全文获取类型
收费全文 | 1716篇 |
免费 | 26篇 |
国内免费 | 1篇 |
专业分类
电工技术 | 2篇 |
综合类 | 1篇 |
化学工业 | 86篇 |
金属工艺 | 9篇 |
机械仪表 | 5篇 |
建筑科学 | 16篇 |
矿业工程 | 2篇 |
能源动力 | 14篇 |
轻工业 | 57篇 |
水利工程 | 14篇 |
无线电 | 69篇 |
一般工业技术 | 111篇 |
冶金工业 | 1264篇 |
原子能技术 | 14篇 |
自动化技术 | 79篇 |
出版年
2023年 | 5篇 |
2022年 | 13篇 |
2021年 | 16篇 |
2020年 | 29篇 |
2019年 | 22篇 |
2018年 | 12篇 |
2017年 | 11篇 |
2016年 | 13篇 |
2015年 | 10篇 |
2014年 | 15篇 |
2013年 | 24篇 |
2012年 | 15篇 |
2011年 | 36篇 |
2010年 | 19篇 |
2009年 | 18篇 |
2008年 | 20篇 |
2007年 | 19篇 |
2006年 | 16篇 |
2005年 | 18篇 |
2004年 | 13篇 |
2003年 | 15篇 |
2002年 | 10篇 |
2001年 | 9篇 |
2000年 | 17篇 |
1999年 | 49篇 |
1998年 | 347篇 |
1997年 | 256篇 |
1996年 | 132篇 |
1995年 | 68篇 |
1994年 | 66篇 |
1993年 | 83篇 |
1992年 | 17篇 |
1991年 | 23篇 |
1990年 | 19篇 |
1989年 | 16篇 |
1988年 | 20篇 |
1987年 | 9篇 |
1986年 | 17篇 |
1985年 | 17篇 |
1983年 | 11篇 |
1982年 | 11篇 |
1981年 | 22篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 39篇 |
1976年 | 81篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1973年 | 6篇 |
排序方式: 共有1743条查询结果,搜索用时 10 毫秒
131.
N Lydakis-Simantiris P Dorlet DF Ghanotakis GT Babcock 《Canadian Metallurgical Quarterly》1998,37(18):6427-6435
Depletion of Ca2+ and/or Cl- ions from PSII membranes blocks the electron-transfer reactions that precede O2 evolution on the oxidizing side of the enzyme. Illumination of these inhibited preparations at 273 K generates a paramagnetic species that is detectable by low-temperature (T < 20 K) EPR as a signal in the g = 2 region, 90-230 G wide, depending on the treatment that PSII has undergone. This signal has recently been assigned to YZ* in magnetic interaction with the manganese cluster in its S2 state [Gilchrist et al. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 9545-9549]. This view, however, is not universal, owing, in part, to the fact that its spectroscopic properties depend on the preparation and the experimental conditions used for its study and, in part, to uncertainties as to the room temperature behavior of YZ* in inhibited preparations. Here, we report time-resolved and conventional EPR data showing that, at room temperature and at 273 K, YZ* can be accumulated in its 20 G form in high yields in both Ca2+-depleted and acetate-inhibited preparations, and that the kinetics of its decay match the decay kinetics of the low-temperature signal generated in corresponding samples. The properties of the YZ* signal, however, are shown to depend on the polypeptide content, the temperature, and the electron donors and acceptors present in the sample under examination. Our results support assignment of the EPR signal in inhibited preparations to S2 YZ* and demonstrate a protective role of the 17 and 23 kDa extrinsic polypeptides for the manganese cluster against externally added reductants. 相似文献
132.
