The ternary or quaternary layered compounds called MAB phases are frequently mentioned recently together with the well-known MAX phases.However,MAB phases are generally referred to layered transition metal borides,whi...The ternary or quaternary layered compounds called MAB phases are frequently mentioned recently together with the well-known MAX phases.However,MAB phases are generally referred to layered transition metal borides,while MAX phases are layered transition metal carbides and nitrides with different types of crystal structure although they share the common nano-laminated structure characteristics.In order to prove that MAB phases can share the same type of crystal structure with MAX phases and extend the composition window of MAX phases from carbides and nitrides to borides,two new MAB phase compounds Zr_(2)SeB and Hf_(2)SeB with the Cr_(2)AlC-type MAX phase(211 phase)crystal structure were discovered by a combination of first-principles calculations and experimental verification in this work.First-principles calculations predicted the stability and lattice parameters of the two new MAB phase compounds Zr_(2)SeB and Hf_(2)SeB.Then they were successfully synthesized by using a thermal explosion method in a spark plasma sintering(SPS)furnace.The crystal structures of Zr_(2)SeB and Hf_(2)SeB were determined by a combination of the X-ray diffraction(XRD),scanning electron microscopy(SEM),and high-resolution transmission electron microscopy(HRTEM).The lattice parameters of Zr_(2)SeB and Hf_(2)SeB are a=3.64398Å,c=12.63223Åand a=3.52280Å,c=12.47804Å,respectively.And the atomic positions are M at 4f(1/3,2/3,0.60288[Zr]or 0.59889[Hf]),Se at 2c(1/3,2/3,1/4),and B at 2a(0,0,0).And the atomic stacking sequences follow those of the Cr_(2)AlC-type MAX phases.This work opens up the composition window for the MAB phases and MAX phases and will trigger the interests of material scientists and physicists to explore new compounds and properties in this new family of materials.展开更多
A 314-type MAB phase V_(3)PB_(4)with hexagonal crystal structure is synthesized by self-propagating high temperature combustion synthesis(SHS),with the help of the full first-principles predictions for the phase stabi...A 314-type MAB phase V_(3)PB_(4)with hexagonal crystal structure is synthesized by self-propagating high temperature combustion synthesis(SHS),with the help of the full first-principles predictions for the phase stability and adiabatic combustion temperature of SHS.Using XRD and TEM,V_(3)PB_(4)crystallizes in the space group of P6m2,with the lattice parameters a?3.030Åand c=9.148Å,of much interest,well with the predicted one.Furthermore,the electronic structure,chemical bonding,and elastic properties of hex-V_(3)PB_(4)are predicted by first-principles.No bandgap around Fermi energy indicates its electronic conductor.And the strong covalent bonding is present between the B and V atoms with,significantly,much weaker V-P bond.With the help of the theoretical model of bond stiffness,the significantly high ratio of bond stiffness of weakest bonds to the strongest ones(0.873)of hex-V_(3)PB_(4)indicates its poor damage tolerance and fracture toughness.The high bond stiffness results in its high moduli in comparison with other MAB phases.As the number of inserted P atoms increases,the engineering elastic modulus decrease,without the price of an increase in density.展开更多
Mo2AlB2 is a new MAB phase that has been observed in thin foils of Mo Al B during TEM observation and in Na OH etched Mo Al B samples. However, the structural characteristics, chemical bonding and properties of this n...Mo2AlB2 is a new MAB phase that has been observed in thin foils of Mo Al B during TEM observation and in Na OH etched Mo Al B samples. However, the structural characteristics, chemical bonding and properties of this new compound have not been investigated. In this work, geometry optimized crystal structure of Mo2AlB2 is obtained and its stability, elastic and thermal dynamic properties are investigated. Mo2AlB2 is stable in Al lean conditions, which is consistent with the exiting experimental results. It is also gauged as a damage tolerant or quasi-ductile ceramic based on the low Pugh’s shear to bulk modulus ratio(G/B = 0.544) and positive Cauchy pressure in all three crystallographic directions, which is underpinned by the metallic bonding. Mo2AlB2 also exhibits high stiffness which is attributed to the strong B–B covalent bond chains within its crystal structure. Due to the anisotropic chemical bonding, Mo2AlB2 has anisotropic thermal expansion coefficients αa= 6.19 × 10^–6 K1, αb= 12.13 × 10^-6 K^–1, αc= 6.66 × 10^-6 K^–1,respectively, along a, b and c directions in the temperature range between 300 and 1500 K. The heat capacity from 300 to 1500 K can be described as Cp= 120.32 + 0.01648 T-2.597 × 10^6 T^-2(J·mol^–1·K^–1).The elastic constants decrease almost linearly with temperature. The elastic constants representing the resistance to principle deformation(c11, c22 and c33) decrease in faster rates than those representing shear deformation resistance(c44, c55 and c66). Correspondingly, bulk and Young’s modulus decrease in faster rates than shear modulus. In light of the structure-property relations of Mo2AlB2, it is suggested that future damage tolerant ceramics can be designed by putting stiff covalent bonding units into soft metallic bonding box to obtain both high stiffness and quasi-ductility.展开更多
