A thermal active restrained shrinkage ring test to study the early age concrete behaviour of massive structures

In massive concrete structures, cracking may occur during hardening, especially if autogenous and thermal strains are restrained. The concrete permeability due to this cracking may rise significantly and thus increase leakage (in tank, nuclear containment…) and reduce the durability.The restrained shrinkage ring test is used to study the early age concrete behaviour (delayed strains evolution and cracking). This test shows, at 20 °C and without drying, for a concrete mix which is representative of a French nuclear power plant containment vessel (w/c ratio equal to 0.57), that the amplitude of autogenous shrinkage (about 40 μm/m for the studied concrete mix) is not high enough to cause cracking. Indeed, in this configuration, thermal shrinkage is not significant, whereas this is a major concern for massive structures.Therefore, an active test has been developed to study cracking due to restrained thermal shrinkage. This test is an evolution of the classical restrained shrinkage ring test. It allows to take into account both autogenous and thermal shrinkages. Its principle is to create the thermal strain effects by increasing the temperature of the brass ring (by a fluid circulation) in order to expand it. With this test, the early age cracking due to restrained shrinkage, the influence of reinforcement and construction joints have been experimentally studied. It shows that, as expected, reinforcement leads to an increase of the number of cracks but a decrease of crack widths. Moreover, cracking occurs preferentially at the construction joint.     

Influence of the nature of aggregates on the behaviour of concrete subjected to elevated temperature

An experimental study is carried out on concretes composed of three different types of aggregates: semi crushed silico-calcareous, crushed calcareous and rolled siliceous. For each aggregate type, two water/cement ratios (W/C), 0.6 and 0.3 are studied. Aggregates and concrete specimens were subjected to 300, 600 and 750 °C heating–cooling cycles. We analyse the evolution of thermal, physical and mechanical properties of concrete in terms of behaviour and physical characteristic evolutions of aggregates with temperature. The study of thermal behaviour of aggregates showed the importance of initial moisture state for the flints. The crystallisation and microstructure of quartz play an important role in the thermal stability of siliceous aggregates. The residual mechanical behaviour of concrete varies depending on the aggregate and the influence of aggregates is also dependent on paste composition. This study allowed to better understand the influence of chemical and mineralogical characteristics of aggregates on the thermomechanical behaviour of concrete.     

食品专业化学动力学创新实验教学探索

针对传统化学动力学实验项目难以满足食品专业教学需求的实际情况,我们通过龙葵红色素热降解反应,开展食品专业化学动力学创新实验教学探索。通过实验教学,学生了解了天然食品色素的热稳定性,掌握了色素热降解动力学参数的测定方法,加深了对化学动力学知识的理解,实现了实验项目与专业问题的有机结合,有效地调动了学习积极性。教改在培养学生创造性思维和解决专业实际问题能力方面,收到了十分理想的效果。     

A mechanistic method for scrutinising LIFT ignition data

The lateral ignition and flame spread test (LIFT) standard (ASTM E 1321‐97a) requires the Thermal Response correlation to be scrutinized for data points that violate the zero heat loss requirement, but the standard gives no guidance on how this should be done. The fundamentals of linear regression were reviewed and an unbiased and mechanistic algorithm for scrutinising LIFT ignition data without human intervention was developed. The algorithm produced reasonable results compared with human interpretation of exemplar test data taken from the literature. Copyright © 2010 John Wiley & Sons, Ltd.     

The effect of synthesis conditions on the physicochemical properties of magnesium aluminate materials

Magnesium aluminate-based materials were prepared by applying different methods: (i) mechanochemical milling of the initial mixture of magnesium and aluminium nitrate powders (in appropriate stoichiometric amounts) followed by heat treatment at temperatures of 650 °C and 850 °C and (ii) melting of the mixture of nitrate precursors at 240 °C followed by thermal treatment at 650 °C, 750 °C and 850 °C. The effect of synthesis method on the structure and morphology of the obtained solids was studied by using various techniques such as: nitrogen adsorption-desorption isotherms, powder XRD, IR spectroscopy and SEM. It was shown that the mechanochemical milling performed before calcination procedure leads to obtaining of nanocrystalline magnesium aluminate spinel phase at lower temperature of 650 °C in comparison with the method using thermal treatment only (at 750 °C). The obtained nanomaterials exhibit mesoporous structure.     

