全文获取类型
收费全文 | 11902篇 |
免费 | 911篇 |
国内免费 | 21篇 |
专业分类
电工技术 | 131篇 |
综合类 | 8篇 |
化学工业 | 2691篇 |
金属工艺 | 431篇 |
机械仪表 | 567篇 |
建筑科学 | 272篇 |
矿业工程 | 10篇 |
能源动力 | 391篇 |
轻工业 | 1162篇 |
水利工程 | 37篇 |
石油天然气 | 21篇 |
无线电 | 1594篇 |
一般工业技术 | 2348篇 |
冶金工业 | 1962篇 |
原子能技术 | 130篇 |
自动化技术 | 1079篇 |
出版年
2023年 | 96篇 |
2022年 | 169篇 |
2021年 | 280篇 |
2020年 | 206篇 |
2019年 | 262篇 |
2018年 | 298篇 |
2017年 | 343篇 |
2016年 | 402篇 |
2015年 | 353篇 |
2014年 | 498篇 |
2013年 | 776篇 |
2012年 | 661篇 |
2011年 | 825篇 |
2010年 | 594篇 |
2009年 | 561篇 |
2008年 | 561篇 |
2007年 | 435篇 |
2006年 | 351篇 |
2005年 | 327篇 |
2004年 | 320篇 |
2003年 | 305篇 |
2002年 | 263篇 |
2001年 | 214篇 |
2000年 | 197篇 |
1999年 | 233篇 |
1998年 | 615篇 |
1997年 | 456篇 |
1996年 | 272篇 |
1995年 | 205篇 |
1994年 | 199篇 |
1993年 | 193篇 |
1992年 | 116篇 |
1991年 | 100篇 |
1990年 | 95篇 |
1989年 | 94篇 |
1988年 | 66篇 |
1987年 | 57篇 |
1986年 | 56篇 |
1985年 | 77篇 |
1984年 | 68篇 |
1983年 | 46篇 |
1982年 | 47篇 |
1981年 | 44篇 |
1980年 | 45篇 |
1979年 | 38篇 |
1978年 | 33篇 |
1977年 | 76篇 |
1976年 | 108篇 |
1975年 | 43篇 |
1973年 | 32篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
The feasibility of a new alloy design concept utilizing the principle of ‘tungsten bronze effect’ is critically evaluated for the development of metallic bipolar plates for proton exchange membrane fuel cell (PEMFC). An austenitic stainless steel (ASS) is modified with W and La to improve the stability of the passive film in an acidic environment as well as to reduce the contact resistance by the tungsten bronze effect. The experimental ASS containing W and La was evaluated in a simulated PEMFC environment of H3PO4 and H2SO4 solutions at 80 °C, and the electrical property was evaluated by performing a contact resistance test. The test results show that the ASS modified with W and La has good passive film stability for corrosion resistance and low contact resistance. The X-ray photoelectron spectroscopy (XPS) analysis clearly suggests the possibility of the tungsten bronze effect from the change in valency state of W6+ to W5+ in the passive film formed on the modified ASS. The feasibility of a new alloy design concept utilizing the ‘tungsten bronze effect’ is well demonstrated; however, more study is highly required for the development of metallic bipolar plates of PEMFC. 相似文献
92.
AG Heriot D Kumar V Thomas M Young J Pilcher AE Joseph 《Canadian Metallurgical Quarterly》1998,85(12):1713-1715
BACKGROUND: The use of fine-needle aspiration cytology (FNAC) in the diagnosis of colonic lesions was investigated. METHODS: Some 22 patients (median age 71 years) with a colonic lesion identified on abdominal ultrasonography underwent ultrasonographically-guided FNAC using a 21-G needle. The sample was checked immediately by a cytopathologist for adequacy. RESULTS: Eighteen patients had colonic carcinoma; aspiration cytology detected malignant epithelial cells consistent with colonic carcinoma in 17 patients and severely dysplastic cells in one patient. The sensitivity and specificity of ultrasonographically-guided FNAC in the diagnosis of colonic carcinoma was 94 and 100 per cent respectively. The remaining four patients had a diagnosis of ileocaecal tuberculosis, ileocaecal Crohn's disease, and metastatic adenocarcinoma in the liver with no identifiable primary (two patients). One demonstrated granulomata, grew acid-fast bacilli and the patient was treated for tuberculosis. One had inflammatory cells and the patient was found to have Crohn's disease on histology. The remaining two patients had confirmed metastatic adenocarcinoma in the liver on aspiration cytology but suspected colonic lesions were found to be benign on cytological examination and no primary lesion was subsequently demonstrated. There were no complications of FNAC and patients complained of minimal discomfort. There has been no evidence of tumour recurrence with a median follow-up of 12 (range 1-25) months. CONCLUSION: Ultrasonographically-guided FNAC is a valid method for the diagnosis of colonic tumours. 相似文献
93.
