Does ICT matter for effectiveness and efficiency in mathematics education?

ICT infrastructure investments in educational institutions have been one of the key priorities of education policy during the last decade. Despite the attention, research on the effectiveness and efficiency of ICT is inconclusive. This is mainly due to small-scale research with weak identification strategies which lack a proper control group. Using the 2011 ‘Trends in International Mathematics and Science Study’ (TIMSS) data, we define by a Mahalanobis matching a control group with similar student, teacher, school and regional characteristics. The results indicate that accounting or not accounting for these characteristics, may considerably alter the estimated impact of ICT. This suggests that a correction for characteristics related to the student population, teaching staff, administrative personnel and school management is warranted in the evaluation of the impact of ICT.     

不同环境对AZ61镁合金大气腐蚀的影响

AZ61镁合金大气腐蚀的评估不但能为材料设计提供依据,还能为实验室大气模拟腐蚀实验提供对比参数。利用XRD、SEM、EDS和XPS研究AZ61镁合金板材在万宁、西双版纳和江津三地挂片(2个月)的腐蚀速度、表面腐蚀形貌及产物构成,结果表明,AZ61镁合金板材在3种气候环境下的腐蚀速度排序为:万宁江津西双版纳,且近海大气(万宁)的腐蚀速度成倍增加,年腐蚀率达到17μm/a;AZ61合金在万宁的近海大气环境中形成的表面腐蚀层较厚且疏松,表面龟裂明显,表面腐蚀产物中有来自于含盐大气结晶的氯离子存在;AZ61合金在西双版纳大气中形成的腐蚀产物层较薄;AZ61合金在江津工业大气中的腐蚀产物层的厚度介于西双版纳和万宁之间;表面腐蚀层综合分析表明,万宁挂片中有水合碱式碳酸镁存在。     

挤压态ZM61-Sn合金的显微组织及高温力学性能

利用金相显微镜(OM)、X射线衍射(XRD)、扫描电镜(SEM)和高温拉伸对挤压态ZM61-xSn(x=2, 4, 8 wt. %)合金的显微组织,高温力学性能和断裂机制进行了研究。结果表明添加Sn 元素可有效细化合金组织且细化效果随Sn 含量的增加而增强。挤压态ZM61-xSn(x=2, 4, 8 wt. %)合金的平均晶粒尺寸分别为11, 8和4 μm。随 Sn 含量的增加,合金的力学性能先升高后降低。 在所有的实验合金中ZM61-4Sn合金的强度最高,当在180 ℃下进行拉伸实验时,其极限抗拉强度和屈服强度分别为216和173 MPa。合金的延伸率随Sn 含量的增加而增加,当拉伸温度为300 ℃时,ZM61-xSn(x=2, 4, 8wt. %)合金的延伸率分别为183.8%, 235.8% 和258.6%。ZM61-4Sn合金具有最好的强度和塑性的结合。试样最后的断裂主要由局部缩颈引起以及试样的主要断裂机制为显微孔洞的聚集。当在260和300 ℃下拉伸时,合金发生了不完全的动态再结晶。     

时效处理AZ61铸造镁合金中孪晶的形成机理

研究了AZ61铸造镁合金时效处理后组织中大量楔形孪晶的形成机理,并分析了时效处理对合金力学性能的影响。结果表明,时效处理前合金中存在大量的板条状亚结构和晶粒内部适当的Al的摩尔浓度梯度是导致孪晶形成的主要因素,时效处理后合金的屈服强度和抗拉强度分别由铸态的88.79MPa和189.73MPa上升至109.18MPa和250.03MPa,伸长率由6.93%上升至9.40%,合金的强度和韧性均得到改善。     

轧制温度对AZ61合金组织和电极性能的影响

为了探讨轧制温度对AZ61镁合金板材微观组织的影响,以及对电化学性能和放电性能的遗传效应,本文利用X射线衍射仪、光学显微镜和扫描电子显微镜对AZ61镁合金的微观组织结构和放电形貌进行了表征,采用极化曲线和组装镁空气电池放电方法对AZ61镁合金的电化学及放电性能进行了测试。研究表明:在300~400 ℃范围内,随着轧制温度的升高,AZ61镁合金的腐蚀电位先正移后负移再正移,腐蚀电流密度先减小后增大再减小。在350 ℃下轧制的AZ61镁合金具有最优良的耐腐蚀性能,其腐蚀电位为-1.470 V,腐蚀电流为8.415×10^-6 A/cm²。在电流密度为10 mA/cm²条件下,轧制温度为350 ℃的AZ61镁合金的放电效率和比容量均达到峰值,分别为55.97%和1 253.13 mAh/g,且其放电形貌较为光滑并伴随少量凹坑。轧制温度可以有效细化合金的微观组织结构,减少β相的含量,有利于该合金耐腐蚀性的提高和放电参数的增大。     

Benefits and costs of transport

Abstract. This article aims to bring together insights from a broad body of recent literature concerned with the nature, the measurement and policy implications of benefits and costs of transport. It is argued that, for various reasons, transport cannot be treated as an'ordinary'economic sector, and the policy implications of a number of the sector's peculiarities are addressed. Explicit attention is given to spatial aspects and network elements, internal and external benefits and costs, and efficiency aspects and equity considerations in policy making.