全文获取类型
收费全文 | 6365篇 |
免费 | 628篇 |
国内免费 | 34篇 |
专业分类
系统科学 | 1087篇 |
丛书文集 | 2篇 |
教育与普及 | 3篇 |
理论与方法论 | 285篇 |
现状及发展 | 1056篇 |
研究方法 | 131篇 |
综合类 | 4437篇 |
自然研究 | 26篇 |
出版年
2020年 | 5篇 |
2019年 | 5篇 |
2018年 | 732篇 |
2017年 | 739篇 |
2016年 | 437篇 |
2015年 | 41篇 |
2014年 | 21篇 |
2013年 | 20篇 |
2012年 | 323篇 |
2011年 | 1039篇 |
2010年 | 845篇 |
2009年 | 477篇 |
2008年 | 557篇 |
2007年 | 826篇 |
2006年 | 54篇 |
2005年 | 103篇 |
2004年 | 159篇 |
2003年 | 210篇 |
2002年 | 108篇 |
2001年 | 40篇 |
2000年 | 43篇 |
1999年 | 20篇 |
1996年 | 4篇 |
1992年 | 10篇 |
1991年 | 10篇 |
1990年 | 4篇 |
1989年 | 10篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 10篇 |
1976年 | 5篇 |
1975年 | 9篇 |
1974年 | 5篇 |
1973年 | 9篇 |
1972年 | 3篇 |
1971年 | 9篇 |
1970年 | 9篇 |
1969年 | 7篇 |
1968年 | 5篇 |
1967年 | 8篇 |
1966年 | 5篇 |
1965年 | 7篇 |
排序方式: 共有7027条查询结果,搜索用时 593 毫秒
961.
Fracture behavior and microstructure analysis of Al2O3–MgO–CaO castables for steel-ladle purging plugs
下载免费PDF全文
![点击此处可从《矿物冶金与材料学报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Three different castables based on the Al2O3–MgO–CaO system were prepared as steel-ladle purging plug refractories: corundum- based low-cement castable (C-LCC), corundum-spinel-based low-cement castable (C-S-LCC), and corundum-spinel no-cement castable (C-S-NCC) (hydratable alumina (ρ-Al2O3) bonded). The fracture behavior at room temperature was tested by the method of “wedge-splitting” on samples pre-fired at different temperatures; the specific fracture energy G′f and notched tensile strength σNT were obtained from these tests. In addition, the Young’s modulus E was measured by the method of resonance frequency of damping analysis (RFDA). The thermal stress resistance parameter R′′′′ calculated using the values of G′f, σNT, and E was used to evaluate the thermal shock resistance of the materials. According to the microstructure analysis results, the sintering effect and the bonding type of the matrix material were different among these three castables, which explains their different fracture behaviors. 相似文献
962.
Effects of stress ratio on the temperature-dependent high-cycle fatigue properties of alloy steels
下载免费PDF全文
![点击此处可从《矿物冶金与材料学报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
This paper addresses the effects of stress ratio on the temperature-dependent high-cycle fatigue (HCF) properties of alloy steels 2CrMo and 9CrCo, which suffer from substantial vibrational loading at small stress amplitude, high stress ratio, and high frequency in the high-temperature environments in which they function as blade and rotor spindle materials in advanced gas or steam turbine engines. Fatigue tests were performed on alloy steels 2CrMo and 9CrCo subjected to constant-amplitude loading at four stress ratios and at four and three temperatures, respectively, to determine their temperature-dependent HCF properties. The interaction mechanisms between high temperature and stress ratio were deduced and compared with each other on the basis of the results of fractographic analysis. A phenomenological model was developed to evaluate the effects of stress ratio on the temperature-dependent HCF properties of alloy steels 2CrMo and 9CrCo. Good correlation was achieved between the predictions and actual experiments, demonstrating the practical and effective use of the proposed method. 相似文献
963.
Microstructure evolution and thixoforming behavior of 7075 aluminum alloy in the semi-solid state prepared by RAP method
下载免费PDF全文
![点击此处可从《矿物冶金与材料学报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The effects of isothermal treatments on the microstructural evolution and coarsening rate of semi-solid 7075 aluminum alloy produced via the recrystallization and partial remelting (RAP) process were investigated. Samples of 7075 aluminum alloy were subjected to cold extrusion, and semi-solid treatment was carried out for 5–30 min at temperatures ranging from 580 to 605°C. A backward-extrusion experiment was conducted to investigate liquid segregation during the thixoforming process. The results revealed that obvious grain coarsening and spheroidization occurred during prolonged isothermal treatments. In addition, higher soaking temperatures promoted the spheroidization and coarsening process because of the increased liquid fraction and the melting of second phases. Segregation of the liquid phase caused by the difference in fluidity between the liquid and the solid phases was observed in different regions of the thixoformed specimens. 相似文献
964.
