全文获取类型
收费全文 | 2762篇 |
免费 | 5篇 |
国内免费 | 3篇 |
专业分类
电工技术 | 3篇 |
化学工业 | 58篇 |
金属工艺 | 7篇 |
机械仪表 | 10篇 |
建筑科学 | 11篇 |
能源动力 | 8篇 |
轻工业 | 71篇 |
水利工程 | 2篇 |
石油天然气 | 1篇 |
无线电 | 21篇 |
一般工业技术 | 44篇 |
冶金工业 | 2484篇 |
原子能技术 | 4篇 |
自动化技术 | 46篇 |
出版年
2022年 | 3篇 |
2021年 | 2篇 |
2020年 | 4篇 |
2019年 | 5篇 |
2018年 | 6篇 |
2017年 | 5篇 |
2016年 | 2篇 |
2014年 | 4篇 |
2013年 | 13篇 |
2012年 | 10篇 |
2011年 | 14篇 |
2010年 | 9篇 |
2009年 | 9篇 |
2008年 | 8篇 |
2007年 | 11篇 |
2006年 | 18篇 |
2005年 | 20篇 |
2004年 | 16篇 |
2003年 | 16篇 |
2002年 | 10篇 |
2001年 | 6篇 |
1999年 | 84篇 |
1998年 | 748篇 |
1997年 | 469篇 |
1996年 | 287篇 |
1995年 | 156篇 |
1994年 | 142篇 |
1993年 | 155篇 |
1992年 | 14篇 |
1991年 | 29篇 |
1990年 | 24篇 |
1989年 | 31篇 |
1988年 | 26篇 |
1987年 | 19篇 |
1986年 | 14篇 |
1985年 | 26篇 |
1984年 | 2篇 |
1983年 | 10篇 |
1982年 | 17篇 |
1981年 | 13篇 |
1980年 | 30篇 |
1979年 | 11篇 |
1978年 | 10篇 |
1977年 | 66篇 |
1976年 | 166篇 |
1975年 | 9篇 |
1974年 | 4篇 |
1973年 | 2篇 |
1971年 | 4篇 |
1954年 | 1篇 |
排序方式: 共有2770条查询结果,搜索用时 15 毫秒
71.
GM Howard TV Nguyen M Harris PJ Kelly JA Eisman 《Canadian Metallurgical Quarterly》1998,13(8):1318-1327
This study was designed to assess the relative contributions of genetic and environmental factors to the variation and covariation of quantitative ultrasound (QUS) measurements and their relationships to bone mineral density (BMD). Forty-nine monozygotic (MZ) and 44 dizygotic (DZ) female twins between 20 and 83 years of age (53 +/- 13 years, mean +/- SD) were studied. Digital (phalangeal) QUS (speed of sound [SOS]) and calcaneal QUS (broadband ultrasound attenuation [BUA] and velocity of sound [VOS]) were measured using a DBM Sonic 1200 ultrasound densitometer and a CUBA ultrasound densitometer, respectively. Femoral neck (FN), lumbar spine (LS), and total body (TB) BMD were measured using dual-energy X-ray absorptiometry. Familial resemblance and hence heritability (proportion of variance of a trait attributable to genetic factors) were assessed by analysis of variance, univariate, and multivariate model-fitting genetic analyses. In both QUS and BMD parameters, MZ twins were more alike than DZ pairs. Estimates of heritability for age- and weight-adjusted BUA, VOS, and SOS were 0.74, 0.55, and 0.82, respectively. Corresponding indices of heritability for LS, FN, and TB BMD were 0.79, 0.77, and 0.82, respectively. In cross-sectional analysis, both BUA and SOS, but not VOS, were independently associated with BMD measurements. However, analysis based on intrapair differences suggested that only BUA was related to BMD. Bivariate genetic analysis indicated that the genetic correlations between BUA and BMD ranged between 0.43 and 0.51 (p < 0.001), whereas the environmental correlations ranged between 0.20 and 0.28 (p < 0.01). While the genetic correlations within QUS and BMD measurements were significant, factor analysis indicates that common genes affect BMD at different sites. Also, individual QUS measurements appear to be influenced by some common sets of genes rather than by environmental factors. Significant environmental correlations were only found for BMD measurements and ranged between 0.50 and 0.65 (p < 0.001). These data suggest that QUS and BMD measurements are highly heritable traits. While it appears that there is a common set of genes influencing both QUS and BMD measurements, specific genes yet to be identified appear to have greater effects than that of shared genes in each trait. 相似文献
72.
