全文获取类型
收费全文 | 58418篇 |
免费 | 5434篇 |
国内免费 | 2979篇 |
专业分类
电工技术 | 4152篇 |
技术理论 | 5篇 |
综合类 | 4122篇 |
化学工业 | 8964篇 |
金属工艺 | 3468篇 |
机械仪表 | 3460篇 |
建筑科学 | 5134篇 |
矿业工程 | 1484篇 |
能源动力 | 1758篇 |
轻工业 | 3675篇 |
水利工程 | 1039篇 |
石油天然气 | 3082篇 |
武器工业 | 501篇 |
无线电 | 7201篇 |
一般工业技术 | 7234篇 |
冶金工业 | 2473篇 |
原子能技术 | 761篇 |
自动化技术 | 8318篇 |
出版年
2024年 | 195篇 |
2023年 | 1027篇 |
2022年 | 1540篇 |
2021年 | 2457篇 |
2020年 | 1780篇 |
2019年 | 1472篇 |
2018年 | 1677篇 |
2017年 | 1868篇 |
2016年 | 1655篇 |
2015年 | 2296篇 |
2014年 | 2788篇 |
2013年 | 3433篇 |
2012年 | 3741篇 |
2011年 | 3970篇 |
2010年 | 3597篇 |
2009年 | 3327篇 |
2008年 | 3456篇 |
2007年 | 3303篇 |
2006年 | 3319篇 |
2005年 | 2805篇 |
2004年 | 1992篇 |
2003年 | 1794篇 |
2002年 | 1894篇 |
2001年 | 1683篇 |
2000年 | 1462篇 |
1999年 | 1554篇 |
1998年 | 1152篇 |
1997年 | 1013篇 |
1996年 | 989篇 |
1995年 | 806篇 |
1994年 | 658篇 |
1993年 | 484篇 |
1992年 | 434篇 |
1991年 | 322篇 |
1990年 | 213篇 |
1989年 | 177篇 |
1988年 | 157篇 |
1987年 | 93篇 |
1986年 | 62篇 |
1985年 | 54篇 |
1984年 | 33篇 |
1983年 | 18篇 |
1982年 | 30篇 |
1981年 | 16篇 |
1980年 | 17篇 |
1979年 | 8篇 |
1978年 | 2篇 |
1959年 | 2篇 |
1951年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
以1-甲基咪唑和氯代正丁烷为原料,合成1-丁基-3-甲基咪唑氯盐离子液体;以醋酸锌[Zn(Ac)2]、硫酸锌(ZnSO4)和氯化锌(ZnCl2)为锌源,在1-丁基-3-甲基咪唑氯盐离子液体和丙氨酸体系中与硝酸铈反应,经水热合成法制备得到Ce掺杂的纳米ZnO。采用扫描电子显微镜(SEM)、紫外-可见光吸收光谱(UV-Vis)、X射线衍射仪(XRD)、X射线光电子能谱分析(XPS)和红外光谱(FT-IR)对产品进行表征。以亚甲基蓝(MB)为目标降解物,采用UV-Vis检测,考察了Ce掺杂的纳米ZnO的光催化活性。研究表明,焙烧温度对光催化的晶体结构和光催化活性产生较大的影响;2%Ce/ZnO、焙烧温度为500℃、催化时间为30 min、亚甲基蓝用量0.05 g、pH值为10时降解率可达99.5%以上。 相似文献
52.
本文研究了砂类型、砂率、石粉含量和抗压强度对机制砂混凝土耐磨性的影响,建立了磨损量的多因素计算模型。结果表明:由于含石粉及具有更高的粗糙度和坚固性,石灰岩与辉绿岩机制砂制备的C30、C40混凝土耐磨性比河砂混凝土提高20%以上;在0.40~0.44范围内选取较低的砂率可获得较优的耐磨性;利用石粉含量为5%~11%(质量分数)的机制砂制备混凝土,石粉含量为9%时可获得最佳的混凝土耐磨性,微观分析表明此时混凝土密实度最佳;通过灰色系统理论确定了耐磨性影响因素的影响程度排序为:砂率R3>压碎值R2>粗糙度R1>抗压强度R5>石粉含量R4>0.6;对比验证表明提出的混凝土磨损量多因素计算模型具有较高的预测精度和良好的适用性。 相似文献
53.
