全文获取类型
收费全文 | 93516篇 |
免费 | 7532篇 |
国内免费 | 4048篇 |
专业分类
电工技术 | 3818篇 |
技术理论 | 8篇 |
综合类 | 8146篇 |
化学工业 | 17242篇 |
金属工艺 | 2093篇 |
机械仪表 | 2127篇 |
建筑科学 | 13438篇 |
矿业工程 | 5121篇 |
能源动力 | 5422篇 |
轻工业 | 5432篇 |
水利工程 | 18798篇 |
石油天然气 | 7974篇 |
武器工业 | 215篇 |
无线电 | 2457篇 |
一般工业技术 | 5990篇 |
冶金工业 | 3418篇 |
原子能技术 | 1225篇 |
自动化技术 | 2172篇 |
出版年
2024年 | 280篇 |
2023年 | 1058篇 |
2022年 | 2007篇 |
2021年 | 2471篇 |
2020年 | 2757篇 |
2019年 | 2325篇 |
2018年 | 2209篇 |
2017年 | 2770篇 |
2016年 | 3001篇 |
2015年 | 3006篇 |
2014年 | 5607篇 |
2013年 | 5662篇 |
2012年 | 6939篇 |
2011年 | 6995篇 |
2010年 | 5153篇 |
2009年 | 5406篇 |
2008年 | 4765篇 |
2007年 | 6267篇 |
2006年 | 5916篇 |
2005年 | 5420篇 |
2004年 | 4240篇 |
2003年 | 3882篇 |
2002年 | 3372篇 |
2001年 | 2684篇 |
2000年 | 2265篇 |
1999年 | 1736篇 |
1998年 | 1296篇 |
1997年 | 1117篇 |
1996年 | 860篇 |
1995年 | 799篇 |
1994年 | 573篇 |
1993年 | 431篇 |
1992年 | 348篇 |
1991年 | 274篇 |
1990年 | 214篇 |
1989年 | 220篇 |
1988年 | 127篇 |
1987年 | 148篇 |
1986年 | 107篇 |
1985年 | 104篇 |
1984年 | 97篇 |
1983年 | 59篇 |
1982年 | 32篇 |
1981年 | 12篇 |
1980年 | 20篇 |
1979年 | 18篇 |
1978年 | 6篇 |
1977年 | 10篇 |
1976年 | 8篇 |
1951年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
The growth of nominally undoped GaSb layers by atmospheric pressure metalorganic vapor phase epitaxy on GaSb and GaAs substrates
is studied. Trimethylgallium and trimethylantimony are used as precursors for the growth at 600°C in a horizontal reactor.
The effect of carrier gas flow, V/III-ratio, and trimethylgallium partial pressure on surface morphology, electrical properties
and photoluminescence is investigated. The optimum values for the growth parameters are established. The carrier gas flow
is shown to have a significant effect on the surface morphology. The optimum growth rate is found to be 3–8 μm/ h, which is
higher than previously reported. The 2.5 μm thick GaSb layers on GaAs are p-type, having at optimized growth conditions room-temperature
hole mobility and hole concentration of 800 cm2 V−1 s−1 and 3·1016 cm-3, respectively. The homoepitaxial GaSb layer grown with the same parameters has mirror-like surface and the photoluminescence
spectrum is dominated by strong excitonic lines. 相似文献
92.
J. Shin Y. Hsu T. C. Hsu G. B. Stringfellow R. W. Gedridge 《Journal of Electronic Materials》1995,24(11):1563-1569
GalnSb alloys as well as the constituent binaries InSb and GaSb have been grown by organometallic vapor phase epitaxy using
the new antimony precursor trisdimethylaminoantimony (TDMASb) combined with conventional group III precursors trimethylindium
(TMIn) and trimethylgallium (TMGa). InSb layers were grown at temperatures between 275 and 425°C. The low values of V/III
ratio required to obtain good morphologies at the lowest temperatures indicate that the pyrolysis temperature is low for TDMASb.
In fact, at the lowest temperatures, the InSb growth efficiency is higher than for other antimony precursors, indicating the
TDMASb pyrolysis products assist with TMIn pyrolysis. A similar, but less pronounced trend is observed for GaSb growth at
temperatures of less than 500°C. No excess carbon contamination is observed for either the InSb or GaSb layers. Ga1-xInxSb layers with excellent morphologies with values of x between 0 and 0.5 were grown on GaSb substrates without the use of
graded layers. The growth temperature was 525°C and the values of V/III ratio, optimized for each value of x, ranged between
1.25 and 1.38. Strong photoluminescence (PL) was observed for values of x of less than 0.3, with values of halfwidth ranging
from 13 to 16 meV, somewhat smaller than previous reports for layers grown using conventional precursors without the use of
graded layers at the interface. The PL intensity was observed to decrease significantly for higher values of x. The PL peak
energies were found to track the band gap energy; thus, the luminescence is due to band edge processes. The layers were all
p-type with carrier concentrations of approximately 1017 cm3. Transmission electron diffraction studies indicate that the Ga0.5In0.5 Sb layers are ordered. Two variants of the Cu-Pt structure are observed with nearly the same diffracted intensities. This
is the first report of ordering in GalnSb alloys. 相似文献
93.
