全文获取类型
收费全文 | 8633篇 |
免费 | 139篇 |
国内免费 | 6篇 |
专业分类
电工技术 | 73篇 |
综合类 | 4篇 |
化学工业 | 1096篇 |
金属工艺 | 114篇 |
机械仪表 | 207篇 |
建筑科学 | 233篇 |
矿业工程 | 33篇 |
能源动力 | 188篇 |
轻工业 | 525篇 |
水利工程 | 47篇 |
石油天然气 | 11篇 |
武器工业 | 2篇 |
无线电 | 640篇 |
一般工业技术 | 1172篇 |
冶金工业 | 3679篇 |
原子能技术 | 118篇 |
自动化技术 | 636篇 |
出版年
2021年 | 64篇 |
2020年 | 49篇 |
2019年 | 52篇 |
2018年 | 59篇 |
2017年 | 69篇 |
2016年 | 72篇 |
2015年 | 60篇 |
2014年 | 106篇 |
2013年 | 237篇 |
2012年 | 161篇 |
2011年 | 255篇 |
2010年 | 180篇 |
2009年 | 186篇 |
2008年 | 212篇 |
2007年 | 207篇 |
2006年 | 216篇 |
2005年 | 153篇 |
2004年 | 166篇 |
2003年 | 159篇 |
2002年 | 141篇 |
2001年 | 144篇 |
2000年 | 135篇 |
1999年 | 212篇 |
1998年 | 1078篇 |
1997年 | 594篇 |
1996年 | 479篇 |
1995年 | 283篇 |
1994年 | 237篇 |
1993年 | 296篇 |
1992年 | 122篇 |
1991年 | 135篇 |
1990年 | 121篇 |
1989年 | 137篇 |
1988年 | 101篇 |
1987年 | 108篇 |
1986年 | 120篇 |
1985年 | 121篇 |
1984年 | 104篇 |
1983年 | 88篇 |
1982年 | 84篇 |
1981年 | 118篇 |
1980年 | 116篇 |
1979年 | 93篇 |
1978年 | 83篇 |
1977年 | 132篇 |
1976年 | 221篇 |
1975年 | 79篇 |
1974年 | 47篇 |
1973年 | 68篇 |
1972年 | 46篇 |
排序方式: 共有8778条查询结果,搜索用时 500 毫秒
71.
Thirteen scenarios were used to measure and compare the perceptions of elder abuse and help-seeking behaviors of African-American, Caucasian American, and Korean-American elderly women. Significant group differences existed in their perceptions of elder abuse with regard to six scenarios, and the Korean-American women were substantially less likely to perceive a given situation as abusive than the other groups. The three groups also showed significant differences in their intended use of formal and informal sources of help in the case of elder abuse. 相似文献
72.
Williams K.A. White I.H. Burns D. Sibbett W. 《Quantum Electronics, IEEE Journal of》1996,32(11):1988-1994
Following a comparison of picosecond-pulse generation techniques, feedback schemes are reported for the generation of picosecond pulse trains with improved jitter for both multicontact and conventional single-contact InGaAsP-InP lasers. Subpicosecond jitter is achieved for Q-switched laser sources using a novel optoelectronic feedback scheme. The use of resonant electrical feedback is shown to improve the timing jitter of gain-switched pulses by up to six times. Pulse-to-pulse timing jitter as low as 250 fs is demonstrated for a hybrid of optical and electrical feedback schemes. Limits for timing jitter in diode lasers are established for optoelectronic, electrical, and optical feedback schemes, and the key picosecond pulse generation schemes are compared in terms of timing jitter for the first time 相似文献
73.
74.
75.
A Brewaeys P Devroey FM Helmerhorst EV Van Hall I Ponjaert 《Canadian Metallurgical Quarterly》1995,10(10):2731-2735
This study investigated the development of family relationships in lesbian families who conceived after donor insemination (DI). The main characteristics of this unknown family structure were analysed. An anonymous donor was used to conceive, the family unit consisted of two mothers and a father was non-existent, and both women had a homosexual orientation. A total of 50 lesbian couples who conceived after DI with children aged between 1 and 2 years participated in this study, using a standardized interview created for this study. After the birth of their child, 56% of the lesbian mothers (n = 100) would have wanted the identity of the donor to be registered, while 10% would have done so at the time of the insemination. Opinions differed in 12 of the 50 couples: the biological mother was in favour of identity registration while the social mother was not. Both women considered themselves a parent of the child with equally shared responsibilities. Mothers were open about the special features of their family structure with children and within their immediate social network. Only 30% disclosed their lesbian identity in a broader social environment. The features of these newly created families may influence the psychological development of the children and therefore make long-term follow-up studies indispensable. 相似文献
76.
