Based on the first-principles method,we predict two new stable BN allotropes:C12-BN and O16-BN,which belong to cubic and orthorhombic crystal systems,respectively.It is confirmed that both the phases are thermally and...Based on the first-principles method,we predict two new stable BN allotropes:C12-BN and O16-BN,which belong to cubic and orthorhombic crystal systems,respectively.It is confirmed that both the phases are thermally and dynamically stable.The results of molecular dynamics simulations suggest that both the BN phases are highly stable even at high temperatures of 1000 K.In the case of mechanical properties,C12-BN has a bulk modulus of 359 GPa and a hardness of 43.4 GPa,making it a novel superhard material with potential technological and industrial applications.Electronic band calculations reveal that both C12-BN and O16-BN are insulators with direct band gaps of 3.02 e V and 3.54 e V,respectively.The XRD spectra of C12-BN and O16-BN are also simulated to provide more information for possible experimental observation.Our findings enrich the BN allotrope family and are expected to stimulate further experimental interest.展开更多
Using the first-principles method of the plane-wave pseudo-potential, the structural properties of the newly-discovered willemite-Ⅱ Si3N4 （wⅡ phase） and post-phenacite Si3N4 （δ phase） are investigated. The α p...Using the first-principles method of the plane-wave pseudo-potential, the structural properties of the newly-discovered willemite-Ⅱ Si3N4 （wⅡ phase） and post-phenacite Si3N4 （δ phase） are investigated. The α phase is predicted to undergo a first-order α→wⅡ phase transition at 18.6 GPa and 300 K. Within the quasi-harmonic approximation （QHA）, the α→wⅡ phase boundary is also obtained. When the well-known β→γ transition is suppressed by some kinetic reasons, the β→δ phase transformation could be observed in the phase diagram. Besides, the temperature dependences of the cell volume,thermal expansion coefficient, bulk modulus, specific heat, entropy and Debye temperature of the involved phases are determined from the non-equilibrium free energies. The thermal expansion coefficients of wⅡ-Si3N4 show no negative values in a pressure range of 0-30 GPa, which implies that the wⅡ-Si3N4 is mechanically stable. More importantly, the δ-Si3N4 is found to be a negative thermal expansion material. Further experimental investigations may be required to determine the physical properties of wⅡ- and δ-Si3N4 with higher reliability.展开更多
In order to solve the problems of low thermal conductivity and easy liquid leakage of a stearic acid(SA),the composite phase change material(PCM)was prepared by adding boron nitride(BN)and expanded graphite(EG)to melt...In order to solve the problems of low thermal conductivity and easy liquid leakage of a stearic acid(SA),the composite phase change material(PCM)was prepared by adding boron nitride(BN)and expanded graphite(EG)to melted SA,and its thermal conductivity,crystal structure,chemical stability,thermal stability,cycle stability,leakage characteristics,heat storage/release characteristics,and temperature response characteristics were char-acterized.The results showed that the addition of BN and EG significantly improved the thermal conductivity of the material,and they efficiently adsorbed melted SA.The maximum load of SA was 76 wt.%and there was almost no liquid leakage.Moreover,the melting enthalpy and temperature were 154.20 J·g^(−1) and 67.85℃,re-spectively.Compared with pure SA,the SA/BN/EG composite showed a lower melting temperature and a higher freezing temperature.In addition,when the mass fraction of BN and EG was 12 wt.%,the thermal conductivity of the composite was 6.349 W·m^(−1)·K^(−1),which was 18.619 times that of SA.More importantly,the composite showed good stability for 50 cycles of heating and cooling,and the SA/BN/EG-12 hardly decomposes below 200℃,which implies that the working performance of the composite PCM is relatively stable within the tem-perature range of 100℃.Therefore,the composite can exhibit excellent thermal stability in the field of building heating.展开更多
The nanolaminated MAB phases have attracted great research interests in recent years due to their similarities to MAX phases,which display both metallic and ceramic-like properties.In the present work,a newly discover...The nanolaminated MAB phases have attracted great research interests in recent years due to their similarities to MAX phases,which display both metallic and ceramic-like properties.In the present work,a newly discovered MAB phase Cr4AlB4 was investigated by first principles calculations.Energy evaluations indicate that Cr4AlB4 can be synthetized in Al lean condition,which can further transform to Cr2AlB2 in Al rich condition.The full set of elastic properties and their dependences on temperature,ideal strengths under different tensile and shear deformations,and thermal expansions of Cr4AlB4 were predicted.The results reveal that the properties of Cr4AlB4 are dominated by the layered crystal structure and weak bonding nature between Al and Cr2B2 layers,including low elastic stiffness and large thermal expansion along[010]direction(the stacking direction of Al and Cr2B2 layers),low shear resistances in(010)plane,and preferentially cleavage along and/or shear in(010)plane.Therefore,it suggests that Cr4AlB4 displays similar mechanical properties to MAX phases,including readily machinable,thermal shock resistant,and damage tolerant.In combination with the fact that Cr,Al and B all can form dense oxides to protect the material from further oxidation,Cr4AlB4 is regarded as a promising high temperature ceramic.展开更多
