全文获取类型
收费全文 | 19296篇 |
免费 | 834篇 |
国内免费 | 140篇 |
学科分类
工业技术 | 20270篇 |
出版年
2024年 | 103篇 |
2023年 | 421篇 |
2022年 | 998篇 |
2021年 | 1167篇 |
2020年 | 915篇 |
2019年 | 948篇 |
2018年 | 1226篇 |
2017年 | 960篇 |
2016年 | 934篇 |
2015年 | 604篇 |
2014年 | 844篇 |
2013年 | 1522篇 |
2012年 | 899篇 |
2011年 | 1078篇 |
2010年 | 864篇 |
2009年 | 814篇 |
2008年 | 724篇 |
2007年 | 581篇 |
2006年 | 473篇 |
2005年 | 373篇 |
2004年 | 278篇 |
2003年 | 250篇 |
2002年 | 192篇 |
2001年 | 179篇 |
2000年 | 171篇 |
1999年 | 170篇 |
1998年 | 294篇 |
1997年 | 242篇 |
1996年 | 232篇 |
1995年 | 171篇 |
1994年 | 159篇 |
1993年 | 151篇 |
1992年 | 106篇 |
1991年 | 136篇 |
1990年 | 100篇 |
1989年 | 99篇 |
1988年 | 82篇 |
1987年 | 91篇 |
1986年 | 79篇 |
1985年 | 89篇 |
1984年 | 75篇 |
1983年 | 71篇 |
1982年 | 64篇 |
1981年 | 72篇 |
1980年 | 50篇 |
1979年 | 36篇 |
1978年 | 31篇 |
1977年 | 29篇 |
1976年 | 35篇 |
1971年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
Development of nano indium tin oxide (ITO) grains by alkaline hydrolysis of In(III) and Sn(IV) salts
Indium tin oxide (ITO) nano powders of different compositions (In: Sn = 90: 10, 70: 30 and 50: 50) were prepared by heat treatment
(300-450°C) of mixed hydroxides of In(III) and Sn(IV). The hydroxides were obtained by the reaction of aq. NH3 with mixed aq. solutions of In(NO3)3 and SnCl4. FTIR and TG/DTA studies revealed that powders existed as In(OH)3H2O—SnO3H2H2O in the solid state and then they transformed to In2O3—SnO2 via some metastable intermediates after 300°C. Cubic phase of In2O3 was identified by XRD for the oxides up to 30% of Sn. Particle size measurements of the solid dispersed in acetone and SEM
study for microstructure showed that the oxides were in the nano range (55-75 nm) whereas the size range determined from Debye-Scherrer
equation were 11–24 nm. 相似文献
42.
43.
We consider a system comprising a finite number of nodes, with infinite packet buffers, that use unslotted ALOHA with Code
Division Multiple Access (CDMA) to share a channel for transmitting packetised data. We propose a simple model for packet
transmission and retransmission at each node, and show that saturation throughput in this model yields a sufficient condition
for the stability of the packet buffers; we interpret this as the capacity of the access method. We calculate and compare
the capacities of CDMA‐ALOHA (with and without code sharing) and TDMA‐ALOHA; we also consider carrier sensing and collision
detection versions of these protocols. In each case, saturation throughput can be obtained via analysis of a continuous time
Markov chain. Our results show how saturation throughput degrades with code‐sharing. Finally, we also present some simulation
results for mean packet delay. Our work is motivated by optical CDMA in which “chips” can be optically generated, and hence
the achievable chip rate can exceed the achievable TDMA bit rate which is limited by electronics. Code sharing may be useful
in the optical CDMA context as it reduces the number of optical correlators at the receivers. Our throughput results help
to quantify by how much the CDMA chip rate should exceed the TDMA bit rate so that CDMA‐ALOHA yields better capacity than
TDMA‐ALOHA.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
44.
45.
Power semiconductor devices find wide application in modern power electronic converters. Protection of these devices against overload/short circuit conditions is of paramount importance. Present day protection topologies employing different circuits have invariably one main drawback in that the fault current reaches the set value before action is initiated to trip the system. This poses a severe stress on the device. Hence an adequate safety margin has to be necessarily provided to prevent excessive device stresses and care has to be taken to see that the device is operated well within its safe operating areas. The present paper proposes a method wherein the slope or rate of rise of the fault current is detected and once the slope exceeds the set reference, action is initiated to trip the system much before the fault current reaches dangerous levels. The method provides a fast means of detection of overload and short circuit currents and can be conveniently adopted for the protection of devices in power transistor/IGBT based inverters against short circuited load conditions or shoot through faults. The possible reduction of stresses in the power devices are also highlighted 相似文献
46.
47.
A new finite element crack growth algorithm has been developed to simulate dynamic fracture. In this algorithm, pseudo elements with very high initial density are placed below the crack plane and the density is reduced to zero in a gradual manner as the crack passes the element. A number of linear elastic and elasto-viscoplastic problems have been carried out to test the new algorithm. The results are compared with some of the existing crack growth models. 相似文献
48.
49.
OBJECTIVE: To compare skinfold thickness measurements with bioelectrical impedance analysis (BIA) as a measure of body fat for use in a survey of children (the National Study of Health and Growth). DESIGN: Part cross-sectional, part repeated measurement study. SETTING: A junior school in Bath. SUBJECTS: 42 boys and 33 girls aged from 9 to 11 years. INTERVENTIONS: Measurements of BIA, height, weight, and triceps, biceps, subscapular and suprailiac skinfolds. RESULTS: All measurements were highly repeatable with intraclass correlation coefficients > 0.90. The level of agreement between estimates of percentage of body fat derived from prediction equations based on impedance or skinfold measurements respectively was poor: the mean difference (impedance estimate minus skinfold estimate) was 4.67% (95% range -3.47 to 12.82) for boys and 7.81% (95% range 1.27 to 14.34) for girls. The two estimates were found to correlate highly (r = 0.83 for boys and r = 0.81 for girls) because weight, used to convert estimates of fat-free mass derived from impedance to fat mass, was highly correlated with impedance and moderately highly correlated with skinfold thicknesses. The correlations of resistance (R) and (H)2/R with skinfold thicknesses were very low. There was a moderate correlation of R and H2/R with log(weight-for-height index), but lower than that of log(weight-for-height index) with each of the skinfolds. CONCLUSIONS: As currently available equations for converting impedance-based estimates of total body water to fat mass are not fully developed for use in children of varying ages, estimates of body fat calculated from skinfold thickness measurements remain preferable in epidemiological studies of children's health and growth. 相似文献
50.
Poly(vinylbenzyltrimethylammonium chloride)‐graft‐cotton cellulose, an anion‐exchange matrix, was synthesized by a mutual radiation‐induced grafting technique with a 60Co γ‐radiation source. The grafted matrix was characterized by grafting yield estimation, elemental analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy. The grafting yield decreased with the increase in the dose rate. However, the grafting yield and nitrogen content of grafted samples increased almost linearly with an increase in the total irradiation dose. To evaluate the performance of the grafted anion‐exchange matrix, the protein adsorption and elution behavior were investigated in a continuous column process under various experimental conditions, with bovine serum albumin used as a model protein. The binding and elution behavior of the anion‐exchange matrix depended on different experimental parameters, such as the grafting yield, ionic strength, pH of the medium, and amount of protein loaded. From a breakthrough curve, the equilibrium binding capacity and elution percentage of the grafted anion‐exchange matrix were estimated to be 40 mg/g and 94%, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5512–5521, 2006 相似文献