全文获取类型
收费全文 | 126326篇 |
免费 | 13903篇 |
国内免费 | 7693篇 |
专业分类
电工技术 | 9131篇 |
技术理论 | 3篇 |
综合类 | 11217篇 |
化学工业 | 15093篇 |
金属工艺 | 8778篇 |
机械仪表 | 10951篇 |
建筑科学 | 17578篇 |
矿业工程 | 4399篇 |
能源动力 | 4000篇 |
轻工业 | 10036篇 |
水利工程 | 5369篇 |
石油天然气 | 6156篇 |
武器工业 | 2105篇 |
无线电 | 10016篇 |
一般工业技术 | 14819篇 |
冶金工业 | 4900篇 |
原子能技术 | 1601篇 |
自动化技术 | 11770篇 |
出版年
2024年 | 285篇 |
2023年 | 2020篇 |
2022年 | 3438篇 |
2021年 | 4372篇 |
2020年 | 4656篇 |
2019年 | 4011篇 |
2018年 | 3789篇 |
2017年 | 4751篇 |
2016年 | 4984篇 |
2015年 | 5307篇 |
2014年 | 7655篇 |
2013年 | 8105篇 |
2012年 | 9596篇 |
2011年 | 10065篇 |
2010年 | 7253篇 |
2009年 | 7331篇 |
2008年 | 6715篇 |
2007年 | 8150篇 |
2006年 | 7421篇 |
2005年 | 6272篇 |
2004年 | 5139篇 |
2003年 | 4294篇 |
2002年 | 3684篇 |
2001年 | 3109篇 |
2000年 | 2572篇 |
1999年 | 2139篇 |
1998年 | 1746篇 |
1997年 | 1452篇 |
1996年 | 1383篇 |
1995年 | 1134篇 |
1994年 | 938篇 |
1993年 | 746篇 |
1992年 | 629篇 |
1991年 | 408篇 |
1990年 | 420篇 |
1989年 | 297篇 |
1988年 | 250篇 |
1987年 | 142篇 |
1986年 | 145篇 |
1985年 | 118篇 |
1984年 | 106篇 |
1983年 | 71篇 |
1982年 | 67篇 |
1981年 | 39篇 |
1980年 | 100篇 |
1979年 | 76篇 |
1965年 | 41篇 |
1964年 | 53篇 |
1959年 | 49篇 |
1955年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
《Journal of dairy science》2022,105(6):5471-5492
The detection of reproductive tract disease (RTD) 3 wk postpartum is important because of its effect on subsequent reproductive outcomes. Numerous methods for the diagnosis of RTD are described, some of which are more practical and instantaneous in terms of diagnosis. Two of these methods involve identification of purulent vaginal discharge (PVD) and evidence of ultrasonographic uterine changes indicative of endometritis (UE). The objectives of our retrospective observational study were (1) to assess the association of PVD or UE score at the prebreeding examination (PBE) with the hazard of pregnancy within the subsequent breeding season; (2) to determine the test sensitivity (Se) and specificity (Sp) at the point of sampling of both tests using a Bayesian latent class model; and (3) to determine the effect of varying positivity thresholds on test accuracy. To achieve these objectives, we analyzed an initial data set of 5,049 PBE from 2,460 spring-calved cows in 8 herds between 2014 and 2018. Each PBE was conducted once between 25 and 86 d in milk. At each PBE, vaginal discharge was obtained with a Metricheck device (Simcro) whereas uterine contents were assessed using transrectal ultrasonography. Purulent vaginal discharge was scored on a scale of 0 to 3 depending on discharge character, and UE was scored on a scale of 0 to 4 depending on the presence and consistency of intraluminal fluid. Cows with scores of ≥2 in either test had received treatment. Fertility data were available from 4,756 PBE after data exclusion. The association between PVD or UE score at the PBE and subsequent hazard of pregnancy was analyzed using a Cox proportional hazards model. Cows with a PVD score of 2 or 3 were less likely to conceive than cows with a PVD score 0 [score 2 hazard ratio (HR) = 0.74; 95% confidence interval (CI): 0.59–0.94; score 3 HR = 0.65; 95% CI: 0.51–0.84]. Cows with a UE score of 1, 2, 3, or 4 were less likely to conceive than cows with a UE score of 0 (score 1 HR = 0.82; 95% CI: 0.73–0.93; score 2 HR = 0.79; 95% CI: 0.62–1.00; score 3 HR = 0.43; 95% CI: 0.43–0.90; score 4 HR = 0.39; 95% CI: 0.26–0.58). To determine the Se and Sp of PVD or UE score for diagnosis of RTD at the time of PBE, a Bayesian latent class model was fitted on 2,460 individual cow PBE. Flat priors were used for the Se and