基金supported by PhD research startup foundation of Civil Aviation University of China(Grant No.2020KYQD94)。
文摘Based on the first-principles method,we predict two new stable BN allotropes:C12-BN and O16-BN,which belong to cubic and orthorhombic crystal systems,respectively.It is confirmed that both the phases are thermally and dynamically stable.The results of molecular dynamics simulations suggest that both the BN phases are highly stable even at high temperatures of 1000 K.In the case of mechanical properties,C12-BN has a bulk modulus of 359 GPa and a hardness of 43.4 GPa,making it a novel superhard material with potential technological and industrial applications.Electronic band calculations reveal that both C12-BN and O16-BN are insulators with direct band gaps of 3.02 e V and 3.54 e V,respectively.The XRD spectra of C12-BN and O16-BN are also simulated to provide more information for possible experimental observation.Our findings enrich the BN allotrope family and are expected to stimulate further experimental interest.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11005088 and 11105115)the Key Project of Henan Educational Commit-tee,China(Grant No.12A140010)the Special Foundation for Young Teacher of Xinyang Normal University,China(Grant No.2011084)
文摘Using the first-principles method of the plane-wave pseudo-potential, the structural properties of the newly-discovered willemite-Ⅱ Si3N4 （wⅡ phase） and post-phenacite Si3N4 （δ phase） are investigated. The α phase is predicted to undergo a first-order α→wⅡ phase transition at 18.6 GPa and 300 K. Within the quasi-harmonic approximation （QHA）, the α→wⅡ phase boundary is also obtained. When the well-known β→γ transition is suppressed by some kinetic reasons, the β→δ phase transformation could be observed in the phase diagram. Besides, the temperature dependences of the cell volume,thermal expansion coefficient, bulk modulus, specific heat, entropy and Debye temperature of the involved phases are determined from the non-equilibrium free energies. The thermal expansion coefficients of wⅡ-Si3N4 show no negative values in a pressure range of 0-30 GPa, which implies that the wⅡ-Si3N4 is mechanically stable. More importantly, the δ-Si3N4 is found to be a negative thermal expansion material. Further experimental investigations may be required to determine the physical properties of wⅡ- and δ-Si3N4 with higher reliability.
基金This research was supported by the National Natural Science Foundation of China(No.51766012)the Natural Science Foundation of Inner Mongolia(No.2019MS05025)+1 种基金the Inner Mongolia Science and Technology Major Project(No.2019ZD014,No.2021ZD0030)the Science and Technology Research Project of Inner Mongolia Autonomous Region(No.2021GG0252).
文摘In order to solve the problems of low thermal conductivity and easy liquid leakage of a stearic acid(SA),the composite phase change material(PCM)was prepared by adding boron nitride(BN)and expanded graphite(EG)to melted SA,and its thermal conductivity,crystal structure,chemical stability,thermal stability,cycle stability,leakage characteristics,heat storage/release characteristics,and temperature response characteristics were char-acterized.The results showed that the addition of BN and EG significantly improved the thermal conductivity of the material,and they efficiently adsorbed melted SA.The maximum load of SA was 76 wt.%and there was almost no liquid leakage.Moreover,the melting enthalpy and temperature were 154.20 J·g^(−1) and 67.85℃,re-spectively.Compared with pure SA,the SA/BN/EG composite showed a lower melting temperature and a higher freezing temperature.In addition,when the mass fraction of BN and EG was 12 wt.%,the thermal conductivity of the composite was 6.349 W·m^(−1)·K^(−1),which was 18.619 times that of SA.More importantly,the composite showed good stability for 50 cycles of heating and cooling,and the SA/BN/EG-12 hardly decomposes below 200℃,which implies that the working performance of the composite PCM is relatively stable within the tem-perature range of 100℃.Therefore,the composite can exhibit excellent thermal stability in the field of building heating.
基金supported by National Natural Science Foundation of China under Grant No.U1435206 and No.51672064.
文摘The nanolaminated MAB phases have attracted great research interests in recent years due to their similarities to MAX phases,which display both metallic and ceramic-like properties.In the present work,a newly discovered MAB phase Cr4AlB4 was investigated by first principles calculations.Energy evaluations indicate that Cr4AlB4 can be synthetized in Al lean condition,which can further transform to Cr2AlB2 in Al rich condition.The full set of elastic properties and their dependences on temperature,ideal strengths under different tensile and shear deformations,and thermal expansions of Cr4AlB4 were predicted.The results reveal that the properties of Cr4AlB4 are dominated by the layered crystal structure and weak bonding nature between Al and Cr2B2 layers,including low elastic stiffness and large thermal expansion along[010]direction(the stacking direction of Al and Cr2B2 layers),low shear resistances in(010)plane,and preferentially cleavage along and/or shear in(010)plane.Therefore,it suggests that Cr4AlB4 displays similar mechanical properties to MAX phases,including readily machinable,thermal shock resistant,and damage tolerant.In combination with the fact that Cr,Al and B all can form dense oxides to protect the material from further oxidation,Cr4AlB4 is regarded as a promising high temperature ceramic.
