全文获取类型
收费全文 | 4817篇 |
免费 | 8篇 |
国内免费 | 2篇 |
专业分类
电工技术 | 3篇 |
化学工业 | 90篇 |
金属工艺 | 13篇 |
机械仪表 | 5篇 |
建筑科学 | 5篇 |
能源动力 | 1篇 |
轻工业 | 17篇 |
石油天然气 | 1篇 |
无线电 | 25篇 |
一般工业技术 | 35篇 |
冶金工业 | 4579篇 |
原子能技术 | 2篇 |
自动化技术 | 51篇 |
出版年
2022年 | 7篇 |
2021年 | 4篇 |
2019年 | 5篇 |
2018年 | 4篇 |
2017年 | 6篇 |
2016年 | 3篇 |
2015年 | 4篇 |
2013年 | 11篇 |
2012年 | 9篇 |
2011年 | 14篇 |
2010年 | 7篇 |
2009年 | 11篇 |
2008年 | 10篇 |
2007年 | 10篇 |
2006年 | 9篇 |
2005年 | 10篇 |
2004年 | 5篇 |
2003年 | 7篇 |
2002年 | 3篇 |
2001年 | 6篇 |
1999年 | 154篇 |
1998年 | 1378篇 |
1997年 | 750篇 |
1996年 | 522篇 |
1995年 | 271篇 |
1994年 | 246篇 |
1993年 | 258篇 |
1992年 | 36篇 |
1991年 | 80篇 |
1990年 | 65篇 |
1989年 | 90篇 |
1988年 | 79篇 |
1987年 | 68篇 |
1986年 | 71篇 |
1985年 | 61篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 23篇 |
1981年 | 31篇 |
1980年 | 49篇 |
1979年 | 3篇 |
1978年 | 10篇 |
1977年 | 124篇 |
1976年 | 268篇 |
1975年 | 7篇 |
1974年 | 2篇 |
1970年 | 3篇 |
1968年 | 2篇 |
1966年 | 2篇 |
1955年 | 3篇 |
排序方式: 共有4827条查询结果,搜索用时 15 毫秒
41.
In Saccharomyces cerevisiae, an HO endonuclease-induced double-strand break can be repaired by at least two pathways of nonhomologous end joining (NHEJ) that closely resemble events in mammalian cells. In one pathway the chromosome ends are degraded to yield deletions with different sizes whose endpoints have 1 to 6 bp of homology. Alternatively, the 4-bp overhanging 3' ends of HO-cut DNA (5'-AACA-3') are not degraded but can be base paired in misalignment to produce +CA and +ACA insertions. When HO was expressed throughout the cell cycle, the efficiency of NHEJ repair was 30 times higher than when HO was expressed only in G1. The types of repair events were also very different when HO was expressed throughout the cell cycle; 78% of survivors had small insertions, while almost none had large deletions. When HO expression was confined to the G1 phase, only 21% were insertions and 38% had large deletions. These results suggest that there are distinct mechanisms of NHEJ repair producing either insertions or deletions and that these two pathways are differently affected by the time in the cell cycle when HO is expressed. The frequency of NHEJ is unaltered in strains from which RAD1, RAD2, RAD51, RAD52, RAD54, or RAD57 is deleted; however, deletions of RAD50, XRS2, or MRE11 reduced NHEJ by more than 70-fold when HO was not cell cycle regulated. Moreover, mutations in these three genes markedly reduced +CA insertions, while significantly increasing the proportion of both small (-ACA) and larger deletion events. In contrast, the rad5O mutation had little effect on the viability of G1-induced cells but significantly reduced the frequency of both +CA insertions and -ACA deletions in favor of larger deletions. Thus, RAD50 (and by extension XRS2 and MRE11) exerts a much more important role in the insertion-producing pathway of NHEJ repair found in S and/or G2 than in the less frequent deletion events that predominate when HO is expressed only in G1. 相似文献
42.