H Shinozuka T Ohmura SL Katyal AI Zedda GM Ledda-Columbano A Columbano 《Canadian Metallurgical Quarterly》1996,23(6):1572-1577
Several proteins from culture supernatants of Streptococcus sobrinus were able to bind avidly to Sephadex G-75. The proteins could be partially eluted from the Sephadex by low-molecular-weight alpha-1,6 glucan or fully eluted by 4 M guanidine hydrochloride. Elution profiles were complex, yielding proteins of 16, 45, 58 to 60, 90, 135, and 145 kDa, showing that the wild-type strain possessed multiple glucan-binding proteins. Two mutants of Streptococcus sobrinus incapable of aggregation by high-molecular-weight alpha-1,6 glucan were isolated. One mutant was spontaneous, from a cell suspension to which glucan had been added, whereas the other was induced by ethyl methanesulfonate. Both mutants were devoid of a 60-kDa protein, as shown by gel electrophoresis of culture supernatants and whole cells. Amino acid analysis showed that the 58- to 60-kDa protein and the 90-kDa protein were distinct, although both were N-terminally blocked. Both mutants retained their ability to adhere to glass in the presence of sucrose and to ferment mannitol and sorbitol. Both mutants retained their glucosytransferase activities, as shown by activity gels. Western blots (immunoblots), employing antibody against a glucan-binding protein of Streptococcus mutans, failed to reveal cross-reactivity with S. sobrinus proteins. The results show that even though S. sobrinus produces several proteins capable of binding alpha-1,6 glucans, the 60-kDa protein is probably the lectin needed for glucan-dependent cellular aggregation. 相似文献
73.
74.
Recent work in many laboratories has revealed that cytokines are important mediators of inflammation, host defense, and tissue injury in a variety of neurological diseases. A role for astrocytes and microglia in these diseases has been considered pivotal, since both cell types readily produce and respond to cytokines in vitro and show morphologic and immunocytochemical evidence for activation in vivo. Although much of the work documenting these events has been generated in rodent systems, our laboratory has focused on human cell culture systems to define the nature of the activating signals for human microglia and astrocytes and their responses to activating cytokines and growth factors and evidence for activation. The results have shown that interleukin-1 (IL-1) is a potent activator of human astrocytes and induces cytokines such as tumor necrosis factor alpha and interleukin-6, and is a potent activator of nitric oxide generation in astrocytes. Astrocytes also promote microglial growth and differentiation through production of colony-stimulating factors, an activity that is enhanced following activation with IL-1. This review will summarize the human glia data generated in this and other laboratories and present hypotheses how glia may participate in certain human central nervous system diseases. 相似文献
75.
GM Evins DN Cameron JG Wells KD Greene T Popovic S Giono-Cerezo IK Wachsmuth RV Tauxe 《Canadian Metallurgical Quarterly》1995,172(1):173-179
Since the Latin American cholera epidemic began in 1991, 447 isolates of Vibrio cholerae O1 from the Western Hemisphere have been assayed by multilocus enzyme electrophoresis (MEE) to determine allelic variation among 16 enzyme-encoding genes. Two electrophoretic types (ETs) were identified among toxigenic isolates from Latin America: 323 were ET 4, the ET associated with the Latin American epidemic, and 29 were ET 3. Twenty-three of these ET 3 isolates had a distinctive antimicrobial resistance pattern also seen in isolates imported into the United States from Latin America and Southeast Asia. These resistant isolates had an identical ribotype and nearly identical pulsed-field gel electrophoresis (PFGE) patterns. Most nontoxigenic isolates analyzed were not precursors or descendants of toxigenic epidemic strains. MEE provided a population genetic frame-work for the interpretation of PFGE and ribotype data from the isolates in this study. All three methods identified 2 distinct strains of toxigenic V. cholerae O1 currently epidemic in Latin America. 相似文献
76.
