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排序方式: 共有106条查询结果,搜索用时 15 毫秒
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32.
C. Herrero Latorre J. Barciela GarcíaAuthor VitaeS. García MartínAuthor Vitae R.M. Peña CrecenteAuthor Vitae 《Analytica chimica acta》2013
Selenium is an essential element for the normal cellular function of living organisms. However, selenium is toxic at concentrations of only three to five times higher than the essential concentration. The inorganic forms (mainly selenite and selenate) present in environmental water generally exhibit higher toxicity (up to 40 times) than organic forms. Therefore, the determination of low levels of different inorganic selenium species in water is an analytical challenge. Solid-phase extraction has been used as a separation and/or preconcentration technique prior to the determination of selenium species due to the need for accurate measurements for Se species in water at extremely low levels. The present paper provides a critical review of the published methods for inorganic selenium speciation in water samples using solid phase extraction as a preconcentration procedure. On the basis of more than 75 references, the different speciation strategies used for this task have been highlighted and classified. The solid-phase extraction sorbents and the performance and analytical characteristics of the developed methods for Se speciation are also discussed. 相似文献
33.
Assessment technique for evaluating military vehicular impacts to vegetation in the Mojave desert 总被引:1,自引:0,他引:1
A new plant-damage assessment technique was developed. The technique consists of linear transects the width of a military vehicle’s tracks located in existing tracks in the soil (usually during a prior training rotation period of 30 days or since the last rain or wind storm). Measurements of vegetation within the tracks are used to determine the area of plant parts impacted. The percent of the plant parts damaged and the percent expected to recover are estimated. The technique documents prior-damage classes based on estimates of damage that plants have apparently experienced previously (as assessed from field indicators of damage such as plant shape and height). The technique was used to evaluate different vehicle types (rubber-tire wheels vs. metal tracks) in six areas at the NTC with different soils and training intensity levels. The technique provides tabular data that can be sorted and queried to show a variety of trends related to military vehicular impacts. It also is suitable for assessing other non-military off-road traffic impacts. The study reports: (1) differences in plant sensitivity to different vehicle track types, (2) plant cover and density by species and training area after prolonged impact, (3) the degree to which rubber tire wheels have less impact than metal tracks, and (4) mean percent survival was inversely proportional to the degree of prior damage received by the vegetation (i.e., plants previously impacted have lower survival than plants not previously impacted). 相似文献
34.
This study describes Singapore students’ (N = 607) performance on two tasks in a recently developed Mathematics Processing Instrument (MPI). The MPI comprised tasks sourced from Australia's NAPLAN and Singapore's PSLE. This study also examines students’ use of the model method to solve the two tasks. The model method is a visual problem-solving heuristic prevalently used in Singapore classrooms. The study found that students who solved the tasks using a visual method predominantly used the model method as a visual problem-solving strategy. Another interesting observation was the hindrance of successful problem solving caused by the persistence of prototypical images of model drawings. Implications include encouraging teachers to get their students to identify problem situations where the model method will both work and not work well, and making the role of the generator in the model method explicit in the mathematics textbooks. 相似文献
35.
Melissa C. Gilbert 《School science and mathematics》2014,114(7):339-348
This study examined the productive disposition of pre‐algebra students who demonstrated similar knowledge of the focal content but varied in other academic behaviors expected in the Common Core State Standards for Mathematics (CCSSM). Specifically, the study considered students' attention to precision when critiquing a peer's work. The comprehensive definition of productive disposition used included task values (interest, utility), an ability belief (efficacy), three personal achievement goals, and negative emotions. As hypothesized, the 61 students who provided a more precise critique reported higher productive disposition (in particular, significantly higher mastery‐approach personal achievement goals and less frequent negative emotions) than the 79 students who provided a basic critique. These findings illustrate how productive disposition can inform assessments of mathematical competence within the CCSSM recently implemented across the United States. 相似文献
36.
