Temperature control curve is the key to achieving temperature control and crack prevention of high concrete dam during construction,and its rationality depends on the accurate measurement of temperature stress.With th...Temperature control curve is the key to achieving temperature control and crack prevention of high concrete dam during construction,and its rationality depends on the accurate measurement of temperature stress.With the simulation testing machine for the temperature stress,in the present study,we carried out the deformation process tests of concrete under three temperature curves:convex,straight and concave.Besides,we not only measured the early-age elastic modulus,creep parameters and stress process,but also proposed the preferred type.The results show that at early age,higher temperature always leads to greater elastic modulus and smaller creep.However,the traditional indoor experiments have underestimated the elastic modulus and creep development at early age,which makes the calculated value of temperature stress too small,thus increasing the cracking risk.In this study,the stress values of the three curves calculated based on the strain and early-age parameters are in good agreement with the temperature stress measured by the temperature stress testing machine,which verifies the method accuracy.When the temperature changes along the concave curve,the law of stress development is in consistent with that of strength.Under this condition,the stress fluctuation is small and the crack prevention safety of the concave type is higher,so the concave type is better.The test results provide a reliable basis and support for temperature control curve design and optimization of concrete dams.展开更多
China’s Ministry of Commerce(MOFCOM)and the Ministry of Industry and Information Technology(MIIT)have designed a stress-test for China’s labor-intensive industries,
A step stress test is carried out to study the reliability characteristics of an AlGaN/GaN high electron mobility transistor（HEMT）.An anomalous critical drain-to-gate voltage with a negative temperature coefficient ...A step stress test is carried out to study the reliability characteristics of an AlGaN/GaN high electron mobility transistor（HEMT）.An anomalous critical drain-to-gate voltage with a negative temperature coefficient is observed in the stress sequence,beyond which the HEMT device starts to recover from degradation induced by early lower voltage stress.While the performance degradation featuring the drain current slump stems from electron trapping in the surface or bulk states during low-to-medium bias stress,the recovery is attributed to high field induced electron detrapping.The carrier detrapping mechanism could be helpful for lessening the trapping-related performance degradation of a GaN-based HEMT.展开更多
In this study,nitrogen doped electrochemically exfoliated reduced graphene oxide and carbon black supported platinum(Pt/Nr EGO_(2)-CB_(3))has been prepared to enhance the performance and durability of hightemperature ...In this study,nitrogen doped electrochemically exfoliated reduced graphene oxide and carbon black supported platinum(Pt/Nr EGO_(2)-CB_(3))has been prepared to enhance the performance and durability of hightemperature PEMFCs with lower Pt loading.On the one hand,Pt/Nr EGO_(2)-CB_(3)with the strong interaction between the Pt and nitrogen(N)prevent agglomeration of Pt particles and Pt particles is 5.46±1.46 nm,which is smaller than that of 6.78±1.34 nm in Pt/C.Meanwhile,ECSA of Pt/Nr EGO_(2)-CB_(3)decrease 13.65%after AST,which is much lower than that of 97.99%in Pt/C.On the other hand,the Nr EGO flakes in MEAac act as a barrier to mitigate phosphoric acid redistribution,which improves the formation of triple-phase boundaries(TPBs)and gives stable operation of the MEAacwith a lower decay rate of 0.02 mV h^(-1)within100 h.After steady-state operation,the maximum power density of Pt/Nr EGO_(2)-CB_(3)(0.411 W cm^(-2))is three times higher than that of conventional Pt/C(0.134 W cm^(-2))in high-temperature PEMFCs.After AST,the mass transfer resistance of Pt/Nr EGO_(2)-CB_(3)electrode(0.560Ωcm^(2))is lower than that in Pt/C(0.728Ωcm^(2)).展开更多
A reasonable unsupported roof distance(URD) when advancing underground coal mine roadways can contribute greatly to safe and rapid roadway development.A mechanical model of the roof,using the relationship between the ...A reasonable unsupported roof distance(URD) when advancing underground coal mine roadways can contribute greatly to safe and rapid roadway development.A mechanical model of the roof,using the relationship between the roof stress distribution and URD,obtained by the difference method,and roof stability according to the in-situ roof stress and rock mass strength was developed.We subsequently designed a proper range of URD,developed a testing method of URD with the function of mining protection,evaluated roof stability through analyzing the test data and then determined a reasonable URD.Considering the factors of the geological conditions,the immediate roof stability and the efficiency of the labor arrangement system,the URD of the advancing roadway of 9802 working face in Zhangshuanglou coal mine was determined to be 6 m using the proposed method.The results show that,when a 2 m length of roadway was reinforced by temporary support and high pre-stressed bolt support after the roadway advancement of 6 m per cycle,the speed and the security of the roadway development can be achieved and the advance rate can reach more than 400 m per month.展开更多
基金National Key R&D Plan Project(No.2021YFC3090102)。
文摘Temperature control curve is the key to achieving temperature control and crack prevention of high concrete dam during construction,and its rationality depends on the accurate measurement of temperature stress.With the simulation testing machine for the temperature stress,in the present study,we carried out the deformation process tests of concrete under three temperature curves:convex,straight and concave.Besides,we not only measured the early-age elastic modulus,creep parameters and stress process,but also proposed the preferred type.The results show that at early age,higher temperature always leads to greater elastic modulus and smaller creep.However,the traditional indoor experiments have underestimated the elastic modulus and creep development at early age,which makes the calculated value of temperature stress too small,thus increasing the cracking risk.In this study,the stress values of the three curves calculated based on the strain and early-age parameters are in good agreement with the temperature stress measured by the temperature stress testing machine,which verifies the method accuracy.When the temperature changes along the concave curve,the law of stress development is in consistent with that of strength.Under this condition,the stress fluctuation is small and the crack prevention safety of the concave type is higher,so the concave type is better.The test results provide a reliable basis and support for temperature control curve design and optimization of concrete dams.