TD Bradshaw S Wrigley DF Shi RJ Schultz KD Paull MF Stevens 《Canadian Metallurgical Quarterly》1998,77(5):745-752
OBJECTIVE: To evaluate the results of cervical cytology screening in the National Breast and Cervical Cancer Early Detection Program and to compare the findings with results from other screening programs. METHODS: We analyzed data on 312,858 women aged 18 years and older who received one or more Papanicolaou smears, and follow-up if indicated, from October 1991 through June 1995 at screening sites across the United States providing comprehensive National Breast and Cervical Cancer Early Detection Program services. RESULTS: Of the women screened, more than half were 40 years or older; slightly less than half (44%) were of racial and ethnic minorities. During the first screening cycle, 3.8% of Papanicolaou tests were reported as abnormal (squamous intraepithelial lesion [SIL] or squamous cell cancer); proportions of abnormals decreased with increasing age. The age-adjusted rate of biopsy-confirmed cervical intraepithelial neoplasia (CIN) II or worse among women screened was 7.4 per 1000 Papanicolaou tests; rates of CIN were highest among young women, but cancer rates peaked among women in their 50s and 60s. The percentages of first screening cycle-Papanicolaou tests interpreted as high-grade SIL and squamous cell carcinoma associated with biopsy-confirmed CIN II or worse (the positive predictive value) were 56.0% for CIN II/III and 3.7% for invasive cancer. Of the 150 invasive cancers diagnosed, 54.0% were classified as local disease. CONCLUSION: Observed results emphasize the duality of cervical neoplasia-CIN in younger women and invasive cancer in older women. This finding points to the importance of reaching both younger and older women for cervical cancer screening. 相似文献
133.
In this paper, the isothermal crystallization of nylon 6 in the presence of Kevlar 129 fiber was investigated by polarized optical microscopy (POM). The formation of a transcrystalline domain was found to be mainly controlled by crystallization conditions, such as the temperature of the isothermal crystallization, residual time at melting temperature and the cooling rate of the melt. The nucleation rate of nylon 6 on the fibers was mainly affected by the crystallization temperature. The interfacial transcrystallinity of nylon 6 occurred on the surface of Kevlar 129 fiber in the temperature range 130–190 °C. The reason for the formation of interfacial transcrystalline morphology is discussed from the molecular level, based on the understanding of the packing mode of nylon 6 chains around fibers and the interaction between matrix and fibers. It was found that the lattice matching and hydrogen‐bonding between nylon 6 and poly(p‐phenylene terephthalamide) (PPTA) crystals play an important role in the epitaxial crystallization. Copyright © 2004 Society of Chemical Industry 相似文献
134.
D Awasthi DF Church D Torbati ME Carey WA Pryor 《Canadian Metallurgical Quarterly》1997,47(6):575-81; discussion 581-2
BACKGROUND: Free radicals may be involved in the pathophysiology of traumatic brain injury (TBI) through oxidative damage of neurovascular structures. Endogenous antioxidants, such as ascorbate and alpha-tocopherol, may play a critical role in combating these oxidative reactions and their oxidized products can serve as an important index of oxidative stress. METHODS: We used electron spin resonance (ESR) spectroscopy and in vivo spin trapping (reaction of an organic compound with free radical species) to detect the possible generation of free radicals after TBI. Injury was inflicted by a weight drop technique over the head (5.7 kg-cm). Rats were intravenously infused with either 1 mL, 0.1 M of the spin trap, alpha-phenyl-N-tert-butyl nitrone (PBN), or an equivalent volume of saline immediately before TBI or sham-injury. Animals were divided into four groups: (1) Group I: PBN-infused sham-injured, (2) Group II: PBN-infused injured, (3) Group III: saline-infused sham-injured, and (4) Group IV: saline-infused injured. Additional groups of saline-infused uninjured, saline-infused, and PBN-infused injured animals were used for histopathology. Sixty minutes after TBI or sham-injury, rats were again anesthetized and decapitated. The brains were removed within 1 minute, homogenized, and extracted for lipids. The extracts were analyzed by ESR spectroscopy. Brain ascorbic acid (AA) concentration was determined spectrophotometrically, using the ascorbate oxidase assay. RESULTS: No PBN spin adduct signals (indicating trapped free radical species) were visible 60 minutes after TBI. All groups of rats showed an ascorbyl free radical signal. The ascorbyl signal intensity (AI) was, however, significantly higher in the injured rats, while the brain (AA) was significantly reduced. In addition, the ratio of AI/AA, which eliminates the effect of variable ascorbate concentrations in the brain, was also significantly higher in the injured animals. CONCLUSIONS: We conclude that 60 minutes following TBI there was a significantly increased level of oxidative stress in the brain. This may reflect formation of free radical species with subsequent interaction with ascorbate (antioxidant) during the 60 minute period. The lack of PBN spin adduct signals 1 hour after TBI may indicate that free radical generation is time dependent and might be detectable earlier or later than the 60 minute period. 相似文献
135.
