全文获取类型
收费全文 | 16587篇 |
免费 | 719篇 |
国内免费 | 871篇 |
专业分类
系统科学 | 1050篇 |
丛书文集 | 443篇 |
教育与普及 | 341篇 |
理论与方法论 | 57篇 |
现状及发展 | 74篇 |
研究方法 | 37篇 |
综合类 | 16173篇 |
自然研究 | 2篇 |
出版年
2024年 | 93篇 |
2023年 | 222篇 |
2022年 | 404篇 |
2021年 | 394篇 |
2020年 | 336篇 |
2019年 | 224篇 |
2018年 | 228篇 |
2017年 | 259篇 |
2016年 | 359篇 |
2015年 | 635篇 |
2014年 | 809篇 |
2013年 | 862篇 |
2012年 | 923篇 |
2011年 | 1017篇 |
2010年 | 974篇 |
2009年 | 999篇 |
2008年 | 1139篇 |
2007年 | 1069篇 |
2006年 | 959篇 |
2005年 | 831篇 |
2004年 | 591篇 |
2003年 | 587篇 |
2002年 | 723篇 |
2001年 | 684篇 |
2000年 | 535篇 |
1999年 | 452篇 |
1998年 | 255篇 |
1997年 | 271篇 |
1996年 | 228篇 |
1995年 | 195篇 |
1994年 | 211篇 |
1993年 | 127篇 |
1992年 | 123篇 |
1991年 | 116篇 |
1990年 | 97篇 |
1989年 | 81篇 |
1988年 | 67篇 |
1987年 | 44篇 |
1986年 | 26篇 |
1985年 | 10篇 |
1984年 | 5篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1971年 | 1篇 |
1967年 | 1篇 |
1965年 | 1篇 |
1964年 | 1篇 |
1958年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
961.
中国南方岩溶地区脆弱的生态环境及石漠化过程 总被引:9,自引:0,他引:9
中国南方岩溶地区分布有面积广泛的碳酸盐岩,发育多种岩溶地貌。由于该地区的特殊地质、水文、气候等自然因子的影响,其岩溶生态环境非常脆弱,环境容量低,敏感度高,抗干扰能力弱。当人类对森林进行破坏使森林的“生物泵”作用消失后,引发出一系列的生态问题:岩溶生态环境开始恶化,植被走向逆向演替等。最后出现大面积的石山,发生石漠化现象。可见,森林在岩溶生态系统中的作用是十分重要的,是影响岩溶生态系统的关键因子。 相似文献
962.
目前关于人类性别在数量上大致均等的一般解释难以自圆其说。作者依据从进化论思想推导出的配偶供需量均等原则,不仅很好地解释了这一“大致均等”的现象,而且揭示了更深剌的“精确均等”内涵。它告诉人们:男性比女性在数量上稍多一点的根本原因,是因为男性的平均寿命比女性稍短一点。文章不仅定性而且定量地对此现点进行了说明。 相似文献
963.
胡明 《重庆三峡学院学报》2003,19(5):71-76
当蒋介石发动反革命的"四·一二事件"以后,中共从1927年4月27日召开"五大"到8月7日召开"八七会议",其政策经过了一系列大幅度的戏剧性动荡和摇摆,并造成了革命的失败.虽然作为总书记的陈独秀负有不可推卸的责任,但是共产国际的横加干涉和左右动摇也是造成这一结局的重要原因. 相似文献
964.
对于一般情形下的Chwa & Hakimi模型,在不以“t-可诊断性”和“相信大多数”作前提假设的情况下,找到了求全体相容故障模式的具体方法——方程诊断算法,实践证明,诊断效果理想。 相似文献
965.
杨树不同杂交组合苗期性状遗传变异 总被引:4,自引:0,他引:4
对T120×S3107等8个杂交组合及各组合内无性系间的苗高、胸径、分枝情况等性状变异进行了研究。结果表明,组合间和无性系间在各性状上存在着显著的变异,苗高、胸径、侧枝的广义遗传力分别为77.75%,52.64%,56.96%。各杂交组合苗高和胸径平均值均超过对照T120和I 69。I 69×S3244,I 69×I 63组合的苗高和胸径性状表现都较好。因此,从这批材料中选出优良无性系是有可能的。 相似文献
966.
