首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1755篇
  免费   9篇
  国内免费   1篇
电工技术   17篇
综合类   1篇
化学工业   297篇
金属工艺   37篇
机械仪表   43篇
建筑科学   66篇
矿业工程   43篇
能源动力   24篇
轻工业   161篇
水利工程   16篇
无线电   127篇
一般工业技术   263篇
冶金工业   503篇
原子能技术   24篇
自动化技术   143篇
  2023年   9篇
  2021年   38篇
  2020年   27篇
  2019年   19篇
  2018年   21篇
  2017年   21篇
  2016年   22篇
  2015年   28篇
  2014年   32篇
  2013年   54篇
  2012年   49篇
  2011年   81篇
  2010年   48篇
  2009年   66篇
  2008年   62篇
  2007年   74篇
  2006年   52篇
  2005年   43篇
  2004年   26篇
  2003年   38篇
  2002年   32篇
  2001年   32篇
  2000年   26篇
  1999年   42篇
  1998年   140篇
  1997年   92篇
  1996年   58篇
  1995年   49篇
  1994年   31篇
  1993年   40篇
  1992年   23篇
  1991年   15篇
  1990年   17篇
  1989年   16篇
  1988年   13篇
  1987年   13篇
  1986年   11篇
  1985年   20篇
  1984年   10篇
  1983年   13篇
  1982年   25篇
  1980年   12篇
  1979年   12篇
  1978年   13篇
  1977年   26篇
  1976年   34篇
  1975年   14篇
  1974年   9篇
  1973年   13篇
  1970年   10篇
排序方式: 共有1765条查询结果,搜索用时 15 毫秒
51.
52.
Objective: To evaluate cognitive-behavioral therapy to enhance medication adherence and reduce depression (CBT-AD) in individuals with HIV. Design: A two arm, randomized, controlled, cross-over trial comparing CBT-AD to enhanced treatment as usual only (ETAU). ETAU, which both groups received, included a single-session intervention for adherence and a letter to the patient's provider documenting her or his continued depression. The intervention group also received 10 to 12 sessions of CBT-AD. Main Outcome Measures: Adherence to antiretroviral therapy as assessed by Medication Event Monitoring Systems (MEMs) and depression as assessed by blinded structured evaluation. Results: At the acute outcome assessment (3-months), those who received CBT-AD evidenced significantly greater improvements in medication adherence and depression relative to the comparison group. Those who were originally assigned to the comparison group who chose to cross over to CBT-AD showed similar improvements in both depression and adherence outcomes. Treatment gains for those in the intervention group were generally maintained at 6- and 12-month follow-up assessments. By the end of the follow-up period, those originally assigned CBT-AD demonstrated improvements in plasma HIV RNA concentrations, though these differences did not emerge before the cross-over, and hence there were not between-groups differences. Conclusions: CBT-AD is a potentially efficacious approach for individuals with HIV struggling with depression and adherence. Replication and extension in larger efficacy trials are needed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
53.
Nuclear magnetic resonance (NMR) spectroscopy has been used effectively in the analysis of elastomeric, soft materials and has been proven to be both sensitive to micro- and macroscopic changes associated with “aging” mechanisms. Traditional analyses, however, rely on empirical formulae containing a large number of (often arbitrary) independent variables. The resulting ambiguity can be circumvented largely by developing models of NMR observables that are based on basic polymer physics. We compare two such models, one previously published and one derived herein, along with two empirical expressions that describe the proton transverse magnetization decay associated with complex polymer networks. One particular extracted parameter, the proton-proton residual dipolar coupling (RDC), can be directly related to network topology, and a comparison of the extracted RDCs reveals high consistency among the models. An expression derived from the properties of a static Gaussian chain can minimize the number of parameters necessarily to describe the solid-like, networked proton population to a single independent parameter, the average RDC, Davg. The distribution of RDCs derived via this methodology is qualitatively similar to those derived from previously published multiple quantum techniques, although quantitative differences between the derived RDCs persist, suggesting that further analysis is necessary.  相似文献   
54.
The synthesis of the bis-cyclometalated complex [{Ir(μ-Cl)(ccpz)2}2] (1, cppz = 1-chloro-4-(4-chlorophenyl)-phthalazine) from hydrated iridium(III) chloride and the ligand ccpz in refluxing ethoxyethanol is described. Compound 1 was characterized by spectroscopic means and the crystal structure was determined by X-ray crystallography. The absorption and emission spectra exhibit the new compound as a red-emitting phosphorescent complex.  相似文献   
55.
Participatory modeling workshops were held in Sonora, México, with the goal of developing water resources management strategies in a water-stressed basin. A model of the water resources system, consisting of watershed hydrology, water resources infrastructure, and groundwater models, was developed deliberatively in the workshops, along with scenarios of future climate and development. Participants used the final version of the water resources systems model to select management strategies. The performance of the strategies was based on the reliability of meeting current and future demands at a daily time scale over a year’s period. Pre- and post-workshop surveys were developed and administered. The survey questions focused on evaluation of participants’ modeling capacity and the utility and accuracy of the models. The selected water resources strategies and the associated, expected reliability varied widely among participants. Most participants could be clustered into three groups with roughly equal numbers of participants that varied in terms of reliance on expanding infrastructure vs. demand modification; expectations of reliability; and perceptions of social, environmental, and economic impacts. The wide range of strategies chosen and associated reliabilities indicate that there is a substantial degree of uncertainty in how future water resources decisions could be made in the region. The pre- and post-survey results indicate that participants believed their modeling abilities increased and beliefs in the utility of models increased as a result of the workshops.  相似文献   
56.
This paper describes the use of the dual isotope method involving δ(15)N and δ(18)O measurements of dissolved nitrates to assess the origin and fate of groundwater nitrate at the Klju? groundwater source, Serbia. A sampling campaign was conducted in September 2007 during flow conditions obtaining groundwater from observation wells and river water fed by a shallow aquifer hosted in alluvial (sandy-gravel) sediments. Nitrate isotope ratios ranged from +5.3 to +16.9‰ and δ(18)O(NO(3)) values varied from -2.3 to +5.0‰. Two major contamination sources were identified with isotopic compositions characteristic for nitrate derived from nitrification of soil organic nitrogen (+5.3 to +7.8‰ for δ(15)N) resulting in nitrate concentrations of 33.6 and 78.8 mg/L and nitrate derived from animal wastes or human sewage, e.g. via septic systems, yielding δ(15)N values of +9.9 to +11.9‰ and elevated nitrate concentrations of 31.2-245.8 mg/L. The occurrence of nitrification and denitrification was also revealed based on concentration and isotope data for dissolved nitrate.  相似文献   
57.
In a series of 3 experiments, 68 undergraduates learned to a criterion of 2 errorless trials to recite the 1st 12 or 18 responses in a pattern based on Base 3. In Exp I, the pattern was presented as letters (w, d, r, dw, dd,?.?.?.) or as numerals (0, 1, 2, 10, 11,?.?.?.). In Exps II and III, the pattern was presented as letters, and Ss were given a letter-to-digit conversion list (w?=?0, d?=?1, r?=?2) either before or after learning. On a posttest, Ss who knew that the pattern related to numerals during learning performed better on transfer tasks such as addition or further counting in Base 3. Results suggest that different rule systems were acquired even though the same levels of performance on the behavioral objective were achieved. (25 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
58.
    