Dietary exposure to acrylamide in a group of Japanese adults based on 24-hour duplicate diet samples
Junko Kawahara Yazhi Zheng Miho Terui Akiko Shinohara Kaori Uyama Miyuki Yoneyama 《Food additives & contaminants. Part A, Chemistry, analysis, control, exposure & risk assessment》2019,36(1):15-25
Acrylamide is a probable human carcinogen and known human neurotoxin that can be generated in food through heating. Using a mathematical modelling approach, our previous study estimated long-term average dietary exposure to acrylamide in the Japanese people; however, the validity of these estimates remained unknown. Here, we aimed to obtain a more accurate estimate of acrylamide exposure that would reflect the usual practice of heat processing and consumption of foods in the population. We collected duplicate diet samples and dietary records during 24 h from a group of Japanese adults. A total of 110 duplicate diet samples were analysed for acrylamide by LC-MS/MS. Data from individual dietary records were used to examine the association between dietary acrylamide exposure and consumption of selected food groups (e.g., coffee, tea, confectioneries, and vegetables prepared at high temperature [deep-frying, stir-frying, sautéing, and baking]). Of the 110 homogenised diet samples, 108 contained detectable levels of acrylamide. Dietary exposure to acrylamide ranged from 8 to 1582 ng/kg body weight (bw)/day, with the mean value of 215 ng/kg-bw/day and median value of 143 ng/kg-bw/day. This mean value was higher than the value we previously estimated for Japanese adults using a mathematical approach. Multiple linear regression analysis showed log dietary acrylamide exposure was significantly associated with consumption of coffee and vegetables prepared at high temperature during 24-hr of sampling (adj. R2 = 0.250, p < 0.001). We revealed significant difference in dietary acrylamide exposure between participants who had coffee and vegetables prepared at high temperature (median, 169 ng/kg-bw/day; range, 35–1224 ng/kg-bw/day, n = 42) and those who had none of them (median, 75 ng/kg-bw/day; range, 8–311 ng/kg-bw/day, n = 15) (Steel-Dwass test, p < 0.05). 相似文献
55.
WeiJia Luo LingXia Li Shihui Yu Qianyu Guo Bowen Zhang Zheng Sun 《Ceramics International》2018,44(11):12414-12419
Low-loss (Zn1-xNix)ZrNbTaO8 (0.02?≤?x?≤?0.10) ceramics possessing single wolframite structure are initiatively synthesized by solid-state route. Based on the results of Rietveld refinement, complex chemical bond theory is used to establish the correlation between structural characteristics and microwave performance in this ceramic system. A small amount of Ni2+ (x?=?0.06) in A-site with the fixed substitution of Ta5+ in B-site can effectually raise the Q?×?f value of ZnZrNb2O8 ceramic, embodying a dense microstructure and high lattice energy. The dielectric constant and τf are mainly affected by bond ionicity and the average octahedral distortion. The (Zn0.94Ni0.06)ZrNbTaO8 ceramic sample sintered at 1150?°C for 3?h exhibits an outstanding combination of microwave dielectric properties: εr =?27.88, Q?×?f?=?128,951?GHz, τf =?–39.9?ppm/°C. Thus, it is considered to be a candidate material for the communication device applications at high frequency. 相似文献
56.
57.
58.
Wenjie Wu Yan Liu Dong Liu Wenxing Chen Zhaoyi Song Ximin Wang Yamin Zheng Ning Lu Chunxia Wang Junjie Mao Yadong Li 《Nano Research》2021,14(4):998-1003
The demand for high-performance non-precious-metal electrocatalysts to replace the noble metal-based catalysts for oxygen reduction reaction(ORR)is intensively increasing.Herein,single-atomic copper sites supported on N-doped three-dimensional hierarchically porous carbon catalyst(Cu1/NC)was prepared by coordination pyrolysis strategy.Remarkably,the Cu1/NC-900 catalyst not only exhibits excellent ORR performance with a half-wave potential of 0.894 V(vs.RHE)in alkaline media,outperforming those of commercial Pt/C(0.851 V)and Cu nanoparticles anchored on N-doped porous carbon(CuNPs/NC-900),but also demonstrates high stability and methanol tolerance.Moreover,the Cu1/NC-900 based Zn-air battery exhibits higher power density,rechargeability and cyclic stability than the one based on Pt/C.Both experimental and theoretical investigations demonstrated that the excellent performance of the as-obtained Cu1/NC-900 could be attributed to the synergistic effect between copper coordinated by three N atoms active sites and the neighbouring carbon defect,resulting in elevated Cu d-band centers of Cu atoms and facilitating intermediate desorption for ORR process.This study may lead towards the development of highly efficient non-noble metal catalysts for applications in electrochemical energy conversion. 相似文献
59.
AbstractThe impacts of using radio frequency (RF) energy to dry purple-fleshed potatoes were investigated and compared to infrared radiation (IR) and microwave (MW) drying techniques. The gelatinization rate, color, flavor, morphological, and structural characteristics, thermodynamic properties, as well as antioxidant capacity were examined. The results indicated that the drying time of RF (70?min) and MW (21?min) were shorter than IR (105?min). The gelatinization ratio of MW, RF, and IR were 94.4?±?2.0, 88.3?±?1.4, and 64.5?±?1.1%, respectively. The degree of crystallization of purple-fleshed potato powder decreased with all three drying methods. RF-dried potatoes were able to retain higher polyphenols (loss rate: 4.3%), total anthocyanin (3.7%), and total flavonoid content (35.1%), whereas IR showed the lowest retention. The content of polyphenols also decreased significantly. Furthermore, after RF heating, the scavenging of free radicals was higher compared to MW and IR. 相似文献
60.