全文获取类型
收费全文 | 9094篇 |
免费 | 924篇 |
国内免费 | 481篇 |
专业分类
电工技术 | 70篇 |
综合类 | 531篇 |
化学工业 | 2149篇 |
金属工艺 | 3515篇 |
机械仪表 | 248篇 |
建筑科学 | 177篇 |
矿业工程 | 261篇 |
能源动力 | 60篇 |
轻工业 | 704篇 |
水利工程 | 19篇 |
石油天然气 | 138篇 |
武器工业 | 105篇 |
无线电 | 121篇 |
一般工业技术 | 1547篇 |
冶金工业 | 824篇 |
原子能技术 | 13篇 |
自动化技术 | 17篇 |
出版年
2024年 | 40篇 |
2023年 | 171篇 |
2022年 | 262篇 |
2021年 | 296篇 |
2020年 | 300篇 |
2019年 | 295篇 |
2018年 | 309篇 |
2017年 | 375篇 |
2016年 | 318篇 |
2015年 | 344篇 |
2014年 | 513篇 |
2013年 | 547篇 |
2012年 | 640篇 |
2011年 | 760篇 |
2010年 | 612篇 |
2009年 | 581篇 |
2008年 | 465篇 |
2007年 | 706篇 |
2006年 | 768篇 |
2005年 | 486篇 |
2004年 | 421篇 |
2003年 | 265篇 |
2002年 | 225篇 |
2001年 | 159篇 |
2000年 | 124篇 |
1999年 | 90篇 |
1998年 | 57篇 |
1997年 | 61篇 |
1996年 | 50篇 |
1995年 | 41篇 |
1994年 | 39篇 |
1993年 | 37篇 |
1992年 | 26篇 |
1991年 | 29篇 |
1990年 | 20篇 |
1989年 | 12篇 |
1988年 | 7篇 |
1987年 | 8篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1976年 | 2篇 |
1951年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Behavior of Magnesium‐Alloys for Automotive Applications under Mechanical and Environmental Loading: Influence of Passivating Films and Mechanisms of Local Breakdown To assure an efficient design of components under cyclic loading, all available data concerning fatigue have to be observed. Therefore the influences of manufacturing on the material condition, the mechanical loads and environmental effects have to be analysed. Magnesium‐alloys are of special interest for lightweight applications because of their excellent strength‐density ratio. The corrosion resistance of magnesium‐alloys depends on the same factors that are critical to other metals. The alloys have a good stability to atmospheric exposure and a good resistance to attack by alkali, chromic and hydrofluoric acids. However, because of the electrochemical activity of magnesium, the relative importance of some factors is greatly amplified. The nature and composition of passive films formed on magnesium‐alloys depend on the prevailing conditions, viz. alloy‐composition, passivation potential, pH, electrolyte composition and temperature. Passive films may be damaged by local breakdown. Because of this, magnesium‐alloys suffer a degradation of their properties when exposed to an aqueous environment. The main topic of the present investigations is the verification of mechanisms of the local breakdown of the protecting film. At least two mechanisms are possible for this localization: mechanical breakdown by slip steps and electrochemical breakdown (for e.g. by the effects of chloride ions). Corrosion and passivation of different high purity alloys have been studied in different solutions (neutral, alkaline with specific anions and cations) using electrochemical techniques. The diecasted alloys were tested as produced and machined. The results clarified that depending on alloy/material and surface condition/corrosion environment different mechanisms for electrochemical breakdown of the protecting films are possible. Hence fatigue life under environmental loading is influenced by surface and testing conditions. 相似文献
2.
Significant increases in the activity of vanadium(III) amidinate catalysts for ethylene polymerization have been obtained by immobilization on a MgCl2‐based support prepared by reaction of AlEt3 with a MgCl2/ethanol adduct. Catalyst immobilization and activation on this type of support prevents the rapid decay in activity observed under homogeneous polymerization conditions with unsupported catalysts. Stable polymerization activity is also observed with analogous titanium(III) complexes. Polyethylene with narrow molecular weight distribution and spherical particle morphology is obtained without reactor fouling. Copyright © 2005 Society of Chemical Industry 相似文献
3.
Absorbability of Calcium From Common Beans 总被引:4,自引:0,他引:4
C.M. WEAVER R.P. HEANEY W.R. PROULX SM. HINDERS P.T. PACKARD 《Journal of food science》1993,58(6):1401-1403
Absorption of calcium from white, red, and pinto beans, labeled with 45Ca, was measured in 24 women in a three-way randomized design using a calcium load of 72 mg and milk as the referent. Fractional absorption for the three beans averaged 0.219 ± 0.047 and did not differ by type. Milk calcium absorption at the same load was more than two times higher, 0.451 ± 0.088 (P < 0.001). Oxalate content averaged 0.34%, and phytate averaged 1.7%, a stoichiometric excess relative to calcium. To evaluate the relation of phytate to reduced absorbability, labeled pinto beans were pre-treated with phytase and fed to 10 subjects. Fractional absorption rose, averaging 0.318 ± 0.071, (P <0.01 vs. untreated beans), but was significantly below that of milk. The difference was partly accounted for by phytate, with the remainder probably due to relatively high oxalate. 相似文献
4.