MJ Sherratt DF Holmes CA Shuttleworth CM Kielty 《Canadian Metallurgical Quarterly》1997,29(8-9):1063-1070
We have applied scanning transmission electron microscopy to intact native fibrillin-containing microfibrils isolated from foetal bovine elastic tissues in order to derive new insights into microfibril organisation. This technique provides quantitative data on the mass per unit length and axial mass distribution of unstained, unshadowed macromolecules. Scanning transmission electron microscopy of microfibrils from aorta, skin and nuchal ligament revealed that the beads corresponded to peaks of mass and the interbead regions to troughs of mass. These major features of axial mass distribution were characteristic of all microfibrils examined. Tissue-specific and age-dependent variations in mass were identified in microfibrils that were structurally comparable by rotary shadowing electron microscopy. Increased microfibril mass correlated with increasing gestational age. The additional mass was associated predominantly at, or close to, the bead. Some microfibril populations exhibited pronounced assymetry in their axial mass distribution. These data indicate that intact native microfibrillar assemblies from developing elastic tissues are heterogeneous in composition. Loss of mass following chondroitinase ABC or AC lyase treatment confirmed the presence of chondroitin sulphate in nuchal ligament microfibrillar assemblies. 相似文献
136.
Harry Cohen Gongkang Fu Wassem Dekelbab Fred Moses 《Canadian Metallurgical Quarterly》2003,8(5):312-322
Truck weight-limit regulations have significant influence on truck operating weights. These regulations directly influence loads applied to highway facilities, such as bridges and pavements. “Truck weight” herein collectively refers to a vehicle’s gross weight, axle weights, and axle configuration. Truck load spectra as a result of truck weight limits are important to bridge engineering in many respects, such as that of determining requirements for evaluation and design of bridges for both strength and fatigue. This paper’s objective is to present a new method for predicting truck weight spectra resulting from a change in truck weight limits. This method is needed to estimate impacts of the change on highway bridges such as accelerated fatigue accumulation. Historical and recent truck weight data are used to test and illustrate the proposed method, and the results show its good prediction capability. This method is also applied here to an example of estimating the impact on steel bridge fatigue due to a possible increase in the gross-vehicle-weight limit from 356 kN (80 kips) on five axles to 431 kN (97 kips) on six axles. Also included is an investigation of the AASHTO fatigue truck model for steel bridge evaluation. Results show that the current fatigue truck model may become invalid under the studied scenario of truck weight-limit increase. 相似文献
137.
The bispectrum of complex signals: definitions and properties 总被引:4,自引:0,他引:4
The symmetry properties and the relationship between all forms of third-order cumulants of complex signals are investigated. It is shown that all cumulants (for different position of the complex conjugate) are related by simple transformations. Autoregressive modeling of complex-valued signals using third-order cumulants is also investigated. It is shown that modeling of complex-valued signals requires a different approach from modeling of real-valued signals 相似文献
138.
139.
The differential diagnosis of all patients with altered mental status must include drug toxicity. In particular, intentional or unintentional overdosing and/or poisoning are common emergency department presenting complaints. A comprehensive approach to managing these patients must incorporate aggressive information gathering, a careful physical examination looking for toxic syndromes, and diagnostic testing. Proper decontamination is the key to effective management, as is the use of specific antidotes when indicated. 相似文献
140.
Stoica P. Moses R.L. Soderstrom T. Li J. 《Signal Processing, IEEE Transactions on》1991,39(6):1360-1368
The asymptotic properties of sinusoidal frequency estimators based on the high-order Yule-Walker (HOYW) equations were analyzed recently. The results of that analysis are used to propose two classes of frequency estimators; one class uses singular value decomposition, and the other uses a sparse matrix solution. Both classes entail two estimation steps: the first step generates initial estimates which are used to obtain an optimal weighting matrix, and the second step generates an optimally weighted estimate. Each two-step method produces asymptotically minimum variance estimates over all estimators of their class. The implementation of the proposed estimators is described in detail, and numerical examples are presented to evaluate their performance 相似文献