, when analysed in isolation. For France, Italy and Spain we conclude that adding retail confidence does not improve the indicator model once consumer confidence has been included. For the UK the reverse case is obtained. For the remaining four countries we show that combining consumer sentiment and retail trade confidence into a composite indicator leads to optimal results. Our results suggest that incorporating information from retail trade surveys may offer significant benefits for the analysis of short‐term prospects of consumption. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
59.
筒型基础系缆平台沉/拔过程侧摩阻力原型测试 总被引:7,自引:1,他引:6
介绍了1999年首次安装在渤海锦州9-3地区多筒基础平台安装施工过程中的侧摩阻力原型测试研究,描述了简型基础上沉和上拔过程中筒壁侧摩阻力的测试系统和测试方法,给出了主要测试结果并与现行规范进行了对比分析,这项成果对筒型基础平台的设计和施工具有直接的参考意义。 相似文献
60.
本文采用常规方法研究西藏6个分布于海拔4,240~4,850m 的高山草原土的放线菌数量,对从6个土样中分离纯化所得的454株放线菌中选出具有代表性的84株菌进行分离鉴定。结果表明:西藏同一土类不同土样中放线菌数量差异很大;84株代表性菌可分为9个属,链霉菌属(76.2%)占优势。 相似文献