全文获取类型
收费全文 | 3480篇 |
免费 | 60篇 |
国内免费 | 2篇 |
专业分类
电工技术 | 33篇 |
综合类 | 1篇 |
化学工业 | 365篇 |
金属工艺 | 65篇 |
机械仪表 | 47篇 |
建筑科学 | 91篇 |
矿业工程 | 4篇 |
能源动力 | 39篇 |
轻工业 | 226篇 |
水利工程 | 14篇 |
石油天然气 | 6篇 |
无线电 | 349篇 |
一般工业技术 | 353篇 |
冶金工业 | 1668篇 |
原子能技术 | 51篇 |
自动化技术 | 230篇 |
出版年
2021年 | 23篇 |
2019年 | 16篇 |
2018年 | 29篇 |
2017年 | 29篇 |
2016年 | 39篇 |
2015年 | 21篇 |
2014年 | 35篇 |
2013年 | 110篇 |
2012年 | 70篇 |
2011年 | 92篇 |
2010年 | 53篇 |
2009年 | 70篇 |
2008年 | 93篇 |
2007年 | 93篇 |
2006年 | 85篇 |
2005年 | 70篇 |
2004年 | 89篇 |
2003年 | 61篇 |
2002年 | 79篇 |
2001年 | 59篇 |
2000年 | 53篇 |
1999年 | 95篇 |
1998年 | 483篇 |
1997年 | 242篇 |
1996年 | 193篇 |
1995年 | 96篇 |
1994年 | 102篇 |
1993年 | 113篇 |
1992年 | 43篇 |
1991年 | 53篇 |
1990年 | 43篇 |
1989年 | 44篇 |
1988年 | 42篇 |
1987年 | 39篇 |
1986年 | 43篇 |
1985年 | 48篇 |
1984年 | 28篇 |
1983年 | 40篇 |
1982年 | 25篇 |
1981年 | 40篇 |
1980年 | 31篇 |
1979年 | 19篇 |
1978年 | 30篇 |
1977年 | 63篇 |
1976年 | 107篇 |
1975年 | 19篇 |
1973年 | 14篇 |
1972年 | 14篇 |
1971年 | 20篇 |
1969年 | 14篇 |
排序方式: 共有3542条查询结果,搜索用时 281 毫秒
51.
R Hui JR Ball RD Macmillan FS Kenny OW Prall DH Campbell AL Cornish RA McClelland RJ Daly JF Forbes RW Blamey EA Musgrove JF Robertson RI Nicholson RL Sutherland 《Canadian Metallurgical Quarterly》1998,17(8):1053-1059
The EMS1 and CCND1 genes at chromosome 11q13 are amplified in about 15% of primary breast cancers but appear to confer different phenotypes in ER positive and ER negative tumours. Since there are no published data on EMS1 expression in large series of breast cancers we examined the relationship of EMS1 expression with EMS1 gene copy number and expression of mRNAs for cyclin D1 and ER. In a subset of 129 patients, where matched tumour RNA and DNA was available, EMS1 mRNA overexpression was associated predominantly with gene amplification (P = 0.0061), whereas cyclin D1 mRNA overexpression was not (P = 0.3142). In a more extensive series of 351 breast cancers, there was no correlation between cyclin D1 and EMS1 expression in the EMS1 and cyclin D1 overexpressors (P = 0.3503). Although an association between EMS1 mRNA expression and ER positivity was evident (P = 0.0232), when the samples were divided into quartiles of EMS1 or cyclin D1 mRNA expression, the increase in the proportion of ER positive tumours in the ascending EMS1 mRNA quartiles was not statistically significant (P = 0.0951). In marked contrast there was a significant stepwise increase in ER positivity in ascending quartiles of cyclin D1 mRNA (P = 0.030). A potential explanation for this difference was provided by the observation that in ER positive breast cancer cells oestradiol treatment resulted in increased cyclin D1 gene expression but was without effect on EMS1. The relationship between EMS1 expression and clinical outcome was examined in a subset of 234 patients with median follow-up of 74 months. High EMS1 expression was associated with age > 50 years (P = 0.0001), postmenopausal status (P = 0.0008), lymph node negativity (P = 0.019) and an apparent trend for worse prognosis in the ER negative subgroup. These data demonstrate that overexpression of EMS1 mRNA is largely due to EMS1 gene amplification, is independent of cyclin D1 and ER expression and, in contrast to cyclin D1, is not regulated by oestrogen. Independent overexpression of these genes may confer different phenotypes and disease outcomes in breast cancer as has been inferred from recent studies of EMS1 and CCND1 gene amplification. 相似文献
52.
