全文获取类型
收费全文 | 2014篇 |
免费 | 83篇 |
国内免费 | 23篇 |
学科分类
工业技术 | 2120篇 |
出版年
2024年 | 5篇 |
2023年 | 10篇 |
2022年 | 30篇 |
2021年 | 47篇 |
2020年 | 43篇 |
2019年 | 45篇 |
2018年 | 44篇 |
2017年 | 28篇 |
2016年 | 44篇 |
2015年 | 58篇 |
2014年 | 62篇 |
2013年 | 136篇 |
2012年 | 101篇 |
2011年 | 117篇 |
2010年 | 100篇 |
2009年 | 85篇 |
2008年 | 108篇 |
2007年 | 95篇 |
2006年 | 106篇 |
2005年 | 102篇 |
2004年 | 90篇 |
2003年 | 98篇 |
2002年 | 48篇 |
2001年 | 45篇 |
2000年 | 27篇 |
1999年 | 31篇 |
1998年 | 43篇 |
1997年 | 40篇 |
1996年 | 30篇 |
1995年 | 30篇 |
1994年 | 19篇 |
1993年 | 26篇 |
1992年 | 30篇 |
1991年 | 20篇 |
1990年 | 21篇 |
1989年 | 21篇 |
1988年 | 13篇 |
1987年 | 10篇 |
1986年 | 13篇 |
1985年 | 11篇 |
1984年 | 6篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1973年 | 6篇 |
1954年 | 3篇 |
排序方式: 共有2120条查询结果,搜索用时 26 毫秒
991.
Alex Bates Sunwook Hwang Santanu Mukherjee Sang C. Lee Osung Kwon Gyeung Ho Choi Sam Park 《International Journal of Hydrogen Energy》2013
Two novel fuel cell designs attempt to improve efficiency and reduce the balance of plant weight by implementing a square hole through the center of the bipolar plates. Air is forced through the square hole for the purpose of oxygen delivery, water removal, and stack cooling. This study demonstrates, for the two novel designs, a more even temperature distribution and hot spots away from the center of the bipolar plates. This reduces the number and size of components required to effectively run the system, thus reducing the weight of the balance of plant. Four simulations are presented in this paper, with inlet gases and initial cell temperature set to 333 K. The maximum temperature for case 1 without cooling is 347.97 K, case 1 with water cooling is 335.29 K, case 2 with forced air cooling is 339.42 K, and case 3 with forced air cooling is 335.13 K. 相似文献
992.
In this paper, we examine an algorithm for estimating the location of packet collisions, in the presence of bandlimited multipath
channels. We propose an improvement to the collision localization algorithm to further enhance its performance to compensate
for the increased impairments of the multipath channel. We then examine the collision localization algorithm in conjunction
with two common wireless LAN standards, 802.11b and 802.11a/g. We show that for the 802.11b standard, the collision localization
algorithm performs well, even in the presence of a multi-path channel. We also show that the 802.11 a/g standards are compatible
with collision localization. However, we will see that the IFFT/FFT operations required to perform OFDM transmission limit
the effectiveness of collision localization. We therefore also investigate collision localization in conjunction with block-based
single carrier transmission, a comparable technology to OFDM, and offers some advantages when used with collision localization.
In addition, we also investigate two applications of collision localization. First, we will show that collision localization
in conjunction with Viterbi decoding with erasures can, in many cases, allow for corrupted packets to be completely recovered
at the receiver. Second, we will also demonstrate that collision localization can be used to combat narrow-band interference,
such as Bluetooth, in 802.11 a/g networks. 相似文献
993.
Thequasi-chemical nonrandom lattice fluid model is capable of describing thermodynamic properties for complex systems containing
associating fluids, polymer, biomolecules and surfactants, but this model fails to reproduce the singular behavior of fluids
in the critical region. In this research, we used the quasi-chemical nonrandom lattice fluid model and combined this model
with a crossover theory to obtain a crossover quasi-chemical nonrandom lattice fluid model which incorporated the critical
scaling laws valid asymptotically close to the critical point and reduced to the original quasi-chemical nonrandom model far
from the critical point. The crossover quasi-chemical nonrandom lattice fluid model showed a great improvement in prediction
of the volumetric properties and second-order derivative properties near the critical region. 相似文献
994.