The optomechanical coupling of quantum dots and flexural mechanical modes is studied in suspended nanophononic strings. The investigated devices are designed and monolithically fabricated on an (Al)GaAs heterostructure. Radio frequency elastic waves with frequencies ranging between f = 250 and 400 MHz are generated as Rayleigh surface acoustic waves on the unpatterned substrate and injected as Lamb waves in the nanophononic string. Quantum dots inside the nanophononic string exhibit a 15‐fold enhanced optomechanical modulation compared to those dynamically strained by the Rayleigh surface acoustic wave. Detailed finite element simulations of the phononic mode spectrum of the nanophononic string confirm that the observed modulation arises from valence band deformation potential coupling via shear strain. The corresponding optomechanical coupling parameter is quantified to 0.15 meV n m 1 . This value exceeds that reported for vibrating nanorods by approximately one order of magnitude at 100 times higher frequencies. Using this value, a derived vertical displacement in the range of 10 nm is deduced from the experimentally observed modulation. The results represent an important step toward the creation of large scale optomechanical circuits interfacing single optically active quantum dots with optical and mechanical waves.  相似文献   
59.
    
Transcatheter embolization is a minimally invasive procedure that uses embolic agents to intentionally block diseased or injured blood vessels for therapeutic purposes. Embolic agents in clinical practice are limited by recanalization, risk of non-target embolization, failure in coagulopathic patients, high cost, and toxicity. Here, a decellularized cardiac extracellular matrix (ECM)-based nanocomposite hydrogel is developed to provide superior mechanical stability, catheter injectability, retrievability, antibacterial properties, and biological activity to prevent recanalization. The embolic efficacy of the shear-thinning ECM-based hydrogel is shown in a porcine survival model of embolization in the iliac artery and the renal artery. The ECM-based hydrogel promotes arterial vessel wall remodeling and a fibroinflammatory response while undergoing significant biodegradation such that only 25% of the embolic material remains at 14 days. With its unprecedented proregenerative, antibacterial properties coupled with favorable mechanical properties, and its superior performance in anticoagulated blood, the ECM-based hydrogel has the potential to be a next-generation biofunctional embolic agent that can successfully treat a wide range of vascular diseases.  相似文献   
60.
    
Conductive polymers largely derive their electronic functionality from chemical doping, processes by which redox and charge‐transfer reactions form mobile carriers. While decades of research have demonstrated fundamentally new technologies that merge the unique functionality of these materials with the chemical versatility of macromolecules, doping and the resultant material properties are not ideal for many applications. Here, it is demonstrated that open‐shell conjugated polymers comprised of alternating cyclopentadithiophene and thiadiazoloquinoxaline units can achieve high electrical conductivities in their native “undoped” form. Spectroscopic, electrochemical, electron paramagnetic resonance, and magnetic susceptibility measurements demonstrate that this donor–acceptor architecture promotes very narrow bandgaps, strong electronic correlations, high‐spin ground states, and long‐range π‐delocalization. A comparative study of structural variants and processing methodologies demonstrates that the conductivity can be tuned up to 8.18 S cm?1. This exceeds other neutral narrow bandgap conjugated polymers, many doped polymers, radical conductors, and is comparable to commercial grades of poly(styrene‐sulfonate)‐doped poly(3,4‐ethylenedioxythiophene). X‐ray and morphological studies trace the high conductivity to rigid backbone conformations emanating from strong π‐interactions and long‐range ordered structures formed through self‐organization that lead to a network of delocalized open‐shell sites in electronic communication. The results offer a new platform for the transport of charge in molecular systems.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号