全文获取类型
收费全文 | 2378篇 |
免费 | 96篇 |
国内免费 | 8篇 |
专业分类
电工技术 | 29篇 |
综合类 | 6篇 |
化学工业 | 534篇 |
金属工艺 | 83篇 |
机械仪表 | 52篇 |
建筑科学 | 60篇 |
矿业工程 | 3篇 |
能源动力 | 168篇 |
轻工业 | 185篇 |
水利工程 | 17篇 |
石油天然气 | 5篇 |
无线电 | 236篇 |
一般工业技术 | 584篇 |
冶金工业 | 197篇 |
原子能技术 | 8篇 |
自动化技术 | 315篇 |
出版年
2024年 | 10篇 |
2023年 | 35篇 |
2022年 | 72篇 |
2021年 | 119篇 |
2020年 | 73篇 |
2019年 | 84篇 |
2018年 | 109篇 |
2017年 | 92篇 |
2016年 | 77篇 |
2015年 | 53篇 |
2014年 | 85篇 |
2013年 | 185篇 |
2012年 | 113篇 |
2011年 | 150篇 |
2010年 | 91篇 |
2009年 | 102篇 |
2008年 | 77篇 |
2007年 | 88篇 |
2006年 | 57篇 |
2005年 | 59篇 |
2004年 | 48篇 |
2003年 | 49篇 |
2002年 | 40篇 |
2001年 | 30篇 |
2000年 | 29篇 |
1999年 | 29篇 |
1998年 | 32篇 |
1997年 | 32篇 |
1996年 | 35篇 |
1995年 | 38篇 |
1994年 | 31篇 |
1993年 | 27篇 |
1992年 | 31篇 |
1991年 | 24篇 |
1990年 | 13篇 |
1989年 | 19篇 |
1988年 | 21篇 |
1987年 | 21篇 |
1986年 | 18篇 |
1985年 | 17篇 |
1984年 | 23篇 |
1983年 | 26篇 |
1982年 | 27篇 |
1981年 | 20篇 |
1980年 | 16篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1976年 | 20篇 |
1975年 | 3篇 |
1971年 | 3篇 |
排序方式: 共有2482条查询结果,搜索用时 0 毫秒
91.
Racemic heavy isotope analogs of 1-O-alkyl-sn-glycero-3-phosphocholine (lysoPAF) and 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine (PAF) were prepared for use as internal standards to facilitate quantitative studies based on mass
spectrometry. Starting from pentadencane-1,15-diol andrac-glycerol-1,2-acetonide, a convergent synthesis of 1-O-[16′-2H3]hexadecyl and 1-O-[18′-2H3]octadecylrac-glycero-3-phosphocholine and their acetyl derivatives is described. Three deuterium atoms were introduced at the terminal
position of the 1-O-alkyl group by displacement of thep-toluensulfonyl group from 1-O-alkyl-15′-p-toluensulfonate and 1-O-alkyl-17′-p-toluensulfonate with [2H3]-methylmagnesium iodide. The 1-O-alkyl-17′-p-toluensulfonate was obtained by reaction of the 1-O-alkyl-15′-p-toluensulfonate with allylmagnesium bromide, followed by reductive ozonolysis and treatment withp-toluenesulfonyl chloride. The hydroxyl group at C-2 was protected by a benzyl group and removed at a late stage in the synthesis.
This provided the corresponding lysoderivatives or allowed preparation of racemic PAF by subsequent acetylation of the free
hydroxy group. The phosphocholine moiety was introduced at glycerol C-3 by reaction with bromoethyldichlorophosphate and trimethylamine.
The synthetic compounds were analyzed by FAB/MS and GC/NICIMS. They were shown to contain less than 0.6% protium impurity. 相似文献
92.
