全文获取类型
收费全文 | 1323篇 |
免费 | 108篇 |
国内免费 | 5篇 |
专业分类
电工技术 | 20篇 |
综合类 | 2篇 |
化学工业 | 369篇 |
金属工艺 | 25篇 |
机械仪表 | 26篇 |
建筑科学 | 38篇 |
能源动力 | 57篇 |
轻工业 | 184篇 |
水利工程 | 5篇 |
石油天然气 | 1篇 |
无线电 | 121篇 |
一般工业技术 | 255篇 |
冶金工业 | 64篇 |
原子能技术 | 5篇 |
自动化技术 | 264篇 |
出版年
2024年 | 3篇 |
2023年 | 34篇 |
2022年 | 93篇 |
2021年 | 117篇 |
2020年 | 48篇 |
2019年 | 46篇 |
2018年 | 71篇 |
2017年 | 59篇 |
2016年 | 72篇 |
2015年 | 59篇 |
2014年 | 57篇 |
2013年 | 109篇 |
2012年 | 99篇 |
2011年 | 105篇 |
2010年 | 68篇 |
2009年 | 70篇 |
2008年 | 60篇 |
2007年 | 51篇 |
2006年 | 35篇 |
2005年 | 21篇 |
2004年 | 28篇 |
2003年 | 18篇 |
2002年 | 7篇 |
2001年 | 8篇 |
2000年 | 12篇 |
1999年 | 8篇 |
1998年 | 14篇 |
1997年 | 19篇 |
1996年 | 14篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1986年 | 3篇 |
1982年 | 1篇 |
1979年 | 1篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有1436条查询结果,搜索用时 15 毫秒
961.
Giovanni Bittante Riccardo Negrini Matteo Bergamaschi Qianlin Ni Nageshvar Patel Hugo Toledo-Alvarado Alessio Cecchinato 《Journal of dairy science》2021,104(3):3210-3220
The use of sexed semen to produce purebred replacement heifers allows a large proportion of dairy cows to be mated to beef sires, and quantitative and qualitative improvements to be made to beef production from dairy herds. The major dairy and beef breeds are undergoing rapid genetic improvement as a result of more efficient selection methods, prompting a need to evaluate the meat production of crossbred beef × dairy cattle produced using current genetics. As part of a large project involving 125 commercial dairy farms, we evaluated the combined use of purebreeding with sexed semen and crossbreeding with semen from beef sires, particularly double-muscled breeds. A survey of 1,530 crossbred calves revealed that, whereas purebred dairy calves are destined almost exclusively for veal production, beef × dairy crossbred calves are also destined for beef production after fattening on either the dairy farm of birth or by specialized fatteners. In veal production, compared with Belgian Blue–sired calves (taken as the reference), double-muscled INRA 95–sired calves had a lighter slaughter weight (303 vs. 346 kg), but a greater dressing percent (62.3 vs. 58.4%). Limousin (also known as Limousine)–sired calves had a smaller average daily gain (1.26 vs. 1.34 kg/d), and lighter slaughter (314 vs. 346 kg) and carcass weights (182 vs. 201 kg). Last, Simmental-sired calves had a similar growth rate, but lighter carcass weight (177 vs. 201 kg), smaller dressing percentage (55.3 vs. 58.4%), and smaller muscularity scores (3.25 vs. 3.72). In the case of young bulls and heifers fattened on the dairy farm of birth, Belgian Blue–, Piemontese (also known as Piedmontese)-, and Limousin-sired calves performed similarly; the only exception was that Piemontese-sired calves had a greater dressing percentage. Belgian Blue– and Limousin-sired calves performed similarly when fattened by specialized beef producers. In both veal and beef production, the effects of dam breed were less important than sire breed. Considering the entire project, we can conclude that the combined use of sexed semen for purebreeding and conventional beef semen for terminal crossbreeding improves meat production from dairy herds, especially when the sires are double-muscled beef breeds. 相似文献
962.
Md Asaduzzaman Md Azizul Haque Shahin Banasaz Ksenia Morozova Giovanna Ferrentino Matteo Scampicchio 《International Journal of Food Science & Technology》2021,56(10):5325-5333
The capacity of a direct injection mass spectrometer based on proton transfer reaction (PTR-MS) to monitor the transient changes of the volatile organic compounds (VOCs) during dulce de leche production was investigated. We found a correlation between the mass fingerprint and some physio-chemical properties of the dulce de leche samples. The intensity of the selected mass fragments related to VOCs was positively correlated with the changes of solids content but negatively with that of water activity. Rheological and textural changes were also highly correlated with the intensity observed for several mass fragments. Hence, this relationship could be useful to predict the textural and rheological changes during heating a complex fluid. Although not all the correlations between physical changes and VOCs formation reflected a direct cause–effect relationship, but those results demonstrated the suitability of the PTR-MS technique to monitor the quality changes even in complex food mixture during thermal processing. 相似文献
963.