文摘China’s Ministry of Commerce(MOFCOM)and the Ministry of Industry and Information Technology(MIIT)have designed a stress-test for China’s labor-intensive industries,
文摘A step stress test is carried out to study the reliability characteristics of an AlGaN/GaN high electron mobility transistor（HEMT）.An anomalous critical drain-to-gate voltage with a negative temperature coefficient is observed in the stress sequence,beyond which the HEMT device starts to recover from degradation induced by early lower voltage stress.While the performance degradation featuring the drain current slump stems from electron trapping in the surface or bulk states during low-to-medium bias stress,the recovery is attributed to high field induced electron detrapping.The carrier detrapping mechanism could be helpful for lessening the trapping-related performance degradation of a GaN-based HEMT.
基金supported by the Engineering and Physical Sciences Research Council(EPSRC)(EP/P009050/1 and EP/S021531/1)Tthe Henry Royce Institute for Advanced Materials,funded through the EPSRC grants(EP/R00661X/1,EP/S019367/1,EP/P025021/1 and EP/P025498/1)。
文摘In this study,nitrogen doped electrochemically exfoliated reduced graphene oxide and carbon black supported platinum(Pt/Nr EGO_(2)-CB_(3))has been prepared to enhance the performance and durability of hightemperature PEMFCs with lower Pt loading.On the one hand,Pt/Nr EGO_(2)-CB_(3)with the strong interaction between the Pt and nitrogen(N)prevent agglomeration of Pt particles and Pt particles is 5.46±1.46 nm,which is smaller than that of 6.78±1.34 nm in Pt/C.Meanwhile,ECSA of Pt/Nr EGO_(2)-CB_(3)decrease 13.65%after AST,which is much lower than that of 97.99%in Pt/C.On the other hand,the Nr EGO flakes in MEAac act as a barrier to mitigate phosphoric acid redistribution,which improves the formation of triple-phase boundaries(TPBs)and gives stable operation of the MEAacwith a lower decay rate of 0.02 mV h^(-1)within100 h.After steady-state operation,the maximum power density of Pt/Nr EGO_(2)-CB_(3)(0.411 W cm^(-2))is three times higher than that of conventional Pt/C(0.134 W cm^(-2))in high-temperature PEMFCs.After AST,the mass transfer resistance of Pt/Nr EGO_(2)-CB_(3)electrode(0.560Ωcm^(2))is lower than that in Pt/C(0.728Ωcm^(2)).
基金supported by the State Key Laboratory of Coal Resources and Sate Mining,China University of Mining and Technology (No.SKLCRSM13X07)the National Natural Science Foundation of China (No.51174195)+1 种基金Chinese National Programs for Fundamental Research and Development (No.2013CB227900)the Fundamental Research Funds for the Central Universities (No.2014XT01)
文摘A reasonable unsupported roof distance(URD) when advancing underground coal mine roadways can contribute greatly to safe and rapid roadway development.A mechanical model of the roof,using the relationship between the roof stress distribution and URD,obtained by the difference method,and roof stability according to the in-situ roof stress and rock mass strength was developed.We subsequently designed a proper range of URD,developed a testing method of URD with the function of mining protection,evaluated roof stability through analyzing the test data and then determined a reasonable URD.Considering the factors of the geological conditions,the immediate roof stability and the efficiency of the labor arrangement system,the URD of the advancing roadway of 9802 working face in Zhangshuanglou coal mine was determined to be 6 m using the proposed method.The results show that,when a 2 m length of roadway was reinforced by temporary support and high pre-stressed bolt support after the roadway advancement of 6 m per cycle,the speed and the security of the roadway development can be achieved and the advance rate can reach more than 400 m per month.