Suppose that a QTL affecting milk production trait or dairy cattle has been identified,and the relative distances between the QTL and the markers are hnown without error.The relative efficiency of pre-selection of young bulls before progeny-testing with marker-assisted BLUP(MBLUP) over conventional progeny-testing(i.e.random selection before progeny-testing)for five generations was studied using Monte Carlo simulation.Different numbers of QTL alleles(two or infinite),initial frequencies of the favorable QTL allele(0.1 and 0.5,when the QTL has two alleles),distances between the adjacent markers(5 cM or 20 cM),and ratios of the QTL variance to the genetic variance(10%,25%,and 50%)were considered in the simulation.The results showed that the relative efficiency of MBLUP was higher when the marker distance was 5 cM than that when the distance was 20 cM.The larger the QTL alleic effect,the lower the MBLUP efficiency.The relative superiority of MBLUP was higher when the QTL had two alleles and the frequency of the favorable QTL allele was low in the base population.The relative superiority of MBLUP decreased with th progress of generation in all cases and the decreasing rate was getting higher with the increase of marker density and QTL effect. 相似文献
967.
QIN Xiaomei HONG Ming ZHANG Gongmu MAI Wenjie LI Fengying YU Richeng LIU Jing & JIN Changqing . Institute of Physics Chinese Academy of Sciences Beijing China . Beijing High Pressure Research Center Beijing China . Institute of High Energy Physics Chinese Academy of Sciences Beijing China Correspondence should be addressed to Jin Changqing 《科学通报(英文版)》2003,48(12):1201-1203
Since the discovery of high temperature supercon-ductivity by Bednorz et al. in 1986[1], many series of cu- prate superconductors have been synthesized. Critical temperature has been improved continuously. But no verdict has been given upon what mechanism causes su- perconductivity. CaCuO2 conductive layers are the parent structure of the high Tc cuprate superconductors, such as Cu-12 (n-1)n[2], Tl-12(n-1)n[3] and Hg-12(n-1)n[4,5] super- conductor families. In 1988, infinite-layer (Ca0.86S… 相似文献
968.
LEI Ming YAO Baoli CHEN Yi HAN Yong WANG Yingli MENKE Neimule ZHENG Yuan WANG Congmin FAN Meigong & CHEN Guofu . State Key Laboratory of Transient Optics Technology Xi抋n Institute of Optics Precision Mechanics Chinese Academy of Sciences Xi抋n China . Technical Institute of Physics Chemistry Chinese Academy of Sciences Beijing China Correspondence should be addressed to Yao Baoli 《科学通报(英文版)》2003,48(15):1548-1550
The optical storage characteristics of a new kind of organic photochromic material棗pyrrylfulgide were experimentally investigated in the established parallel optical data storage system. Using the pyrrylfulgide/PMMA film as a photon-mode recording medium, micro-images and encoded binary digital data were recorded, readout and erased in this parallel system. The storage density currently reaches 3?107 bit/cm2. The recorded information on the film can be kept for years in darkness at room temperature. 相似文献
969.
YUZhengquan FANBaoliang DAIYunping ZHENGMing MINHuiling WANGMeili WANGLili FEIJing LINing 《科学通报(英文版)》2003,48(21):2331-2335
Human lysozyme is a 130-aa (amino acid) alkaline polypeptide, and has both anti-bacterial and anti-viral properties which make it an important component of human natural immunity system. As a first step toward the ultimate goal of improving the anti-bacterial properties of bovine and ovine milk, a transgenic mouse that contains the genomic DNA sequence of the human lysozme gene has been generated for the first time. From 83 mice generated by microinjection, a total of 6 positive transgenic mice were identified by PCR and Southern blot. F1 mice positive for transgene in lines were also detected by PCR. This shows that transgene could be transmitted from founder transgenic mice to their offspring. Recombinant human lysozyme (rHlys) was found in the whey of 3 female positive transgenic mice by Western blot. The highest concentration of rHlys for transgenic mice was 0.2mg/mL. The antibacterial activity of the whey for transgenic mice was highly enhanced up to 0.4 times as much as that of human, while that of non-transgenic mouse was very low. Although the lysozyme activity of transgenic mice is still lower than that of human, the rHlys exhibits the same specific activity as that of human lysozyme. It provides a strong basis for further studies into the possible application of rHlys express in mammary gland. 相似文献
970.