Tina Rapke 《Research in Mathematics Education》2016,18(1):27-42
This article describes a study, from a Canadian technical institute's upgrading mathematics course, where students played a role in developing the final closed-book exam that they sat. The study involved a process where students developed practice exams and solutions keys, students sat each other's practice exams, students evaluated classmates' solutions to the practice exams, and finally the instructor used questions from the practice exams to develop the ‘live’ final exam. Phenomenography is used to analyse interview data and report students' experiences. Through the results, claims are made that students experienced deep approaches to learning and worked as partners in learning, teaching and assessment during the process of developing the final exam with their instructor. 相似文献
37.
Alex Williams 《Accreditation and quality assurance》2008,13(11):633-638
Many, possibly most, analytical measurements are carried out to assess compliance with a specification or a regulation, for
example in the control contaminants in food or the detection of performance enhancing substances in sport. When making an
assessment of compliance the presence of unavoidable measurement uncertainty introduces the risk of making incorrect decisions,
that is of accepting a batch of material which is outside the specification or rejecting one that is within. This often leads
to controversy over whether or not the compliance decision is correct. How to make reliable assessment decisions is described
in the EURACHEM/CITAC Guide “Use of uncertainty information in compliance assessment”. The key is the use of decision rules
that lead to an unambiguous interpretation of the measurement result and its uncertainty. These decision rules need to be
designed to ensure that requirements of the specification or regulation are met and that the risk of making an incorrect decision
is acceptable. Ideally they should form part of the specification or regulation.
Presented at the Measurement Uncertainty Symposium, April 2008, Berlin, Germany. 相似文献
38.
Zbynek Dzuman Milena Zachariasova Zdenka Veprikova Michal Godula Jana Hajslova 《Analytica chimica acta》2015
A new reliable and highly sensitive method based on high performance liquid chromatographic (HPLC) separation and high resolution tandem mass spectrometric detection (HRMS/MS) has been developed and validated for determination of 323 pesticide residues, 55 mycotoxins, and 11 plant toxins represented by pyrrolizidine alkaloids. The method was validated for three matrices, leek, wheat, and tea differing in nature/amount of co-extracts that may cause various matrix effects. For target analytes isolation, optimized QuEChERS-based (quick, easy, cheap, effective, rugged, and safe) extraction procedure was employed. Spectral HRMS/MS library has been established providing an entire spectrum of fragment ions for each analyte, which allows unbiased identification and confirmation of target compounds. The limits of quantification (LOQs) of target analytes were below 10 μg kg−1 for 82%, 81%, and 61% for matrices leek, wheat, and tea, respectively. Recoveries were in the acceptable range (70–120%) according to SANCO/12571/2013 for most of target analytes, except for highly polar ‘masked’ mycotoxin deoxynivalenol-3-glucoside with recoveries 35%, 47%, and 42% for matrices leek, wheat, and tea, respectively. The linearities of calibration curves expressed as coefficients of determination were in the range of 0.9661–1.000, and repeatabilities expressed as relative standard deviations (RSDs) at LOQs lied in the range of 0.25–13.51%. The trueness of the method was verified using several certified reference materials (CRMs) and proficiency test samples. 相似文献
39.
采用热脱附-气相色谱/质谱联机技术(TD-GC/MS)分析了芦苇杆粉末在不同环境温度下的挥发物释放行为。结果表明:芦苇杆粉末在40℃及160℃的挥发物中,主要有益成分分别为主要有害成分的4.43倍和5.13倍,环境友好性良好;在60℃及90℃的挥发物中,主要有益成分比例也比主要有害成分高,90℃时的有益成分为有害成分的2.45倍,环境友好性较好。以芦苇杆粉末为基材,在木质素软化温度点和木质材料碳化温度点之间(150—180℃)实施高压无胶模塑成形工艺制备木质粉末基复合材料必将体现出良好的环境友好性。 相似文献
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