Achieving more meaningful N2 conversion by reducing the energy input and carbon footprint is now being investigated through a method of N2 fixation instead of the Haber-Bosch process.Unfortunately,the electrochemical ...Achieving more meaningful N2 conversion by reducing the energy input and carbon footprint is now being investigated through a method of N2 fixation instead of the Haber-Bosch process.Unfortunately,the electrochemical N2 reduction reaction(NRR)method as a rising approach currently still shows low selectivity(Faradaic efficiency<10%)and high-energy consumption[applied potential at least-0.2 V versus the reversible hydrogen electrode(RHE)].Here,the role of molybdenum aluminum boride single crystals,belonging to a family of ternary transition metal aluminum borides known as MAB phases,is reported for the electrochemical NRR for the first time,at a low applied potential(-0.05 V versus RHE)under ambient conditions and in alkaline media.Due to the unique nano-laminated crystal structure of the MAB phase,these inexpensive materials have been found to exhibit excellent electrocatalytic performances(NH3 yield:9.2μg h^-1cm^-2mgcat^-1.,Faradaic efficiency:30.1%)at the low overpotential,and to display a high chemical stability and sustained catalytic performance.In conjunction,further mechanism studies indicate B and Al as main-group metals show a highly selective affinity to N2 due to the strong interaction between the B 2p/Al 3p band and the N 2p orbitals,while Mo exhibits specific catalytic activity toward the subsequent reduction reaction.Overall,the MAB-phase catalyst under the synergy of the elements within ternary compound can suppress the hydrogen evolution reaction and achieve enhanced NRR performance.The significance of this work is to provide a promising candidate in the future synthesis of ammonia.展开更多
The nanolaminated MAB phases have attracted great research interests due to their unusual combination of metal-like and ceramic-like properties, which is similar to MAX phases. Recently, ordered quaternary MAX phases ...The nanolaminated MAB phases have attracted great research interests due to their unusual combination of metal-like and ceramic-like properties, which is similar to MAX phases. Recently, ordered quaternary MAX phases have been discovered, which enriches the family of MAX phases, and opens a new window to tailor the properties of MAX phases and to develop new MXenes. In the present work, we explored possible ordered quaternary MAB phases with Cr3AlB4 structure(space group: Pmmm) by first-principles calculations. The predictions show that M2M’AlB4 phases with M = Mn, Fe, Co and M’ = Cr, Mo, W exhibit strong tendency of ordering, where M locates at 2t site(0.5, 0.5, z2t) and M’ locates at 1 g site(0, 0.5,0.5). The main driving force of ordering may be the differences in bonding strengths between Al and M elements. Analyses on chemical bonds reveal that bonding strengths increase following the order:Al-Mn < Al-Fe < Al-Co, which is consistent with the prediction that ordering tendency increases when M changes from Mn to Co, as derived from enthalpy differences. The ordered M2M’AlB4 phases with M =Mn or Fe are predicted ferromagnetic and ordered M2M’AlB4 phases display lower shear resistance and possibly better ductility in comparison to Cr3AlB4.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
Fe_(2)AlB_(2) powder material was prepared by the direct reaction of iron,aluminum and boron powders in a tubular furnace.The effects of different Al contents,temperature and raw material pretreatment on the purity of...Fe_(2)AlB_(2) powder material was prepared by the direct reaction of iron,aluminum and boron powders in a tubular furnace.The effects of different Al contents,temperature and raw material pretreatment on the purity of product were studied.The mixed powder with the stoichiometric ratio of 1.5Al/2Fe/2B was processed by CIP (Cold Isostatic Pressing),and then calcined at 1150℃ for 120 min.The product containing a small amount of impurities is treated with alkaline solution to obtain high-purity Fe_(2)Al B_(2) powder.Zr B_(2)-Fe_(2)Al B_(2) composite ceramic was successfully prepared at 1250℃ by hot pressing sintering.The density,hardness and fracture toughness were 96.2%,22±0.3 GPa and 5.78±0.5 MPa·m^(1/2),respectively.展开更多
基金supported by the National Natural Science Foundation of China(52032011 and 52072311)Outstanding Young Scientific and Technical Talents in Sichuan Province(2019JDJQ0009)+1 种基金Fundamental Research Funds for the Central Universities(2682020ZT61,2682021GF013,and XJ2021KJZK042)the Opening Project of State Key Laboratory of Green Building Materials,and the Project of State Key Laboratory of Environment-friendly Energy Materials(20kfhg17).