Sp of UE, whereas informative priors were used for PVD Se (mode = 65%, 5th percentile = 45%) and Sp (mode = 90%, 5th percentile = 80%) and RTD prevalence (mode = 20%, 5th percentile = 10%). Posterior estimates (median and 95% Bayesian probability intervals; BPI) were obtained using ‘rjags' (R Studio). The optimal test thresholds (PVD and UE score ≥1) were selected by assessing the effect of different thresholds on test estimates and using a misclassification cost analysis. Based on these, median (95% BPI) Se for PVD and UE score ≥1 were 44% (29–60%) and 67% (33–100%), respectively. Median Sp for PVD and UE score ≥1 were 90% (86–93%) and 91% (86–93%), respectively. Higher scores in both tests were associated with impaired fertility, and UE scoring with a threshold of ≥1 had the highest test Se and Sp estimates although test Se was conditional on days in milk when the PBE occurred. 相似文献
4.
诱导式卫星欺骗干扰可诱导航空器逐渐偏离预定航迹,难以被发现,因此及时有效地检测干扰是飞行安全的保障。在现有紧组合导航体制基础上,设计了一种基于误差估值累加开环校正的紧组合导航结构,并证明了其性能与传统闭环校正紧组合导航性能等效。在此结构中,将紧组合导航系统与自适应序贯概率比检测方法结合,提出了一种基于误差估值累加开环校正的诱导式欺骗检测方法,融合紧组合导航信息与其他不受欺骗影响的导航信息,构建欺骗检测统计量进行诱导式欺骗检测。仿真结果表明,开环校正结构可避免随时间累加的惯性导航系统误差所导致的组合导航滤波器发散问题,同时欺骗检测方法可进一步提高算法对“最坏”情形下微小诱导式欺骗的检测效果。 相似文献
5.
《Contact lens & anterior eye》2022,45(4):101494
PurposeTo determine if there is diurnal variation in gene expression in normal healthy conjunctival cells.MethodsBulbar conjunctival swab samples were collected from four healthy subjects in the morning and evening of the same day. The two swab samples were taken from one eye of each participant, with a minimum of five hours gap between the two samples. RNA was extracted and analysed using RNA sequencing (RNA-Seq).ResultsA total of 121 genes were differentially expressed between the morning and the evening conjunctival samples, of which 94 genes were upregulated in the morning, and 27 genes were upregulated in the evening. Many of the genes that were upregulated in the morning were involved in defence, cell turnover and regulation of gene expression, while the genes upregulated in the evening were involved in signalling and mucin production.ConclusionsThis study has identified several genes whose expression changes over the course of the day. Knowledge of diurnal variations of conjunctival gene expression provides an insight into the regulatory status of the healthy eye and provides a baseline for examining changes during ocular surface disease. 相似文献
6.
《Mechatronics》2022
A large-scale high-precision scan stage is important equipment in the industrial productions of micro-fabrication such as flat panel display (FPD) lithography systems. Designing controllers for multi-input multi-output (MIMO) systems is time-consuming and needs experience because of the interaction between each axis and many controller tuning parameters. The aim of this study is to develop a peak filter design method based on frequency response data to reduce repetitive disturbance. This data-based approach does not use the model and only uses the frequency response data of the controlled system and the disturbance spectrum calculated from the scanning error data (Contribution 1). The peak filter is designed by convex optimization and satisfies robust stability conditions for six-degree-of-freedom systems (Contribution 2). The control performance of the designed peak filter is experimentally demonstrated with an industrial MIMO large-scale high-precision scan stage in reducing the scanning error of the main stroke of the translation along the -axis (Contribution 3). 相似文献
7.