94.
Two paramount observations exist regarding aneuploidy in human oocytes: its association with maternal age and its more frequent occurrence during meiosis I. Numerous experimental studies have shown that fertilization of postovulatory aged oocytes is coupled with reproductive failure and cytogenetic aberrations in embryos. However, the basic cytogenetic defect(s) of aged oocytes that causes these abnormalities has not been adequately described. The objective of this study was to test the hypothesis that postovulatory oocyte ageing results in increased frequencies of premature centromere separation (PCS) in metaphase II (MII) oocytes and aneuploidy in zygotes. MII oocytes and one-cell zygotes were collected from superovulated mice at different times after ovulation and fertilization. Chromosomes were C-banded and analyzed for structural and numerical aberrations. The frequencies of PCS in oocytes significantly (p < 0.01) increased with time postovulation: 15 h (15 of 529, 2.8%), 20 h (82 of 627, 13.1%), and 25 h (118 of 502, 23.5%). In zygotes, the frequencies of hyperploidy significantly (p < 0.01) increased with time post-fertilization: 0-4 h (0 of 260), 4-8 h (5 of 212, 2.4%), and 8-12 h (8 of 262, 3.1%). These data support the hypothesis that postovulatory ageing results in elevated levels of PCS in oocytes and of aneuploidy in zygotes. The link between PCS and aneuploidy may be random segregation of sister chromatids during anaphase II. 相似文献
95.
The thiazolidinedione analogue troglitazone is an antidiabetic agent that improves insulin resistance in rodents and humans. Although coronary artery disease is common in patients with the insulin resistance syndrome, the effects of troglitazone on smooth muscle cells (SMC) have not been fully elucidated. We therefore examined the effects of troglitazone on cell growth and glucose uptake in human aortic SMC. Mitogen-activated protein (MAP) kinase activity and glucose transporter (Glut) 1 mRNA levels were also studied. In the absence of troglitazone, insulin (10(-7) M) caused a 2-fold increase of DNA synthesis in SMC and troglitazone suppressed the increase of DNA synthesis in a dose-dependent manner. This growth suppression was accompanied by inhibition of MAP kinase activity. On the other hand, troglitazone significantly increased Glut 1 mRNA and enhanced glucose uptake in SMC. These results suggest that troglitazone affects the insulin signaling pathways in SMC and suppresses growth while promoting glucose uptake. Our findings support the application of troglitazone as an inhibitor of SMC proliferation in patients with insulin resistance. 相似文献
96.
J Whittle CJ Lin JR Lave MJ Fine KM Delaney DZ Joyce WW Young WN Kapoor 《Canadian Metallurgical Quarterly》1998,36(7):977-987
OBJECTIVES: The authors describe the relation of provider characteristics to processes, costs, and outcomes of medical care for elderly patients hospitalized for community-acquired pneumonia. METHODS: Using Medicare claims data, Medicare beneficiaries discharged from Pennsylvania hospitals during 1990 with community-acquired pneumonia were identified. Claims data were used to ascertain mortality, readmissions, use of procedures and physician consultations, and the costs of care. The relationship of these measures to provider characteristics was analyzed using regression techniques to adjust for patient characteristics, including comorbidity and microbial etiology. RESULTS: Among 22,294 pneumonia episodes studied, 30-day mortality was 17.0%. After adjusting for patient characteristics, 30-day mortality and readmission rates were unrelated to hospital teaching status or urban location or to physician specialty. Use of procedures and physician consultations was more common and costs were 11% higher among patients discharged from teaching hospitals compared with nonteaching hospitals. Similarly, costs were 15% higher at urban hospitals compared with rural hospitals. General internists and medical subspecialists used more procedures and had higher costs than family practitioners. CONCLUSIONS: Processes and costs of care for community-acquired pneumonia varied by provider characteristics, but neither mortality nor readmission rates did. These differences cannot be explained by clinical variables in the database. Further studies should determine whether less costly patterns of care for pneumonia, and perhaps other conditions, could replace more costly ones without compromising patient outcomes. 相似文献
97.