Effect of machining parameters in ultrasonic vibration cutting 总被引:1,自引:0,他引:1
The ultrasonic vibration cutting (UVC) method is an efficient cutting technique for difficult-to-machine materials. It is found that the UVC mechanism is influenced by three important parameters: tool vibration frequency, tool vibration amplitude and workpiece cutting speed that determine the cutting force. However, the relation between the cutting force and these three parameters in the UVC is not clearly established. This paper presents firstly the mechanism how these parameters effect the UVC. With theoretical studies, it is established that the tool–workpiece contact ratio (TWCR) plays a key role in the UVC process where the increase in both the tool vibration parameters and the decrease in the cutting speed reduce the TWCR, which in turn reduces both cutting force and tool wear, improves surface quality and prolongs tool life. This paper also experimentally investigates the effect of cutting parameters on cutting performances in the cutting of Inconel 718 by applying both the UVC and the conventional turning (CT) methods. It is observed that the UVC method promises better surface finish and improves tool life in hard cutting at low cutting speed as compared to the CT method. The experiments also show that the TWCR, when investigating the effect of cutting speed, has a significant effect on both the cutting force and the tool wear in the UVC method, which substantiates the theoretical findings. 相似文献
93.
N. Z. Mehdizadeh M. Lamontagne C. Moreau S. Chandra J. Mostaghimi 《Journal of Thermal Spray Technology》2005,14(3):354-361
Plasma-sprayed molten molybdenum particles (∼40 μm in diameter) were photographed impinging at high velocity (∼140 m/s) on
a glass substrate at room temperature. An optical sensor detected thermal radiation emitted by a droplet as it approached
the substrate and activated a time delay unit. After a selected time interval, an Nd:YAG laser was triggered, emitting a 5
ns pulse that provided illumination for a charge-coupled device (CCD) camera to photograph the impacting droplet through a
long-range microscope. By varying the delay before pulsing the laser, different stages of droplet deformation were recorded.
Impacting droplets spread into a thin circular film that ruptured and broke into small fragments. An optical detector recording
thermal radiation from the impacting droplet gave a signal that increased as the droplet spread out, reached a maximum when
the liquid film began to rupture, and decreased as portions of the droplet recoiled because of surface tension and then flew
out of view of the photodetector. 相似文献
94.
W. -M. Chien D. Chandra A. K. Helmy J. Franklin C. J. Rawn 《Journal of Phase Equilibria and Diffusion》2005,26(2):115-123
The solid-state phase transitions in ammonium nitrate (AN)-potassium nitrate (KN) system, and the equilibrium AN-KN phase
diagram have been determined by using differential scanning calorimetry and high-temperature in situ x-ray diffractometry.
Sample preparation was performed in a special “dry room” with very low humidity. A single phase region (AN III) with no phase
transitions to 373 K was observed in the composition range 5 to 20% KN; this is critical for use in air bag gas generators.
The high-temperature KN phase (KN I) has a wide range of stability from 20 to 100 wt.% KN. There are one eutectic, two eutectoid,
three peritectoid, and one congruent transformations in this phase diagram. Two new nonstoichiometric phases were found at
lower temperatures in the mid-composition range between the AN and KN terminal solid solutions. Details of the phase equilibria
are presented. 相似文献
95.
Jin-Seok Park David Nkurunziza Vikash Chandra Roy Truc Cong Ho Sung-Yeoul Kim Seung-Chan Lee Byung-Soo Chun 《International Journal of Food Science & Technology》2022,57(8):5090-5101
This study investigated the effect of sequential extraction techniques and pretreatment methods in enhancing the recovery of high-value products from spent coffee ground. Supercritical carbon dioxide (SC-CO2) deoiling was performed to recover non-polar lipids, and subcritical water (SCW) hydrolysis of deoiled spent coffee ground (SCG) was conducted involving pretreatment with ultrasound (US-SCG), and sample particle size reduction (PS-SCG). SCW temperature, solid/liquid mixing ratio, constant pressure and residence time were studied for their effect on extracting reducing sugars (RS), total phenolic content (TPC), total flavonoid content (TFC), 5-HMF, and furfural. Temperature increase enhanced hydrolysis efficiency and promoted RS dehydration into 5-HMF and furfural. At 180 °C and 40 mg/600 mL, TPC, TFC and RS values increased significantly with ultrasound pretreatment correlating with improved antioxidant activities by DPPH and ABTS assays. Highest amount of 5-HMF and furfural was obtained at 210°C (highest temperature studied) with 44.71 and 2.36 mg/100 g. Chlorogenic acid recovery was the highest in the non-deoiled SCG (15.07 mg/100 g), with no significant difference observed in PS-SCG, suggesting an increase due to its reduced particle size. The results of this study provide comprehensive knowledge on the combined effects of integrated engineering systems to boost the valorisation of SCGs into high-value products. 相似文献
96.