The improvement of thermoelectric property of bulk ZnO via ZnS addition: Influence of intrinsic defects

As in any semiconducting solids, intrinsic defects can affect the properties of ZnO, such as the electrical and thermal conductivities. Defect engineering is usually focused on optimizing the materials’ synthesis or annealing parameters, i.e., temperature, atmosphere, etc. Here we report an approach to change the intrinsic defects of ZnO by adding a small amount of ZnS. During the sintering process, ZnS was decomposed. Apart from the formation of SO₂, the decomposed S and Zn can also be simultaneously doped onto O and Zn sites to change the intrinsic defects in ZnO. For instance, some of the S was converted to SO₂ and led to the formation of V_o (oxygen vacancy); meanwhile, Zn may move to the V_Zn (Zn vacancy) site and decrease the concentration of Zn vacancy. Due to the changes in these native defects, the carrier concentration increased and the thermal conductivity decreased when the content of ZnS was increased to x = 0.01. This sample had an optimal zT value, which was twice that of undoped ZnO. However, with further increase in ZnS, the carrier concentration was reduced. These results suggest a method to tune the intrinsic defects of ZnO via doping technology and bring potential opportunities to improve the thermoelectric performance of this oxide.     

YSZ/MoS2 self-lubricating coating fabricated by thermal spraying and hydrothermal reaction

Thermal sprayed ceramic coatings have extensively been used in components to protect them against friction and wear. However, the poor lubricating ability severely limits their application. Herein, yttria-stabilized zirconia (YSZ)/MoS₂ composite coatings were successfully fabricated on steel substrate with the combination of thermal spraying technology and hydrothermal reaction. Results show that the synthetic MoS₂ powders are composed of numbers of ultra-thin sheets (about 7 ~ 8?nm), and the sheet has obvious lamellar structure. After vacuum impregnation and hydrothermal reaction, numbers of MoS₂ powders, look like flowers, generate inside the plasma sprayed YSZ coating. Moreover, the growing point of the MoS₂ flower is the intrinsic micro-pores of YSZ coating. The friction and wear tests under high vacuum environment indicate that the composite coating has an extremely long lifetime (>?100,000 cycles) and possesses a low friction coefficient less than 0.1, which is lower by about 0.15 times than that of YSZ coating. Meanwhile, the composite shows an extremely low wear rate (2.30?×?10^?7 mm³ N^?1 m^?1) and causes slight wear damage to the counterpart. The excellent lubricant and wear-resistant ability are attributed to the formation of MoS₂ transfer films and the ultra-smooth of the worn surfaces of hybrid coatings.     

Influence of interface morphology on erosion failure of thermal barrier coatings

The thermal barrier coating system (TBCs) has complex structure and works in severe service environment. Erosion is one of the main factors causing the failure of TBCs. In the present study, the particle erosion process of atmospheric plasma sprayed (APS) thermal barrier coatings at elevated temperature was simulated by the finite element method. The effects of interface morphology on the penetration depth, particle ricochet velocity and interface stress state were studied, and the key parameters such as particle size, initial velocity and erosion position were also considered. The cosine curve with constant wavelength and varying amplitude was used to represent different interface roughness of TBCs. The results show that the interface morphology has little effect on the penetration depth of top coat (TC) and the particle ricochet velocity. The influence of particle erosion position related to the interface morphology is obvious. Basically, the greater the interface roughness is, the more violent the interfacial stress fluctuation is. During the erosion process, the stress in the middle of the interface is significantly higher than that at other positions. These results facilitate understanding of the particle erosion failure mechanism of APS TBCs. The influence of interface morphology should be considered in erosion research.     

Densification,mechanical and thermal properties of ZrC1 − x ceramics fabricated by two-step reactive hot pressing of ZrC and ZrH2 powders

Densification behavior, mechanical and thermal properties of ZrC_1 ? x ceramics with various C/Zr ratios of 0.6–1.0 have been investigated by two-step reactive hot pressing of ZrC and ZrH₂ powders at 30 MPa and 1500–2100 °C. The two-step reactive hot pressed ZrC_1 ? x ceramic has a higher relative density (> 95.3%) than that (91.9%) of stoichiometric ZrC sintered at 2100 °C. A cubic Zr₂C-type ordered phase forms in the ZrC_1 ? x sample obtained at a ZrC/ZrH₂ molar ratio of 0.6 at a relatively low temperature of 1100 °C. The decrease in C/Zr ratio is beneficial to densification of ZrC_1 ? x ceramic, however, excess grain growth occurs after sintering above densification temperature. The elastic modulus and Vickers hardness decrease with decreasing the C/Zr ratio. With decreasing the C/Zr ratio, both thermal conductivity and specific heat decrease due to the enhanced scattering of conducting phonons and electrons by carbon vacancies.