全文获取类型
收费全文 | 55893篇 |
免费 | 267篇 |
国内免费 | 549篇 |
专业分类
系统科学 | 1242篇 |
丛书文集 | 190篇 |
教育与普及 | 288篇 |
理论与方法论 | 521篇 |
现状及发展 | 31703篇 |
研究方法 | 863篇 |
综合类 | 19711篇 |
自然研究 | 2191篇 |
出版年
2013年 | 807篇 |
2012年 | 613篇 |
2011年 | 2648篇 |
2009年 | 615篇 |
2008年 | 826篇 |
2007年 | 935篇 |
2006年 | 1026篇 |
2005年 | 1180篇 |
2004年 | 2271篇 |
2003年 | 1931篇 |
2002年 | 1578篇 |
2001年 | 1510篇 |
2000年 | 1052篇 |
1999年 | 1020篇 |
1998年 | 646篇 |
1997年 | 776篇 |
1996年 | 533篇 |
1994年 | 687篇 |
1993年 | 693篇 |
1992年 | 945篇 |
1991年 | 805篇 |
1990年 | 899篇 |
1989年 | 690篇 |
1988年 | 649篇 |
1987年 | 684篇 |
1986年 | 758篇 |
1985年 | 896篇 |
1984年 | 784篇 |
1983年 | 686篇 |
1982年 | 808篇 |
1981年 | 829篇 |
1980年 | 904篇 |
1979年 | 1416篇 |
1978年 | 1251篇 |
1977年 | 1230篇 |
1976年 | 1117篇 |
1975年 | 1079篇 |
1974年 | 929篇 |
1973年 | 1178篇 |
1972年 | 1204篇 |
1971年 | 1241篇 |
1970年 | 1322篇 |
1969年 | 1203篇 |
1968年 | 1184篇 |
1967年 | 1049篇 |
1966年 | 888篇 |
1965年 | 737篇 |
1964年 | 485篇 |
1958年 | 526篇 |
1957年 | 435篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Chen Huang Xiaohua Geng Qinfei Ke Xiumei Mo Salem S. Al-Dey Mohamed El-Newehy 《自然科学进展(英文版)》2012,22(2):108-114
A novel type of composite vascular graft was developed via electrospinning in the present investigation.Collagen and chitosan were blended to form the inner and outer layer.Poly(1-lactide-co-caprolacto... 相似文献
982.
983.
新可逆PWM整流器主电路参数设计仿真 总被引:3,自引:0,他引:3
利用美国Analogy公司的SaberDesinger软件构造了三相可逆PWM整流器的试验平台,分析了同步模式下整流器的工作方法.通过仿真得到了三相可逆PWM整流器主电路电感值、开关频率等参数对输入电流总谐波失真(THD)、电源功率因数以及系统输出直流电压的影响,为实际设计中确定主电路的参数提供了可靠依据,对工程中的三相可逆PWM整流器设计有实际意义. 相似文献
984.
工序间在制品库存的优化研究 总被引:7,自引:0,他引:7
通过分析车间的设备状态、生产任务的工艺参数等因素对车间在制品库存在的影响,提出了确定工序间在制品库存的数学模型,该模型以满足生产的条件为前提,将车间在制品库存最小化作为目标,同时建立了在制品库存的管理调度算法,以便控制工序间在制品的库存。 相似文献
985.
基于微机的边界扫描测试主控系统的设计 总被引:8,自引:1,他引:8
分析了边界扫描测试技术的工作机制对测试主控系统的功能需求,提出了一种基于微机PCI总线的低成本边界扫描测试主控系统的硬件设计方案,该系统以PC机为平台,以用CPLD器件实现的JTAG主控器生成满足IEEE1149.1协议的边界扫描测试信号,并用普通的SRAM实现存存器共享,仿真表明,该系统产生的测试信号完全满足IEEE1149.1协议的时序要求,可用于IC和PCB的边界扫描测试,以及改进边界扫描测试的研究和实验。 相似文献
986.
A new method based on the combining of the wavelet theory with the fractal theory and named wavelet fractal peak position method (WFPPM) is introduced to extract the number of the components and the relevant peak positions from overlapping signals in chemistry .The overlapping signal is first transformed into continuous wavelet transform value of time domain in certain dilation range via continuous wavelet transform (CWT),and then changed into capacity dimensions (Dc),The number of the components and the relevant positions of overlapping peaks can be identified easily according to the change of Dc.An investigation concerning the influence of different dilation ranges on the peak positions extracted by WFPPM is also provided .Studies show that the WFPPM is an efficient tool for extracting the peak positions and identifying the number of peaks from unresoloved signals,even when this kind of overlapping is significantly serious,Relative errors of lless than 1.0% in peak positions are found when WFPPM is used in the processing of the cadmium (Ⅱ)-indium(Ⅲ) mixture system .The analytical results demonstrate that the desired peak positions can be extracted conveniently ,accurately and rapidly from an unresoled signal via WFPPM,Tremendous developing and applications based on currently reported WFPPM in extracting overlapping signals would be expected in the near future. 相似文献
987.