Thermodynamic simulation of the effect of slag chemistry on the corrosion behavior of alumina–chromia refractory
下载免费PDF全文
![点击此处可从《矿物冶金与材料学报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Shi-xian Zhao Bin-li Cai Hong-gang Sun Gang Wang Hong-xia Li Xiao-yan Song 《矿物冶金与材料学报》2016,23(12):1458-1465
The corrosion behavior of alumina–chromia refractory against two kinds of industrial slags (coal slag and iron smelting slag) at 1550°C was investigated via thermodynamic simulations. In the proposed simulation model, the initial slag first attacks the matrix and surface aggregates and subsequently attacks the inner aggregates. The simulation results indicate that the slag chemistry strongly affects the phase formation and corrosion behavior of the refractory brick. Greater amounts of alumina were dissolved and spinel solid phases formed when the brick interacted with iron smelting slag. These phenomena, as well as the calculated lower viscosity, may lead to much deeper penetration than that exhibited by coal slag and to more severe corrosion compared to that induced by coal slag. The thermodynamic calculations well match the experimental observations, demonstrating the efficiency of the proposed simulation model for evaluating the corrosion behavior of alumina–chromia refractory. 相似文献
965.
Effect of SrCO<Subscript>3</Subscript> addition on the dynamic compressive strength of ZTA
下载免费PDF全文
![点击此处可从《矿物冶金与材料学报》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Ceramic parts usually experience dynamic load in armor applications. Therefore, studying the dynamic behaviors of ceramics is important. Limited data are available on the dynamic behaviors of ceramics; thus, it is helpful to predict the dynamic strength of ceramics on the basis of their mechanical properties. In this paper, the addition of SrCO3 into zirconia-toughened alumina (ZTA) was demonstrated to improve the fracture toughness of ZTA due to the formation of the SrAl12O19 (SA6) phase. The porosity of ZTA was found to be increased by the addition of SrCO3. These newly formed pores served as the nucleation sites of cracks under dynamic load; these cracks eventually coalesced to form damaged zones in the samples. Although the K IC values of the samples were improved, the dynamic strength was not enhanced because of the increase in porosity; in fact, the dynamic strength of ZTA ceramics decreased with the addition of SrCO3. 相似文献
966.
Immunization with amyloid-beta attenuates Alzheimer-disease-like pathology in the PDAPP mouse. 总被引:121,自引:0,他引:121
D Schenk R Barbour W Dunn G Gordon H Grajeda T Guido K Hu J Huang K Johnson-Wood K Khan D Kholodenko M Lee Z Liao I Lieberburg R Motter L Mutter F Soriano G Shopp N Vasquez C Vandevert S Walker M Wogulis T Yednock D Games P Seubert 《Nature》1999,400(6740):173-177
Amyloid-beta peptide (Abeta) seems to have a central role in the neuropathology of Alzheimer's disease (AD). Familial forms of the disease have been linked to mutations in the amyloid precursor protein (APP) and the presenilin genes. Disease-linked mutations in these genes result in increased production of the 42-amino-acid form of the peptide (Abeta42), which is the predominant form found in the amyloid plaques of Alzheimer's disease. The PDAPP transgenic mouse, which overexpresses mutant human APP (in which the amino acid at position 717 is phenylalanine instead of the normal valine), progressively develops many of the neuropathological hallmarks of Alzheimer's disease in an age- and brain-region-dependent manner. In the present study, transgenic animals were immunized with Abeta42, either before the onset of AD-type neuropathologies (at 6 weeks of age) or at an older age (11 months), when amyloid-beta deposition and several of the subsequent neuropathological changes were well established. We report that immunization of the young animals essentially prevented the development of beta-amyloid-plaque formation, neuritic dystrophy and astrogliosis. Treatment of the older animals also markedly reduced the extent and progression of these AD-like neuropathologies. Our results raise the possibility that immunization with amyloid-beta may be effective in preventing and treating Alzheimer's disease. 相似文献
967.
ZHANG Qiang HUANG JiaQi WEI Fei XU GuangHui WANG Yao QIAN WeiZhong WANG DeZheng 《科学通报(英文版)》2007,52(21):2896-2902
Liquefied petroleum gas (LPG), a cheap industrial material, is used as carbon source to produce carbon nanotube (CNT) arrays on ceramic spherical surface on a large scale in the floating catalyst process. The ceramic spheres provide huge surface area and good mobility, leading to the mass production of CNT arrays continuously. The arrays obtained from the surface are of good alignment, and the purity is as high as 97.5%. With the decrease of the growth temperature, CNTs in the array form with small-diameter of about 13 nm can be obtained. Therefore, with the industrial fuel as carbon source and the ceramic sphere as substrate, CNT arrays can easily be produced on large scale at low cost. 相似文献
968.
Long series of quarterly GDP figures are still not available for many countries. This paper suggests an empirical procedure adapted from Chow and Lin (1971) to derive quarterly estimates from annual GDP figures and produces quarterly GDP by sectors for Malaysia from 1973Q1 onwards. A comparison of these estimates with some univariate interpolations using published quarterly figures for recent years show that the use of related series can produce substantially superior estimates of GDP compared to univariate methods. The data set is available from the authors. Copyright © 1998 John Wiley & Sons, Ltd. 相似文献
969.
Jian CHEN Yunhui LIU Xiping SONG School of Economics ManagementTsinghua University Beijing P.R. China Department of System Engineering Engineering ManagementThe Chinese University of Hong Kong Shatin NT Hong Kong P.R China 《系统科学与系统工程学报(英文版)》2004,13(2):202-218
In this paper we consider a group-buying online auction (GBA) model for a monopolistic manufacturer selling novel products in the uncertain market. Firstly, we introduce the bidder's dominant strategy, after which we optimize the GBA price curve and the production volume together. Finally, we compare the GBA with the traditional posted pricing mechanism and find that the GBA is highly probable to be advantageous over the posted pricing mechanism in some appropriate market environments. 相似文献
970.