L Mosekilde CC Danielsen CH S?gaard JE McOsker TJ Wronski 《Canadian Metallurgical Quarterly》1995,16(2):223-230
We examined the effect of the pineal neurohormone melatonin (MLT) on protection from viral encephalitis. The antiviral activity of MLT was evaluated in normal mice inoculated with Semliki Forest virus (SFV) and in stressed mice injected with the attenuated non-invasive West Nile virus (WN-25). Administration of MLT (s.c.) daily from 3 days before through 10 days after virus inoculation reduced viremia and significantly postponed the onset of disease and death by 7 to 10 days. Moreover, MLT injection reduced mortality of SFV (10 PFU) inoculated mice from 100% to 44%. In mice inoculated with high dose of SFV (100 PFU), MLT postponed death and reduced mortality by 20%. In all of the surviving mice anti-SFV antibodies were detected 22 days after virus inoculation. Infection of mice stressed by either isolation or dexamethasone injection with WN-25 induced mortality of 75% and 50% respectively, which was reduced by MLT administration to 31% and 25%, respectively. The efficiency of MLT in protecting from lethal viral infections warrants further investigations on its mechanisms of action. 相似文献
43.
We investigated predisposing factors for the development of heterotopic ossification in a retrospective study of 637 hip arthroplasties, of which 484 were unilateral, 62 bilateral and 29 revision operations. The frequency of heterotopic ossification after a primary hip arthroplasty was 57 percent. In a univariate analysis, men, patients with hypertrophic arthrosis, and cemented arthroplasty were all at risk of developing heterotopic ossification. After a multivariate analysis, the male sex and the cemented arthroplasty remained as significant factors. In bilateral operations, the contralateral side developed heterotopic ossification in 82 percent when the primary hip operation had already caused ossification. There was no increase in ossifications after the contralateral operation. Half of the revision operations had an increase of heterotopic ossification from 1 to 4 Brooker classes. 相似文献
44.
AL Michaud JA Herrick JE Duplain JL Manson C Hemann P Ilich RJ Donohoe R Hille WA Oertling 《Canadian Metallurgical Quarterly》1998,4(4):235-256
Fourier transform infrared (FTIR) spectra have been obtained from solution samples of the heterocycles uracil, lumazine, and violapterin and reveal interpretable carbonyl stretching frequencies. Spectra of conjugate bases of lumazine and violapterin demonstrate decreases in these carbonyl stretching frequencies upon ionization. Based on isotopic shifts from amide deuterated analogs, semiempirical QCFF/PI calculations were used to assign the vibrational frequencies in the region 1100-1800 cm-1 observed from samples in dimethylsulfoxide (DMSO) and aqueous solutions to specific normal modes. The observed deuterium shifts and the calculations suggest that, in some cases, N-H bending motions are coupled to the C=O stretching motions of the pyrimidine ring. These data suggest that for lumazine anions a change in solvent can significantly change the mixing of the N-H bending and C=O stretching vibrational motions. This implies that vibrational analysis for lumazine species in relatively noninteracting media like nonpolar solvents, mulls or pellets cannot necessarily be transferred to the system when it is dissolved in a polar, hydrogen-bonding solvent such as water. Although other explanations can be offered, our vibrational analysis suggests that the changes in normal mode composition of the predominantly C=O stretching vibrations of lumazine anion on going from dimethylsulfoxide to water solution are consistent with a change in the predominant tautomer of the heterocycle. This change appears to correspond to a shifting of the location of the remaining acidic proton to a different ring nitrogen atom. This interpretation is of interest in view of recent ab initio calculations which suggest that proton shifts may occur during the hydroxylation of lumazine as mediated by the enzyme xanthine oxidase. 相似文献
45.
46.
JE Eberhart-Phillips PD Frederick RC Baron L Mascola 《Canadian Metallurgical Quarterly》1993,82(5):797-801
OBJECTIVE: To describe the effects of measles in pregnancy using a large case series. METHODS: Pregnant women with measles were identified by county health department records, and their hospital and clinic records were reviewed. When available, records for the infants of case patients were also reviewed. RESULTS: Fifty-eight pregnant women with measles were identified. Thirty-five (60%) were hospitalized for measles, 15 (26%) were diagnosed with pneumonia, and two (3%) died of measles complications. Excluding three induced abortions, 18 pregnancies (31%) ended prematurely; five were spontaneous abortions and 13 were preterm deliveries. All but two of the 18 pregnancies that terminated early did so within 14 days of rash onset. Two term infants were born with minor congenital anomalies, but their mothers had measles late in the third trimester. No newborns were diagnosed with congenital measles. CONCLUSIONS: The incidence of death and other complications from measles during pregnancy may be higher than expected for age-comparable, nonpregnant women. Measles in pregnancy may lead to high rates of fetal loss and prematurity, especially in the first 2 weeks after the onset of rash. 相似文献
47.