以地形跟踪/地形回避为基础,提出了用轨迹的侧向仰角代替航迹的高度作为航迹规划的隐蔽性代价的概念.该方法结合航迹的航程代价和飞行器的各种机动能力约束,可以达到对已有航迹的优化作用,同时也可直接进行简单的控制点航迹规划.仿真研究结果表明,该算法满足飞行器飞行的各种条件,是一种可行方法,该方法速度快、规划出的航迹可以有效提升飞行器在飞行过程中的生存能力,可以在用于任务规划的同时,达到对航迹进行优化的目的. 相似文献
988.
RNAi-mediated gene silencing in non-human primates 总被引:2,自引:0,他引:2
Zimmermann TS Lee AC Akinc A Bramlage B Bumcrot D Fedoruk MN Harborth J Heyes JA Jeffs LB John M Judge AD Lam K McClintock K Nechev LV Palmer LR Racie T Röhl I Seiffert S Shanmugam S Sood V Soutschek J Toudjarska I Wheat AJ Yaworski E Zedalis W Koteliansky V Manoharan M Vornlocher HP MacLachlan I 《Nature》2006,441(7089):111-114
The opportunity to harness the RNA interference (RNAi) pathway to silence disease-causing genes holds great promise for the development of therapeutics directed against targets that are otherwise not addressable with current medicines. Although there are numerous examples of in vivo silencing of target genes after local delivery of small interfering RNAs (siRNAs), there remain only a few reports of RNAi-mediated silencing in response to systemic delivery of siRNA, and there are no reports of systemic efficacy in non-rodent species. Here we show that siRNAs, when delivered systemically in a liposomal formulation, can silence the disease target apolipoprotein B (ApoB) in non-human primates. APOB-specific siRNAs were encapsulated in stable nucleic acid lipid particles (SNALP) and administered by intravenous injection to cynomolgus monkeys at doses of 1 or 2.5 mg kg(-1). A single siRNA injection resulted in dose-dependent silencing of APOB messenger RNA expression in the liver 48 h after administration, with maximal silencing of >90%. This silencing effect occurred as a result of APOB mRNA cleavage at precisely the site predicted for the RNAi mechanism. Significant reductions in ApoB protein, serum cholesterol and low-density lipoprotein levels were observed as early as 24 h after treatment and lasted for 11 days at the highest siRNA dose, thus demonstrating an immediate, potent and lasting biological effect of siRNA treatment. Our findings show clinically relevant RNAi-mediated gene silencing in non-human primates, supporting RNAi therapeutics as a potential new class of drugs. 相似文献
989.
A thermal gravimetric method is described for evaluating the kinetics of cell size-dependent decomposition and lifetime estima- tion for microcellular tissue engineering scaffolds made of biodegradable polylactic acid(PLA)foams.PLA foam cell sizes from 550 to 20?m were fabricated experimentally using a solvent-free solid-state foaming technique under saturation pressures from 1 to 5 MPa.The thermal properties of the PLA foams with respect to the cell sizes were measured using thermal gravimetric analy- sis in a nitrogen atmosphere and the activation energy and pre-exponential factor were derived to evaluate the decomposition ki- netics and estimate lifetime.It was found that small cell sizes can be achieved under high saturation pressures and that the thermal stability of PLA decreases after the fabrication process.The cell size-dependent thermal stability and degradation rate indicate that a PLA foam of larger cell sizes has a shorter degradation time,a few tenths that of the PLA raw material,at a temperature of 37°C. The results suggest that it is feasible to optimize fabrication parameters to obtain appropriate cell sizes and lifetimes that satisfy the application requirements for various organs.This study provides the basis for precise scaffold design and quantitative analysis of PLA foams in tissue engineering applications. 相似文献
990.
Da Jiang Yinping Pan Shiyuan Wang Yishi Lin Connor M.Holland John R.Kirtley Xianhui Chen Jun Zhao Lei Chen Shaoyu Yin Yihua Wang 《科学通报(英文版)》2021,(5):425-432
The iron-chalcogenide high temperature superconductor Fe(Se,Te) (FST) has been reported to exhibit complex magnetic ordering and nontrivial band topology which ... 相似文献