文摘The ternary or quaternary layered compounds called MAB phases are frequently mentioned recently together with the well-known MAX phases.However,MAB phases are generally referred to layered transition metal borides,while MAX phases are layered transition metal carbides and nitrides with different types of crystal structure although they share the common nano-laminated structure characteristics.In order to prove that MAB phases can share the same type of crystal structure with MAX phases and extend the composition window of MAX phases from carbides and nitrides to borides,two new MAB phase compounds Zr_(2)SeB and Hf_(2)SeB with the Cr_(2)AlC-type MAX phase(211 phase)crystal structure were discovered by a combination of first-principles calculations and experimental verification in this work.First-principles calculations predicted the stability and lattice parameters of the two new MAB phase compounds Zr_(2)SeB and Hf_(2)SeB.Then they were successfully synthesized by using a thermal explosion method in a spark plasma sintering(SPS)furnace.The crystal structures of Zr_(2)SeB and Hf_(2)SeB were determined by a combination of the X-ray diffraction(XRD),scanning electron microscopy(SEM),and high-resolution transmission electron microscopy(HRTEM).The lattice parameters of Zr_(2)SeB and Hf_(2)SeB are a=3.64398Å,c=12.63223Åand a=3.52280Å,c=12.47804Å,respectively.And the atomic positions are M at 4f(1/3,2/3,0.60288[Zr]or 0.59889[Hf]),Se at 2c(1/3,2/3,1/4),and B at 2a(0,0,0).And the atomic stacking sequences follow those of the Cr_(2)AlC-type MAX phases.This work opens up the composition window for the MAB phases and MAX phases and will trigger the interests of material scientists and physicists to explore new compounds and properties in this new family of materials.
基金supported by the National Natural Science Foundation of China(Grant No.51972080).
文摘A 314-type MAB phase V_(3)PB_(4)with hexagonal crystal structure is synthesized by self-propagating high temperature combustion synthesis(SHS),with the help of the full first-principles predictions for the phase stability and adiabatic combustion temperature of SHS.Using XRD and TEM,V_(3)PB_(4)crystallizes in the space group of P6m2,with the lattice parameters a?3.030Åand c=9.148Å,of much interest,well with the predicted one.Furthermore,the electronic structure,chemical bonding,and elastic properties of hex-V_(3)PB_(4)are predicted by first-principles.No bandgap around Fermi energy indicates its electronic conductor.And the strong covalent bonding is present between the B and V atoms with,significantly,much weaker V-P bond.With the help of the theoretical model of bond stiffness,the significantly high ratio of bond stiffness of weakest bonds to the strongest ones(0.873)of hex-V_(3)PB_(4)indicates its poor damage tolerance and fracture toughness.The high bond stiffness results in its high moduli in comparison with other MAB phases.As the number of inserted P atoms increases,the engineering elastic modulus decrease,without the price of an increase in density.