Anna Shiu Ping Tang Ka Ping Kwong Stephen Wai Cheung Chung Yuk Yin Ho 《Food Additives and Contaminants: Part B: Surveillance Communications》2013,6(1):8-14
Fish is the main source of dietary exposure to methylmercury (MeHg), which is a public health concern owing to its potential neurotoxicity. To evaluate the public health risk, this study estimated the total mercury (tHg) and MeHg exposure from fish intake in Hong Kong secondary school students. Median tHg and MeHg concentrations of 280 samples purchased from different commercial outlets (covering 89 species of whole fish and three types of canned tuna), together with the local food consumption data of secondary school students obtained by semi-quantitative food frequency questionnaire in 2000, were used to estimate dietary exposure from fish intake for the average and high consumer (95th percentile exposure). For tHg, the median concentration was 63 µg kg–1 (range 3–1370 µg kg–1) and estimated exposures ranged 0.5–0.6 µg kg–1 body weight (bw) week–1 for an average consumer and 1.6–1.9 µg kg–1 bw week–1 for a high consumer. For MeHg, median concentration was 48 µg kg–1 (range 3–1010 µg kg–1) and estimated dietary exposures were 0.4–0.5 µg kg–1 bw week–1 for an average consumer and 1.2–1.4 µg kg–1 bw week–1 for a high consumer. These values are below the respective provisional tolerable weekly intake (PTWI) established by the Joint Food and Agriculture Organization/World Health Organization Expert Committee on Food Additives (JECFA). The health risk is greater for high consumers since MeHg exposures may approach or exceed the PTWI when other dietary sources are taken into account. 相似文献
995.
Stephen Wai Cheung Chung Ka Ping Kwong Joan C.W. Yau Waiky W.K. Wong 《Food additives & contaminants. Part A, Chemistry, analysis, control, exposure & risk assessment》2013,30(7):831-840
The aim of this first study was to determine the dietary exposure of antimony, lead, mercury in foodstuffs consumed by secondary school students in Hong Kong. Around 100 composite food items were purchased and then cooked prior to analysis. Antimony was measured by hydrogen generation (HG)/inductively coupled plasma–mass spectrometry (ICP–MS), while lead was determined by ICP–MS. Total mercury was measured by cold vapour atomic absorption spectrometry. The detection limits for antimony, lead and total mercury were 1, 0.6 and 3 µg kg?1, respectively. The dietary intake of antimony, lead and total mercury for an average secondary student were estimated to be 0.252, 1.98 and 0.92 µg (kg bw)?1 week?1, respectively. The dietary intake of antimony, lead and total mercury for high-consumer secondary student were estimated to be 0.567, 5.09 and 2.33 µg (kg bw)?1 week?1, respectively. The main contribution to antimony, lead and mercury were milk, vegetables and seafood, respectively. The Tolerable Daily Intake (TDI) of antimony, as recommended by WHO, is 6 µg (kg bw)?1 week?1, while the Provisional Tolerable Weekly Intakes (PTWI) of lead and mercury, as recommended by JECFA, are 25 and 5 µg (kg bw)?1 week?1, respectively. The estimated exposure values for secondary school students were compared to these safety reference values. For the relevant population, this study confirms the low probability of health risks from these metals via food consumption. 相似文献
996.
Shirley A. Micallef Rachel E. Rosenberg Goldstein Ashish George Laura Ewing Ben D. Tall Marc S. Boyer Sam W. Joseph Amy R. Sapkota 《Food microbiology》2013
Antibiotic-resistant enterococci are important opportunistic pathogens and have been recovered from retail tomatoes. However, it is unclear where and how tomatoes are contaminated along the farm-to-fork continuum. Specifically, the degree of pre-harvest contamination with enterococci is unknown. We evaluated the prevalence, diversity and antimicrobial susceptibilities of enterococci collected from tomato farms in the Mid-Atlantic United States. Tomatoes, leaves, groundwater, pond water, irrigation ditch water, and soil were sampled and tested for enterococci using standard methods. Antimicrobial susceptibility testing was performed using the Sensititre microbroth dilution system. Enterococcus faecalis isolates were characterized using amplified fragment length polymorphism to assess dispersal potential. Enterococci (n = 307) occurred in all habitats and colonization of tomatoes was common. Seven species were identified: Enterococcus casseliflavus, E. faecalis, Enterococcus gallinarum, Enterococcus faecium, Enterococcus avis, Enterococcus hirae and Enterococcus raffinosus. E. casseliflavus predominated in soil and on tomatoes and leaves, and E. faecalis predominated in pond water. On plants, distance from the ground influenced presence of enterococci. E. faecalis from samples within a farm were more closely related than those from samples between farms. Resistance to rifampicin, quinupristin/dalfopristin, ciprofloxacin and levofloxacin was prevalent. Consumption of raw tomatoes as a potential exposure risk for antibiotic-resistant Enterococcus spp. deserves further attention. 相似文献
997.