The influence of NaCl and CO2 on the atmospheric corrosion of magnesium alloy AZ91 is studied in the laboratory. Samples were exposed under carefully controlled air and flow conditions; the relative humidity was 95%, the temperature was 22.0°C and the concentration of CO2 was < 1 ppm or 350 ppm. Different amounts of sodium chloride (0–70 μg/cm2) were added before exposure. The corrosion products were analyzed by gravimetry, ion chromatography, X‐ray diffraction and scanning electron microscopy. Mass gain and metal loss results are reported. The combination of high humidity and NaCl is very corrosive towards AZ91. However, the NaCl‐induced corrosion is inhibited by ambient concentrations of CO2. Exposure in the absence of CO2 gives rise to heavy pitting, with brucite, Mg(OH)2, being the dominant corrosion product. In the presence of CO2 a layer of hydrated magnesium hydroxy carbonate, Mg5(CO3)4(OH)2 · 5 H2O forms. A tentative corrosion mechanism is presented that explains the behavior in the two environments. 相似文献
5.
H. B. Wu Z. T. Fan N. Y. Huang X. P. Dong X. F. Tian 《Journal of Materials Engineering and Performance》2005,14(1):132-135
The magnesium (Mg) alloy low-pressure expendable pattern casting (EPC) process is a newly developed casting technique combining
the advantages of both EPC and low-pressure casting. In this article, metal filling and the effect of the flow quantity of
inert gas on the filling rate in the low-pressure EPC process are investigated. The results showed that the molten Mg alloy
filled the mold cavity with a convex front laminar flow and the metal-filling rate increased significantly with increasing
flow quantity when flow quantity was below a critical value. However, once the flow quantity exceeded a critical value, the
filling rate increased slightly. The influence of the flow quantity of inert gas on melt-filling rate reveals that the mold
fill is controlled by flow quantity for a lower filling rate, and, subsequently, controlled by the evaporation of polystyrene
and the evaporation products for higher metal velocity. Meanwhile, the experimental results showed that the melt-filling rate
significantly affected the flow profile, and the filling procedure for the Mg alloy in the low-pressure EPC process. A slower
melt-filling rate could lead to misrun defects, whereas a higher filling rate results in folds, blisters, and porosity. The
optimized filling rate with Mg alloy casting is 140 to 170 mm/s in low-pressure EPC. 相似文献
6.
Influence of initial textures on dynamic recrystallization and textures in AZ31 magnesium alloys 总被引:1,自引:0,他引:1
1 INTRODUCTIONMagnesiumisthelightestmetallicstructurema terialwithhighspecificstrengthandthereforeiswidelyusedinautomotive ,electronicsandaerospaceindustries[1,2 ] .However ,magnesiumoftenshowsinsufficientplasticityatroomtemperatureduetoitsHCPstructurewithlessindependentsystemsofbasalslip .Toenhanceformabilityofmagnesium ,ahigherdeformingtemperatureisusuallyusedwithtwopur poses .Thefirstistoactivatenewslipsystemsbesidesbasalslip ,sothatmorethanfiveindependentslipsystemscanbeprovided ,be… 相似文献
7.
8.
非木本木质素磺酸镁对水泥净浆和砂浆性能的影响 总被引:1,自引:0,他引:1
用红外光谱法等测定了非木本木质素磺酸镁的结构特征参数,研究了其对水泥净浆和砂浆性能的影响,并与木本木质素磺酸钙进行了对比。非木本木质素磺酸镁结构中的甲基、酚羟基、芳香环比木质素磺酸钙少;特性黏度为 0. 61mL/g,仅是木质素磺酸钙的 61%;磺化度为 0 78mmol/g,比木质素磺酸钙低约 40%。木质素磺酸镁溶液的最低表面张力为 49 82mN/m,起泡性能与木质素磺酸钙相近。非木本木质素磺酸镁对水泥净浆和砂浆减水分散作用为木本木质素磺酸钙的 86 6% ~91 1%。但掺质量分数 0 5%的木质素磺酸镁的砂浆, 28d的抗压强度比掺木质素磺酸钙的高 70 5%。 相似文献
9.
The corners with small radii on cross sections are crucial for forming hydroformed components with polygonal sections.In this paper,warm hydroforming experiments of AZ61A magnesium alloy tubes were carried out to study the forming regularity of round corners by using a demonstration part with square sections.Effects of temperature on radius forming,thinning ratio distribution and microstructure were revealed and a component with relative outer corner radius of 3.0 was obtained by warm hydroforming at 240℃.The minimum thickness of the formed square section was located in the transition position between the corner and the straight wall.The thinning ratio of the round corner increased with the increase of forming temperature.Formability of the magnesium tube was improved by raising temperature under the effect of dynamic recrystallization at 240℃. 相似文献
10.
针对钙镁磷肥生产中高炉探料存在的问题 ,对原自动探料装置的电气控制回路及跟踪装置进行改造 ,增加了探料跟踪最大极限位置 ,获得成功。改造后的自动跟踪探料尺 ,动作准确可靠 ,运行稳定 ,效率高 ,节能降耗 ,直接经济效益每年约 2 5万元 ,提高台时产量间接经济效益每年可达 4 0万元 相似文献