B Marchou N Picot P Chavanet JC Auvergnat M Armengaud P Devilliers JE Cerisier FN Marié JL Excler 《Canadian Metallurgical Quarterly》1993,11(14):1383-1385
To determine whether a 3-week hepatitis B (HB) vaccination could achieve protective immunity, 89 healthy non-immunized young adults received three doses of 20 micrograms each of HBs antigen (GenHevac B, Pasteur) and were randomly assigned to schedule A (n = 44): two doses at day 0, one dose at day 21; or schedule B (n = 45): one dose at days 0, 10 and 21. Seroprotection rates (anti-HBs > or = 10 mIU ml-1) for groups A and B respectively were: 23 and 40% at day 21; and 77 and 91% at day 82 (not significant). Anti-HBs geometric mean titres were higher in group B than in group A (p < 0.05) at days 21 (6.4 versus 3.8) and 82 (77.6 versus 33.5). One year after primary vaccination, the seroprotection rate remained as high as 90% in the vaccinees of group B; after boosting all vaccinees had protective levels of anti-HBs antibodies. Thus 3-week HB vaccination with GenHevac B allowed early and durable protective immunity. 相似文献
48.
M Kaefer JE Audia N Bruchovsky RL Goode KC Hsiao IY Leibovitch JH Krushinski C Lee CP Steidle DM Sutkowski BL Neubauer 《Canadian Metallurgical Quarterly》1996,58(2):195-205
Each amino acid is represented by a vector of numerical measurements for the attributes of volume, area, hydrophilicity, polarity, hydrogen bonding, shape, and charge. Inter-residue distances are then calculated according to common metrics, and we introduce a new clustering objective function derived from information-theoretic considerations. The arguments of the function are the inter-object distances of the things to be clustered: in this case the amino acids. By means of approximating the solution of an integer programming problem, then, the residues are partitioned into clusters. The clusters obtained are compared with groups obtained in substitution/mutation studies and found to be similar. Thus, probably the strongest and most objective evidence to date is supplied for believing that physico-chemical properties account for the viability of substitutions and that the important similarities/differences are explained by a relatively small and simple set of properties. 相似文献
49.
B Brismar JE Akerlund S Sj?stedt C Johansson A T?rnqvist B B?ckstrand H B?ng L And?ker PO Gustafsson N Darle M Anger?s A Falk G Tunevall B Kasholm-Tengve T Skau PO Nystr?m T Gasslander A Hagelb?ck B Olsson-Liljequist AE Eklund CE Nord 《Canadian Metallurgical Quarterly》1996,28(5):507-512
The objective of this study is to describe usual medical management and costs associated with recurrent respiratory infections in subjects with chronic obstructive bronchitis in France. A prospective survey was performed in Autumn 1994 on a national sample of private practice pulmonologists (N = 71). Two hundred forty-four patients, presenting at least one infection of the lower respiratory tract, were included. Bronchitis was the most frequent acute exacerbation observed (94%). Pneumonia concerned 9% of the patients. Biological tests, X-rays and pulmonary function tests were prescribed for, respectively, 59, 65 and 45% of the patients. Following the visit, 15 patients were hospitalized (6%). The direct medical cost per acute exacerbation was estimated 3,289 francs (1994 value) of which 60% were hospital-related. An average 10.4 day sick-leave was prescribed to 21% of patients in employment. For those patients, this sick-leave was associated to an extra-cost of 1,264-1,876 francs for Social Security and of 0-2,553 francs out of pocket per episode varying according to their Benefit Regimen. 相似文献
50.
T Higashiguchi PO Hasselgren K Wagner JE Fischer 《Canadian Metallurgical Quarterly》1993,17(4):307-314
The purpose of this study was to determine the etiologic factors of denture stomatitis. Fifteen subjects with clinical evidence of localized simple denture stomatitis, fifteen subjects without clinical signs of denture stomatitis, and forty-five subjects with clinical evidence of generalized simple denture stomatitis were investigated clinically and mycologically. Subjects were evaluated according to age, sex, duration of denture usage, smoking habits, frequency of denture brushing, overnight denture wearing, pH level of saliva and degree of candidal colonization and candidal formation. Salivary samples and swabs were taken from the palate and the mucosal surfaces of the dentures investigated mycologically in order to identify the yeast colonies. Smears were taken from the palate and investigated in order to identify candidal formation. No statistically significant relationship was found between denture stomatitis and age, sex, duration of denture usage, frequency of denture brushing, overnight denture wearing or pH level of saliva. There was however, a statistically significant relationship between denture stomatitis and denture hygiene, smoking habits, candidal colonization and candidal formation. 相似文献