94.
P. Kurpas A. Oster M. Weyers A. Rumberg K. Knorr W. Richter 《Journal of Electronic Materials》1997,26(10):1159-1163
Reflectance anisotropy spectroscopy (RAS) has been used to study As-by-P exchange during metalorganic vapor phase epitaxy.
The study focuses on the processes occurring during switching from GaAs to GaInP, especially the effect of purging PH3 over a GaAs surface. GaAsP/GaAs superlattices of different periodicity were grown and the P-content was determined by high-resolution
x-ray diffraction and correlated to the RAS spectra. From the temperature dependence of the P-content, an activation energy
of 0.56 eV was estimated for the incorporation mechanism. In addition to the insights into the processes at mixed group-V
heterointerfaces, our study demonstrates the reproducibility of RAS transients that thus can be used for process monitoring. 相似文献
95.
高水固结尾砂充填采矿充填体合理配比的研究 总被引:7,自引:4,他引:3
高水固结尾砂充填采矿是一种新型充填采矿法。试验测定了高水固结尾矿充填体的强度和变形特征;研究分析了影响充填体强度的主要因素、高水充填工艺的特殊要求,以及上向,下向进路开采对充填体强度和稳定性的要求,提出了满足支护强度,工艺及经济需求的充填材料的合理配比范围。 相似文献
96.
Mao Ze yu Chen Chang zhi Department of Hydraulic Engineering Tsinghua University Beijing P.R.China 《水动力学研究与进展(B辑)》1999,(3)
1. INTRODUCTIONThesimulationofstreamwatertemperatureissignificantlyimportantforthestudyofnumericalmodelingofrivericeprocesses[1] .Historically,themathematicalmodelingofthetransportandfateofheatinawaterbodyhasbeenthesubjectofextensivestudyforvariousre… 相似文献
97.
单元系T—p相图的数学结构 总被引:1,自引:0,他引:1
作者以化学元素的稳定单质为基准,推演出了任意物质M_i(相态Ω)的热力学生成活度{相态Ω)的函数形式:?D_Ω数值的大小体现着相态Ω的热力学相对稳定性.根据集合论原理沿D(稳定性)座标取极大值的方法把物理性质互不连续的各个异相态连结在一起,建立了单元系在T-p面上的优势分布方程(PSDE):■作者以H_2O为实例,计算了T-p相图,与实验相图基本一致. 相似文献
98.
工业用水中硅酸根离子分析技术的现状与发展 总被引:1,自引:0,他引:1
综述了工业用水中硅酸根离子的基本分析方法以及分析仪器的开发应用现状;论述了流动注射分析技术(FIA)在工业用水中硅酸根离子分析上的应用并预示了水质分析中在线预富集技术与多组分联合测定技术的发展。 相似文献
99.
一种新型同位素示踪剂载体及应用 总被引:2,自引:0,他引:2
同位素沾污、低注井同位素示踪剂因沉降造成的上返速度慢或无法上返、大孔道地层同位素示踪剂失踪等是影响注水开发油田后期吸水剖面测井资料质量的主要问题。同位素示踪剂载体的物理参数,尤其是其密度,是产生上述问题的重要因素。通过对同位素示踪剂载体技术指标优选而生产的一种新型同位素示踪剂,可以解决低注井同位素上返速度慢的问题,并能有效识别大孔道地层,减少同位素沾污。这种同位素示踪剂载体已在河南油田吸水剖面测井中应用,并见到了很好的效果。 相似文献
100.
Huseyin Selcuk Jeosadaque J Sene Marc A Anderson 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》2003,78(9):979-984
This study addresses the removal of humic acid (HA) dissolved in an aqueous medium by a photoelectrocatalytic process. UV254 removal and the degradation of color (Vis400) followed pseudo‐first order kinetics. Rate constants were 1.1 × 10?1 min?1, 8.3 × 10?2 min?1 and 2.49 × 10?2 min?1 (R2 > 0.97) for UV254 degradation and 1.7 × 10?1 min?1, 6.5 × 10?2 min?1 and 2.0 × 10?2 min?1 for color removal from 5 mg dm?3, 10 mg dm?3 and 25 mg dm?3 HA respectively. Following a 2 h irradiation time, 96% of the color, 98% of the humic acid and 85% of the total organic carbon (TOC) was removed from an initial 25 mg dm?3 HA solution in the photoanode cell. Photocatalytic removal on the same photoanode was also studied in order to compare the two methods of degradation. Results showed that the photoelectrocatalytic method was much more effective than the photocatalytic method especially at high pH values and with respect to UV254 removal. The effect of other important reaction variables, eg pH, external potential and electrolyte concentration, on the photoelectrocatalytic HA degradation was also studied. Copyright © 2003 Society of Chemical Industry 相似文献