Hosseini A. Kwong D. Lin C-.Y. Lee B. S. Chen R. T. 《IEEE journal of selected topics in quantum electronics》2010,16(1):61-69
998.
Yujuan Guo Guang Shao Lanning Li Wenhao Wu Ruihong Li Jingjing Li Jian Song Liqin Qiu Mahavir Prashad Fuk Yee Kwong 《Advanced Synthesis \u0026amp; Catalysis》2010,352(9):1539-1553
A new strategy was developed for the synthesis of a valuable class of α‐aminomethylacrylates via the Baylis–Hillman reaction of different aldehydes with methyl acrylate followed by acetylation of the resulting allylic alcohols and SN2′‐type amination of the allylic acetates. Asymmetric hydrogenation of these diverse olefinic precursors using rhodium(Et‐Duphos) catalysts provided the corresponding β2‐amino acid derivatives with excellent enantioselectivities and exceedingly high reactivities (up to >99.5% ee and S/C=10,000). The first hydrogenation of (Z)‐configurated substrates was studied for the synthesis of β2‐amino acid derivatives. The high influence of the substrate geometry and steric hindrance on the reactivity and enantioselectivity was also disclosed for this reaction. This protocol provides a highly practical, facile and scalable method for the preparation of optically pure β2‐amino acids and their derivatives under mild reaction conditions. 相似文献
999.
The effect of epoxidized natural rubber (ENR) with 50 mol% epoxidation (ENR 50) on the processing behaviors, tensile properties, morphology, and thermal properties of linear‐low‐density polyethylene (LLDPE)/soya powder blends was investigated. The LLDPE was blended with various soya powder contents in a Haake internal mixer at 150°C and a rotor speed of 50 rpm for 10 min. The tensile properties were tested by using an Instron tensometer according to ISO 527. The thermal stability of the blends was determined by using a thermogravimetric analyzer (TGA). The tensile strength and elongation at the breakage point were significantly improved by the addition of ENR 50, as evidenced by morphological analysis using scanning electron microscopy (SEM). On the other hand, the tensile modulus increased with soya powder content up to 20% and decreased thereafter. The crystallinity and crystallization temperatures of the blends decreased with the incorporation of ENR, and the thermal stability of the blends was lower with higher soya powder content. However, ENR 50 improved the thermal stability of LLDPE/soya powder blends. J. VINYL ADDIT. TECHNOL., 2010. © 2010 Society of Plastics Engineers 相似文献
1000.
The group combustion of interacting heptanes liquid droplets are numerically simulated by solving two dimensional unsteady laminar Navier-Stokes equations. The unsteady computations for the time-varying vaporization of multi-droplets are carried out with parameters of the Reynolds number (Re), the separation distance (S) between the droplets, and the oxygen mole-fraction. The n-heptane droplets initially at T0 = 300 K are in hot air of 10 atm at Tg = 1250 K. Multi-droplets are staggeringly arranged at a separation distance ranging from 4 to 15 droplet radius. The Reynolds number, based on the droplet diameter and free stream velocity, is varied from Re = 10 to 50. The oxygen mole-fraction of the surrounding air is changed from 15% to 90%. The time variations of the flame structure, the combustion characteristics, and the burning rates are presented and discussed. These results indicated that the staggered arrangement of the multi-droplets induced combustion characteristics distinct from those of a single droplet. The burning rate of the interacting droplets in the staggered arrangement exhibited a relatively strong dependence on the Re, S, and oxygen mole-fraction. The burning rate of the interacting multi-droplets, non-dimensionalized by that of a single droplet, was found as a function of S and Re. 相似文献