AR Reilein IS Tint NI Peunova GN Enikolopov VI Gelfand 《Canadian Metallurgical Quarterly》1998,142(3):803-813
We used melanophores, cells specialized for regulated organelle transport, to study signaling pathways involved in the regulation of transport. We transfected immortalized Xenopus melanophores with plasmids encoding epitope-tagged inhibitors of protein phosphatases and protein kinases or control plasmids encoding inactive analogues of these inhibitors. Expression of a recombinant inhibitor of protein kinase A (PKA) results in spontaneous pigment aggregation. alpha-Melanocyte-stimulating hormone (MSH), a stimulus which increases intracellular cAMP, cannot disperse pigment in these cells. However, melanosomes in these cells can be partially dispersed by PMA, an activator of protein kinase C (PKC). When a recombinant inhibitor of PKC is expressed in melanophores, PMA-induced pigment dispersion is inhibited, but not dispersion induced by MSH. We conclude that PKA and PKC activate two different pathways for melanosome dispersion. When melanophores express the small t antigen of SV-40 virus, a specific inhibitor of protein phosphatase 2A (PP2A), aggregation is completely prevented. Conversely, overexpression of PP2A inhibits pigment dispersion by MSH. Inhibitors of protein phosphatase 1 and protein phosphatase 2B (PP2B) do not affect pigment movement. Therefore, melanosome aggregation is mediated by PP2A. 相似文献
98.
High-sensitivity titration calorimetry is used to measure changes in enthalpy, heat capacity and protonation for the binding of captopril to the angiotensin I-converting enzyme (ACE; EC 3.4.15.1). The affinity of ACE to captopril is high and changes slightly with the pH, because the number of protons linked to binding is low. The determination of the enthalpy change at different pH values suggests that the protonated group in the captopril-ACE complex exhibits a heat protonation of approximately -30 kJ/mol. This value agrees with the protonation of an imidazole group. The residues which may become protonated in the complex could be two histidines existing in two active sites, which are joined to the amino acids coordinated to Zn2+. Calorimetric measurements indicate that captopril binds to two sites in the monomer of ACE, this binding being enthalpically unfavorable and being dominated by a large positive entropy change. Thus, binding is favored by both electrostatic and hydrophobic interactions. The temperature dependence of the free energy of binding deltaG degrees is weak because of the enthalpy-entropy compensation caused by a large heat capacity change, deltaCp =-4.3+/-0.1 kJ/K/mol of monomeric ACE. The strong favorable binding entropy and the negative deltaCp indicate both a large contribution to binding due to hydrophobic effects, which seem to originate from dehydration of the ligand-protein interface, and slight conformational changes in the vicinity of the active sites. 相似文献
99.
The molecular basis for the profound inflammatory response and the accumulation of hyaluronan in orbital connective tissues seen in thyroid-associated ophthalmopathy is unknown. Moreover, the link between the orbital manifestations of Graves' disease and those in the pretibial skin, localized dermopathy, has yet to be established. We have reported recently that leukoregulin, an activated T lymphocyte-derived cytokine, dramatically induces hyaluronan synthesis and prostaglandin-endoperoxide H synthase 2 in human orbital fibroblasts in culture. In the current studies, utilizing giant two-dimensional gel electrophoresis, we find that orbital fibroblasts express constitutively a protein profile that distinguishes them from skin fibroblasts derived from the abdominal wall and from the pretibium. We further demonstrate that leukoregulin, when present in culture medium for 16 hr, up-regulates a set of orbital fibroblast proteins not present in untreated cultures or in fibroblasts from the abdominal wall. However, some of the same protein inductions are present in the pretibial fibroblasts. These leukoregulin-induced changes in protein expression are completely blocked by dexamethasone (10 nM). Our findings are the first to identify proteins that appear to be expressed and differentially regulated in an anatomical site-restricted manner in orbital and pretibial fibroblasts and seem to establish a molecular link between fibroblasts from the orbit and those in pretibial skin. 相似文献
100.