BACKGROUND: Serum creatinine has been reported in previous studies to be a prognostic indicator for overall mortality, in particular in a hypertensive population. METHODS: The Program on the Surgical Control of the Hyperlipidemias (POSCH) was a randomized, controlled clinical trial. All patients had survived a single myocardial infarction, were normotensive, were not obese, were not having heart failure, and were free of diabetes mellitus and renal disease at entry into the study. POSCH had followed its control group patients (N = 417) for a minimum of 7.0 years. In this group, a prospective post hoc analysis of the relationship of baseline serum creatinine with subsequent overall and atherosclerotic coronary heart disease mortality was performed. RESULTS: The baseline serum creatinine values in the control group patients ranged from 0.7 to 1.9 mg/dL (60 to 170 mumol/L), and were found to be independent predictors (P < .01) of both overall mortality and atherosclerotic coronary heart disease mortality. Each 0.1 mg/dL (9 mumol/L) increment in the baseline serum creatinine increased the relative risk for subsequent overall mortality by 36% and the relative risk for subsequent atherosclerotic coronary heart disease mortality by 47%. CONCLUSIONS: These results demonstrate that a serum creatinine value, obtained in normotensive, nonobese, normoglycemic survivors of a myocardial infarction without preexistent renal disease or heart failure, provides independent prognostic information regarding subsequent overall and atherosclerotic coronary heart disease mortality. 相似文献
77.
Daniel Grosjean Eric Grosjean II Edwin L. Williams 《Israel journal of chemistry》1994,34(3-4):365-373
The atmospheric oxidation of the unsaturated aldehyde 2-ethyl acrolein, CH2=C(C2H5)CHO, has been studied in laboratory experiments involving the reaction of ozone with 2-ethyl acrolein in the dark (with cyclohexane added to scavenge the hydroxyl radical), and the sunlight irradiation of 2-ethyl acrolein with NO in air. The major carbonyl products of the 2-ethyl acrolein reaction with ozone are formaldehyde, acetaldehyde, and the dicarbonyl ethylglyoxal, CH3CH2COCHO. Sunlight irradiation of 2-ethyl acrolein and NO led to the formation of three carbonyls (formaldehyde, acetaldehyde, and ethylglyoxal) and three peroxyacyl nitrates, (RC(O)OONO2), including PAN (R = CH3), PPN (R = C2H5), and the unsaturated compound EPAN (R = CH2=C(C2H5)). Mechanisms are outlined for the reactions of ozone and of the hydroxyl radical with 2-ethyl acrolein. These mechanisms are consistent with the observed carbonyl and peroxyacyl nitrate products. Thermal decomposition, a major atmospheric removal process for peroxyacyl nitrates, has been studied for EPAN. The decomposition rate of EPAN relative to that of PAN is 0.59–0.73 at 292–294 K and 1 atm of air. Atmospheric implications of these results are discussed. 相似文献
78.
Bernard Williams 《Metal Powder Report》2003,58(10):27-28
With commercially available PIM feedstock reportedly unable to fulfill the requirements for the microPIM components under investigation, researchers at the Institute for Materials Research III (IMR) in Karlsruhe, Germany, have developed new binder systems for feedstock based on both nanoceramic and ultrafine metallic powders. In the case of ceramics (zirconia) the powders can be as fine as 300 nm, and for metals in the 3μm-5μm particle size range. The powders are used to develop PIM gearwheels (Figure 1) and other components for a demonstrator micro annular gear pump design (Figure 2); however, the technology is also said to be applicable to micro gears used in watches. 相似文献
79.
Control laws are often designed for linear time-varying processes by solving the algebraic Riccati equation for the optimum control law at each instant of time. Such designs may be called "adiabatic approximations." Although they are not optimum, they can result in closed loop systems which perform well. The stability of systems designed using the adiabatic approximation can be assessed by the "second method of Lyapunov." Stability is assured if a readily computed test matrixF , which depends on the rate of change of the parameters of the system, is negative-definite. 相似文献
80.
Many of the targeted applications for powder-metallurgy materials, particularly in the automotive industry, undergo cyclic
loading. It is, therefore, essential to examine the fatigue mechanisms in these materials. The mechanisms of fatigue-crack
initiation and propagation in ferrous powder-metallurgy components have been investigated. The fatigue mechanisms are controlled
primarily by the inherent porosity present in these materials. Since most, if not all, fatigue cracks initiate and propagate
at the specimen surface, surface replication was used to determine the role of surface porosity in relation to fatigue behavior.
Surface replication provides detailed information on both initiation sites and on the propagation path of fatigue cracks.
The effect of microstructural features such as pore size and pore shape, as well as the heterogeneous microstructure on crack
deflection, was examined and is discussed. Fracture surfaces were examined to elucidate a mechanistic understanding of fatigue
processes in these materials. 相似文献