53.
Bovine rotavirus strain B223 is the North American prototype for group A P8[11] serotype/genotype rotaviruses. Rotaviruses with this serotype/genotype are a prevalent cause of neonatal calf diarrhea and have also been isolated from asymptomatic human infants. On the basis of deduced amino acid sequence of the outer capsid viral protein 4 (VP4), strain B223 lacks a cysteine at position 318 that is conserved among all other rotavirus strains. It has been speculated that this may result in the loss of disulfide bond and a change in the structure of the VP4 that may affect the infectivity and antigenicity of the virus. This paper describes partial sequences of the VP4 gene (nucleotides 613 to 1016 coding for amino acid positions 202 to 335) of 16 bovine rotavirus field isolates with P8[11] that were obtained from calves in Nebraska and Indiana. All the isolates lack a cystein at position 203 and had a conserved valine residue at position 318, thus indicating that the prototype strain B223 is representative of group A rotaviruses with P8[11] serotype/genotype. 相似文献
54.
Head and neck squamous cell carcinoma (HNSCC) is an aggressive malignancy in which multiple independent lesions develop over time throughout the mucosa of the upper aerodigestive tract. Therefore, the comprehensive treatment of this neoplasm must include a chemopreventive arm to hold premalignant lesions in check, a role well-suited to antiangiogenic agents. Retinoic acid (RA) and interferon alpha (IFN-alpha), drugs with known biological activity against HNSCC when used individually, are also inhibitors of angiogenesis. Here we show that they are remarkably synergistic antiangiogenic agents able to inhibit both the growth and the neovascularization of HNSCC injected into the floor of the mouth of nude mice. The mechanism of action of these drugs as antiangiogenic agents was 2-fold. They decreased the angiogenic activity of the tumor cells, and they caused the endothelial cells to become refractory to inducers of angiogenesis. When tumor cells were treated in vitro with IFN-alpha A/D, there was a dramatic drop in their secretion of interleukin-8, the major angiogenic factor produced by these tumors. When combined with RA, which causes tumor cells to secrete an inhibitor of angiogenesis, there was a synergistic inhibition of both tumor cell growth and secreted angiogenic activity. The combination of RA and IFN-alpha also acted synergistically on endothelial cells by reducing their responsiveness to both interleukin-8 and tumor conditioned media. Doses of each drug could be reduced by two logs without loss of activity. When animals bearing human HNSCC tumor cells were treated systemically with a combination of RA and IFN-alpha A/D at doses that were ineffective when used alone, dramatic decreases in both tumor growth and tumor angiogenesis were seen. These data suggest that the use of antiangiogenic mixtures may be a particularly effective way to design future chemoprevention protocols against HNSCC. 相似文献
55.