97.
Subhash Chandra Ghosh Senthilkumar Muthaiah Yao Zhang Xiangya Xu Soon Hyeok Hong 《Advanced Synthesis \u0026amp; Catalysis》2009,351(16):2643-2649
Amides are synthesized directly from alcohols and amines in high yields using an in situ generated catalyst from easily available ruthenium complexes such as the (p‐cymene)ruthenium dichloride dimer, [Ru(p‐cymeme)Cl2]2, or the (benzene)ruthenium dichloride dimer, [Ru(benzene)Cl2]2, an N‐heterocyclic carbene (NHC) ligand, and a nitrogen containing L‐type ligand such as acetonitrile. The phosphine‐free catalyst systems showed improved or comparable activity compared to previous phosphine‐based catalytic systems. The in situ generated catalyst from [Ru(benzene)Cl2]2, an NHC ligand, and acetonitrile showed excellent activity toward reactions with cyclic secondary amines such as piperidine and morpholine. 相似文献
98.
Prashant Singh Pradeep Kumar Anju Katyal Rashmi Kalra Sujata K. Dass Satya Prakash Ramesh Chandra 《Catalysis Letters》2010,134(3-4):303-308
In this present work, we report that phosphotungstic acid provides a simple, efficient and environmentally benign route is a two-component one-pot domino Knoevenagel-type condensation/Michael reaction between 4-coumarin derivative and an aldehyde in water as a solvent in shorter duration with high yields. 相似文献
99.
Dilip Chandra Deb Nath Sri Bandyopadhyay Philip Boughton Aibing Yu Darryl Blackburn Chris White 《应用聚合物科学杂志》2010,117(1):114-121
We prepared biodegradable composite films of poly(vinyl alcohol) (PVA) and fly ash (FA) spanning 5, 10, 15, 20, and 25 wt % concentrations by casting aqueous solutions. The tensile strengths of the composite films were increased proportionally via the addition of FA. The strength of the film was enhanced by 193% with 20% FA compared to the neat PVA control. Further addition of FA deviated from the linear trend. The moduli of the composites also increased proportionally with FA addition to 212% at 20 wt % FA addition compared to the control. The percentage strain at break exponentially decreased with the addition of FA. In the dynamic mechanical behavior, the storage and loss moduli both increased with FA content. The tan δ peaks corresponding to the glass‐transition temperature shifted 5–10°C higher above the control sample (73°C). This shift was attributed to a reduction in the mobility of PVA segments because they were anchored by the FA surface. The reductions in mobility manifested in strong interfacial interactions were indicative of hydrogen bonding. Broadening and reduction in the intensities of the stretching and bending peaks of ? OH, ? CH and ? C?O of PVA in the Fourier transform infrared spectra were observed. This suggested that hydrogen bonding was active between the functional groups in the FA and PVA chains. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 相似文献
100.
Effect of Porosity on Structure,Young's Modulus,and Thermal Conductivity of SiC Foams by Direct Foaming and Gelcasting 下载免费PDF全文
Dulal Chandra Jana Govindan Sundararajan Kamanio Chattopadhyay 《Journal of the American Ceramic Society》2017,100(1):312-322
The study demonstrates the aqueous processing of solid‐state‐sintered SiC foams by gelcasting technique. Aside from increasing strength of green bodies, gelcasting monomers were the source of carbon additive which helped in sintering of SiC foams. Sintered foams with the relative density (RD) between 0.44 and 0.11 were processed by direct foaming of SiC slurries followed by gelcasting and sintering. Structural analysis by X‐ray tomography showed the presence of spherical pores with bimodal pore size distribution and the proportion of large size cell and their interconnectivity increased in low RD foams. SEM study revealed that decreased RD resulted in gradual changes in the strut microstructure from the grains with faceted interface to smooth interfaced grains. The analysis of changes in Young's modulus and thermal conductivity with RD were in agreement with the Ashby model for open cell foams. 相似文献