Cognitive radio networks (CRNs) are composed of cognitive, spectrum-agile devices capable of changing their configurations on the fly based on the spectral environment. This capability opens up the possibility of designing flexible and dynamic spectrum access strategies with the purpose of opportunistically reusing portions of the spectrum temporarily vacated by licensed primary users. On the other hand, the flexibility in the spectrum access phase comes with an increased complexity in the design of communication protocols at different layers. This work focuses on the problem of designing effective routing solutions for multi-hop CRNs, which is a focal issue to fully unleash the potentials of the cognitive networking paradigm. We provide an extensive overview of the research in the field of routing for CRNs, clearly differentiating two main categories: approaches based on a full spectrum knowledge, and approaches that consider only local spectrum knowledge obtained via distributed procedures and protocols. In each category we describe and comment on proposed design methodologies, routing metrics and practical implementation issues. Finally, possible future research directions are also proposed. 相似文献
964.
Ottaviani D Parlani C Citterio B Masini L Leoni F Canonico C Sabatini L Bruscolini F Pianetti A 《International journal of food microbiology》2011,144(3):538-545
The distribution of virulence properties in 142 strains of Aeromonas isolated from diarrhoeic patients, food and surface water in Italy and identified by biochemical and molecular methods was investigated. The virulence properties studied were the presence of genes for the aerolysin (aerA), heat-stable cytotonic enterotoxin (ast), heat-labile cytotonic enterotoxin (alt), cytotoxic enterotoxin (act); and cytotoxicity for Vero cells and adhesion on Hep-2 cells. A. hydrophila and A. caviae were the species most commonly isolated from clinical and environmental samples (9/30; 30.0% and 5/27; 18.5%, respectively) while mesophilic A. salmonicida was most common in food samples (19/80; 23.7%). Out of 142 strains, 86 (60.6%) were positive for at least one of the virulence properties. All the toxin genes were present in 4/18 (22.3%) of clinical strains. Most of the food isolates (54/55; 98.2%) were cytotoxic and most of the environmental strains (12/13; 92.3%) were adhesive. The aerA gene was present in most toxigenic strains (72/86; 83.7%), irrespective of their origin. The growth temperature affected the expression of cytotoxicity and adhesivity. Aeromonas strains from food and surface water frequently had toxin gene patterns similar to those of clinical strains and expressed virulence properties at human body temperature. These findings indicate that aeromonads have the potential to cause human illness and confirm the role of food and water as vehicles for Aeromonas diseases. 相似文献
965.
Matteo Camponeschi Andrea Bevilacqua Pietro Andreani 《Analog Integrated Circuits and Signal Processing》2011,67(1):11-20
In this work a 2.2 GHz quadrature receiver front-end suitable for low-power applications is presented. The low-noise amplifier,
the mixer and the voltage-controlled oscillator are merged into a single stage, making the circuit capable of extreme current
reuse while keeping it still functional at low supply voltage. A careful linear time-variant analysis is proven to be necessary
to accurately predict the conversion gain and the bandwidth of the downconverter. A prototype, implemented in a 90 nm CMOS
technology, validates the theoretical analysis, showing 27 dB of downconversion gain over a 14 MHz base-band bandwidth; the
noise figure is 13 dB with a flicker corner frequency of 200 kHz; the input-referred 1 dB compression point is −23.7 dBm.
The circuit draws only 1.3 mA from a 1.0 V supply. 相似文献
966.
The existing literature on price asymmetries does not systematically investigate the sensitivity of the empirical results to the choice of a particular econometric specification. This paper fills this gap by providing a detailed comparison of the three most popular models designed to describe asymmetric price behavior, namely asymmetric ECM, autoregressive threshold ECM and ECM with threshold cointegration. Each model is estimated on a common monthly data set for the gasoline markets of France, Germany, Italy, Spain and UK over the period 1985–2003. All models are able to capture the temporal delay in the reaction of retail prices to changes in spot gasoline and crude oil prices, as well as some evidence of asymmetric behavior. However, the type of market and the number of countries which are characterized by asymmetric oil–gasoline price relations vary across models. The asymmetric ECM prescribes that long-run price asymmetries are most likely to be found in the second stage of the transmission chain. Conversely, the ECM with threshold cointegration suggests that long-run price asymmetries vary across countries and markets. Short-run price asymmetries are captured by the asymmetric ECM specification and the TAR-ECM. The latter model suggests that all European countries are likely to be affected by asymmetries at the distribution stage, while the results obtained with the asymmetric ECM are mixed. 相似文献
967.