It is clear that the initial analysis of visual motion takes place in the striate cortex, where directionally selective cells are found that respond to local motion in one direction but not in the opposite direction. Widely accepted motion models postulate as inputs to directional units two or more cells whose spatio-temporal receptive fields (RFs) are approximately 90 degrees out of phase (quadrature) in space and in time. Simple cells in macaque striate cortex differ in their spatial phases, but evidence is lacking for the varying time delays required for two inputs to be in temporal quadrature. We examined the space-time RF structure of cells in macaque striate cortex and found two subpopulations of (nondirectional) simple cells, some that show strongly biphasic temporal responses, and others that are weakly biphasic if at all. The temporal impulse responses of these two classes of cells are very close to 90 degrees apart, with the strongly biphasic cells having a shorter latency than the weakly biphasic cells. A principal component analysis of the spatio-temporal RFs of directionally selective simple cells shows that their RFs could be produced by a linear combination of two components; these two components correspond closely in their respective latencies and biphasic characters to those of strongly biphasic and weakly biphasic nondirectional simple cells, respectively. This finding suggests that the motion system might acquire the requisite temporal quadrature by combining inputs from these two classes of nondirectional cells (or from their respective lateral geniculate inputs, which appear to be from magno and parvo lateral geniculate cells, respectively). 相似文献
56.
57.
58.
MR Macdonald JL Freeman MF Hui RK Cheung P Warde NP McIvor J Irish HM Dosch 《Canadian Metallurgical Quarterly》1995,17(6):487-493
The crystal structure of the peptide Boc-Phe-Val-OMe determined by X-ray diffraction methods is reported in this paper. The crystals grown from aqueous methanol are orthorhombic, space group P2(1)2(1)2(1),a = 11.843(2), b = 21.493(4), c = 26.676(4) A3 and V = 6790 A3. Data were collected on a CAD4 diffractometer using MoK alpha radiation (lambda = 0.7107 A) up to Bragg angle theta = 26 degrees. The structure was solved by direct methods and refined by a least-squares procedure to an R value of 6.8% for 3288 observed reflections. There are three crystal-lographically independent peptide molecules in the asymmetric unit. All the three molecules exhibit extended conformation. The sidechain of the Val2 residue shows two different conformations. The conformation of the peptide Boc-Phe-Val-OMe is compared with the conformation of Ac-delta Phe-Val-OH. It is observed that while Boc-Phe-Val-OMe exhibits an extended conformation, Ac-delta Phe-Val-OH shows a folded conformation. The results of this comparison highlight the conformation constraining property of the delta Phe residue. Interestingly, even though Boc-Phe-Val-OMe and Ac-delta Phe-Val-OH are conformationally different, they exhibit similar packing patterns in the solid state. 相似文献
59.
60.
R Kneen NG Pham T Solomon TM Tran TT Nguyen BL Tran J Wain NP Day TH Tran CM Parry NJ White 《Canadian Metallurgical Quarterly》1998,27(4):845-850
PURPOSE: To review the University of Florida experience in treating ependymomas, analyze prognostic factors, and provide treatment recommendations. METHODS AND MATERIALS: Forty-one patients with ependymoma and no metastases outside the central nervous system received postoperative radiotherapy with curative intent between 1966 and 1989. Ten patients had supratentorial lesions, 22 had infratentorial lesions, and 9 had spinal cord lesions. All patients had surgery (stereotactic biopsy, subtotal resection, or gross total resection). Most patients with high-grade lesions received radiotherapy to the craniospinal axis. Low-grade intracranial lesions received more limited treatment. Spinal cord lesions were treated using either partial spine or whole spine fields. RESULTS: Of 32 intracranial tumors, 21 recurred, all at the primary site; no spinal cord tumors recurred. Overall 10-year survival rates were 51% (absolute) and 46% (relapse-free); by tumor site: spinal cord, 100%; infratentorial, 45%; supratentorial, 20% (p = 0.002). On multivariate analysis, tumor site was the only factor that influenced absolute survival (p = 0.0004); other factors evaluated included grade, gender, age, duration of symptoms, resection extent, primary tumor dose, treatment field extent, surgery-to-radiotherapy interval, and days under radiotherapy treatment. CONCLUSIONS: Patients with supratentorial or infratentorial tumors receive irradiation, regardless of grade. Craniospinal-axis fields are used when spinal seeding is radiographically or pathologically evident. Spinal cord tumors are treated using localized fields to the primary site if not completely resected. Failure to control disease at the primary site remains the main impediment to cure. 相似文献