8.
Hamid Ezzatpanah Vicente M. Gómez-López Tatiana Koutchma Farnoush Lavafpour Frank Moerman Mohammad Mohammadi Dele Raheem 《Comprehensive Reviews in Food Science and Food Safety》2022,21(2):904-941
Food- and waterborne viruses, such as human norovirus, hepatitis A virus, hepatitis E virus, rotaviruses, astroviruses, adenoviruses, and enteroviruses, are major contributors to all foodborne illnesses. Their small size, structure, and ability to clump and attach to inanimate surfaces make viruses challenging to reduce or eliminate, especially in the presence of inorganic or organic soils. Besides traditional wet and dry methods of disinfection using chemicals and heat, emerging physical nonthermal decontamination techniques (irradiation, ultraviolet, pulsed light, high hydrostatic pressure, cold atmospheric plasma, and pulsed electric field), novel virucidal surfaces, and bioactive compounds are examined for their potential to inactivate viruses on the surfaces of foods or food contact surfaces (tools, equipment, hands, etc.). Every disinfection technique is discussed based on its efficiency against viruses, specific advantages and disadvantages, and limitations. Structure, genomic organization, and molecular biology of different virus strains are reviewed, as they are key in determining these techniques effectiveness in controlling all or specific foodborne viruses. Selecting suitable viral decontamination techniques requires that their antiviral mechanism of action and ability to reduce virus infectivity must be taken into consideration. Furthermore, details about critical treatments parameters essential to control foodborne viruses in a food production environment are discussed, as they are also determinative in defining best disinfection and hygiene practices preventing viral infection after consuming a food product. 相似文献
9.
The carbon vacancy in high-entropy carbides (HECs) has a significant impact on their physical and chemical properties, yet relevant studies have still been relatively few. In this study, we investigate the surface energies of HECs with variable carbon vacancies through first-principles calculations. The results show that the surface energy of the (1 0 0) surface of the stoichiometric HECs is significantly lower than that of (1 1 1) surface. With the decrease in carbon stoichiometry, the surface energies of both (1 0 0) and (1 1 1) surfaces increase gradually, which is mainly due to the weakening of covalent bonding and the decrease of metal Hirshfeld-I (HI) charges. However, the surface energy of (1 0 0) surface increases more quickly than that of (1 1 1) surface and will exceed that of (1 1 1) surface when the carbon stoichiometry decreases to a certain extent, which is primarily attributed to the greater decrease rate of metal HI charges of (1 0 0) surface. 相似文献
10.
《International Journal of Hydrogen Energy》2022,47(59):24807-24816
Engineering alloy nanostructures with a combination of highly active noble metals (Pt and Pd) and less electronegative non-noble metal (Ni) is found to be crucial for improving surface reactivity by enriching with active Pt sites. Herein, a multi-skeletal PtPdNi nanodendrites (NDs) was successfully formed by a simple one-pot method with structure directing agent. The modification of Pt electronic structure and their interaction due to compressive strain were explored using benchmark characterization techniques, which showed that the PtPdNi NDs possess Pt-enriched surface, corroborating to more active catalyst sites for oxygen reduction reaction (ORR) in acidic medium. The PtPdNi NDs have a higher electrochemical surface area (63 m2 g?1) and an earlier onset potential (1.01 V) than PtPd NDs, PtNi NDs, and commercial Pt/C catalysts, indicating the outstanding ORR performance. The high mass and specific activities, as well as superior durability after accelerated degradation test (ADT), highlight the remarkable electrocatalytic performance of PtPdNi NDs over others. As a result, enhancing Pt utilization through the formation of PtPdNi NDs could be a reliable strategy to improve ORR electrocatalysis for polymer electrolyte membrane fuel cell (PEMFC) applications. 相似文献