Melania Maglio Maria Sartori Alessandro Gambardella Tatiana Shelyakova Valentin Alek Dediu Matteo Santin Yolanda Pieiro Manuel Baobre Lpez Jos Rivas Anna Tampieri Simone Sprio Lucia Martini Alessandro Gatti Alessandro Russo Gianluca Giavaresi Milena Fini 《International journal of molecular sciences》2023,24(1)
The reconstruction of large segmental defects still represents a critical issue in the orthopedic field. The use of functionalized scaffolds able to create a magnetic environment is a fascinating option to guide the onset of regenerative processes. In the present study, a porous hydroxyapatite scaffold, incorporating superparamagnetic Fe3O4 nanoparticles (MNPs), was implanted in a critical bone defect realized in sheep metatarsus. Superparamagnetic nanoparticles functionalized with hyperbranched poly(epsilon-Lysine) peptides and physically complexed with vascular endothelial growth factor (VEGF) where injected in situ to penetrate the magnetic scaffold. The scaffold was fixed with cylindrical permanent NdFeB magnets implanted proximally, and the magnetic forces generated by the magnets enabled the capture of the injected nanoparticles forming a VEGF gradient in its porosity. After 16 weeks, histomorphometric measurements were performed to quantify bone growth and bone-to-implant contact, while the mechanical properties of regenerated bone via an atomic force microscopy (AFM) analysis were investigated. The results showed increased bone regeneration at the magnetized interface; this regeneration was higher in the VEGF-MNP-treated group, while the nanomechanical behavior of the tissue was similar to the pattern of the magnetic field distribution. This new approach provides insights into the ability of magnetic technologies to stimulate bone formation, improving bone/scaffold interaction. 相似文献
968.
Matteo Di Giosia Iryna Polishchuk Eva Weber Simona Fermani Luca Pasquini Nicola M. Pugno Francesco Zerbetto Marco Montalti Matteo Calvaresi Giuseppe Falini Boaz Pokroy 《Advanced functional materials》2016,26(30):5569-5575
Composites, materials composed of two or more materials—metallic, organic, or inorganic—usually exhibit the combined physical properties of their component materials. The result is a material that is superior to conventional monolithic materials. Advanced composites are used in a variety of industrial applications and therefore attract much scientific interest. Here the formation of novel carbon‐based nanocomposites is described via incorporation of graphene oxide (GO) into the crystal lattice of single crystals of calcite. Incorporation of a 2D organic material into single‐crystal lattices has never before been reported. To characterize the resulting nanocomposites, high‐resolution synchrotron powder X‐ray diffraction, electron microscopy, transmission electron microscopy, fluorescence microscopy and nanoindentation tests are employed. A detailed analysis reveals a layered distribution of GO sheets incorporated within the calcite host. Moreover, the optical and mechanical properties of the calcite host are altered when a carbon‐based nanomaterial is introduced into its lattice. Compared to pure calcite, the composite GO/calcite crystals exhibits lower elastic modulus and higher hardness. The results of this study show that the incorporation of a 2D material within a 3D crystal lattice is not only feasible but also can lead to the formation of hybrid crystals exhibiting new properties. 相似文献
969.
In dynamic atomic force microscopy (AFM) the cantilever is vibrated and its dynamics are monitored to probe the sample with nanoscale and atomic resolution. Amplitude and frequency modulation atomic force microscopy (AM-AFM and FM-AFM) have established themselves as the most powerful methods in the field. Nevertheless, it is still debatable whether one or the other technique is preferred in a given medium or experiment. Here, we quantitatively establish and compare the limitations in resolution of both techniques by introducing the concept of spatial horizon (SH) and quantifying it. The SH is the limiting spatial boundary beyond which collective atomic interactions do not affect the detection parameters of a given feedback system. We show that while an FM-AFM feedback can resolve a single atom or atomic defect where an AM feedback might fail, relative contrast is in fact equivalent for both feedback systems. That is, if the AM feedback could detect sufficiently small amplitude shifts and there was no noise, the detection of single atoms or atomic defects would be equivalent in AM-AFM and FM-AFM. 相似文献
970.
In order to explain the unique thermoelectric properties of bulk nanocomposite p-type bismuth antimony telluride, its structural and electrical properties are investigated using transmission electron microscopy (TEM) and atomic force microscopy with a conductive probe (C-AFM). The material is observed to contain both nano- and micro-sized grains with sizes varying from 10 nm to 3 μm. This unique structure promotes phonon scattering, thereby decreasing the thermal conductivity to below 1 W mK(-1) at room temperature. Moreover, the C-AFM data show that the electrical conductivity of nanosized grains is higher than the bulk value and reaches 1600 S cm(-1). This results in a moderate increment of the overall electrical conductivity, thereby increasing the figure of merit (ZT) up to 1.4 at 100 °C. In addition to demonstrating a powerful scanning probe microscopy (SPM) based investigation technique that requires minimal sample preparation, our findings contribute towards better understanding of the enhancement of thermoelectric properties of nanocomposite thermoelectric materials. 相似文献