CTLs from patients with Chediak-Higashi syndrome (CHS) are unable to destroy target cells recognized via the TCR. To determine the mechanism responsible for the loss of cytotoxicity, CD8+ CTL clones have been derived from a patient with CHS. Individual CTL clones show poor killing that can be increased in longer assays. However, in the presence of cycloheximide, the small amount of killing observed is abolished, indicating killing arises from newly synthesized proteins, rather than from proteins stored in granules. In this study, we show that the CHS CTL clones express normal levels of the lytic proteins granzyme A, granzyme B, and perforin, which are processed properly during biosynthesis and targeted correctly to giant lytic granules. Despite the difference in size, CHS and normal lytic granules are similar, in that both contain the lysosomal enzyme cathepsin D and the lytic protein granzyme A, and lack the mannose-6-phosphate receptor (MPR). However, unlike normal CTL clones, the CHS CTL clones are unable to secrete their giant granules in which the lytic proteins are stored. After cross-linking the TCR, CHS CTL clones fail to secrete granzyme A, as assayed by both enzyme release and confocal microscopy. We suggest that the defect in CHS lies in a protein that is involved in membrane fusion and is essential for the secretion of lysosomal compartments in certain hemopoietic cells. 相似文献
77.
78.
GM Guiraudon 《Canadian Metallurgical Quarterly》1994,58(4):1254-1261
The surgical treatment of the Wolff-Parkinson-White syndrome made its appearance in 1968 when Dr W. C. Sealy performed the first direct surgical intervention for ablating an accessory connection in a patient with incessant atrioventricular reentrant tachycardia. The surgical approach fell into disfavor in 1990 when catheter ablation using radiofrequency energy was adopted into widespread use. In this presentation, I will attempt to assess the scientific value of the surgical experience using the scholarly tool, the "retrospectroscope," and also to answer the questions, Was it worth it? What was learned? and What was achieved? We conclude that a large body of scientific knowledge and skill was brought to light by this experience and, of even more importance, passed on for best use to the catheter surgeons. 相似文献
79.
80.
AA al-Helw AA Al-Angary GM Mahrous MM al-Dardari 《Canadian Metallurgical Quarterly》1998,15(3):373-382
A procedure was developed for overexpression of Trypanosoma brucei pyruvate kinase in Escherichia coli. The enzyme was purified to near-homogeneity from the bacterial lysate by first removing nucleic acids and contaminating proteins by protamine sulfate precipitation and subsequent passage over a phosphocellulose column. The purified protein is essentially indistinguishable in its physicochemical and kinetic properties from the enzyme purified from trypanosomes. Furthermore, experiments were undertaken to locate the binding site of the allosteric effector fructose 2,6-bisphosphate. Regulation of pyruvate kinase by this effector is unique to trypanosomes and related protozoan organisms. Therefore, a three-dimensional structure model of the enzyme was made, and a putative effector-binding site could be identified in an interdomain cleft. Four residues in this cleft were mutated, and the mutant proteins were produced and purified, using the same methodology as for the wild-type pyruvate kinase. Some mutants showed only minor changes in the activation by the effector. However, substitution of Arg22 by Gly resulted in a 9.2-fold higher S(0.5) for phosphoenolpyruvate and a significantly smaller kcat than the wild-type enzyme. Furthermore, the apparent affinity of this mutant for the allosteric effectors fructose 1,6-bisphosphate and fructose 2,6-bisphosphate was 8.2- and 5.2-fold lower than that of its wild-type counterpart. Effector binding was also affected, although to a lesser extent, in a mutant Phe463Val. These data indicate that particularly residue Arg22, but also Phe463, are somehow involved in the binding of the allosteric effectors. 相似文献