基金supported financially by the National Natural Sciences Foundation of China (Nos. 51672064 and U1435206)
文摘Mo2AlB2 is a new MAB phase that has been observed in thin foils of Mo Al B during TEM observation and in Na OH etched Mo Al B samples. However, the structural characteristics, chemical bonding and properties of this new compound have not been investigated. In this work, geometry optimized crystal structure of Mo2AlB2 is obtained and its stability, elastic and thermal dynamic properties are investigated. Mo2AlB2 is stable in Al lean conditions, which is consistent with the exiting experimental results. It is also gauged as a damage tolerant or quasi-ductile ceramic based on the low Pugh’s shear to bulk modulus ratio(G/B = 0.544) and positive Cauchy pressure in all three crystallographic directions, which is underpinned by the metallic bonding. Mo2AlB2 also exhibits high stiffness which is attributed to the strong B–B covalent bond chains within its crystal structure. Due to the anisotropic chemical bonding, Mo2AlB2 has anisotropic thermal expansion coefficients αa= 6.19 × 10^–6 K1, αb= 12.13 × 10^-6 K^–1, αc= 6.66 × 10^-6 K^–1,respectively, along a, b and c directions in the temperature range between 300 and 1500 K. The heat capacity from 300 to 1500 K can be described as Cp= 120.32 + 0.01648 T-2.597 × 10^6 T^-2(J·mol^–1·K^–1).The elastic constants decrease almost linearly with temperature. The elastic constants representing the resistance to principle deformation(c11, c22 and c33) decrease in faster rates than those representing shear deformation resistance(c44, c55 and c66). Correspondingly, bulk and Young’s modulus decrease in faster rates than shear modulus. In light of the structure-property relations of Mo2AlB2, it is suggested that future damage tolerant ceramics can be designed by putting stiff covalent bonding units into soft metallic bonding box to obtain both high stiffness and quasi-ductility.
文摘Achieving more meaningful N2 conversion by reducing the energy input and carbon footprint is now being investigated through a method of N2 fixation instead of the Haber-Bosch process.Unfortunately,the electrochemical N2 reduction reaction(NRR)method as a rising approach currently still shows low selectivity(Faradaic efficiency<10%)and high-energy consumption[applied potential at least-0.2 V versus the reversible hydrogen electrode(RHE)].Here,the role of molybdenum aluminum boride single crystals,belonging to a family of ternary transition metal aluminum borides known as MAB phases,is reported for the electrochemical NRR for the first time,at a low applied potential(-0.05 V versus RHE)under ambient conditions and in alkaline media.Due to the unique nano-laminated crystal structure of the MAB phase,these inexpensive materials have been found to exhibit excellent electrocatalytic performances(NH3 yield:9.2μg h^-1cm^-2mgcat^-1.,Faradaic efficiency:30.1%)at the low overpotential,and to display a high chemical stability and sustained catalytic performance.In conjunction,further mechanism studies indicate B and Al as main-group metals show a highly selective affinity to N2 due to the strong interaction between the B 2p/Al 3p band and the N 2p orbitals,while Mo exhibits specific catalytic activity toward the subsequent reduction reaction.Overall,the MAB-phase catalyst under the synergy of the elements within ternary compound can suppress the hydrogen evolution reaction and achieve enhanced NRR performance.The significance of this work is to provide a promising candidate in the future synthesis of ammonia.
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 due to their unusual combination of metal-like and ceramic-like properties, which is similar to MAX phases. Recently, ordered quaternary MAX phases have been discovered, which enriches the family of MAX phases, and opens a new window to tailor the properties of MAX phases and to develop new MXenes. In the present work, we explored possible ordered quaternary MAB phases with Cr3AlB4 structure(space group: Pmmm) by first-principles calculations. The predictions show that M2M’AlB4 phases with M = Mn, Fe, Co and M’ = Cr, Mo, W exhibit strong tendency of ordering, where M locates at 2t site(0.5, 0.5, z2t) and M’ locates at 1 g site(0, 0.5,0.5). The main driving force of ordering may be the differences in bonding strengths between Al and M elements. Analyses on chemical bonds reveal that bonding strengths increase following the order:Al-Mn < Al-Fe < Al-Co, which is consistent with the prediction that ordering tendency increases when M changes from Mn to Co, as derived from enthalpy differences. The ordered M2M’AlB4 phases with M =Mn or Fe are predicted ferromagnetic and ordered M2M’AlB4 phases display lower shear resistance and possibly better ductility in comparison to Cr3AlB4.
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.
基金supported by the grants of CAS Priority Research Program(XDB20010300,XDA21010204)Science and Technology Program of Fujian Province(2017T3001)。
文摘Fe_(2)AlB_(2) powder material was prepared by the direct reaction of iron,aluminum and boron powders in a tubular furnace.The effects of different Al contents,temperature and raw material pretreatment on the purity of product were studied.The mixed powder with the stoichiometric ratio of 1.5Al/2Fe/2B was processed by CIP (Cold Isostatic Pressing),and then calcined at 1150℃ for 120 min.The product containing a small amount of impurities is treated with alkaline solution to obtain high-purity Fe_(2)Al B_(2) powder.Zr B_(2)-Fe_(2)Al B_(2) composite ceramic was successfully prepared at 1250℃ by hot pressing sintering.The density,hardness and fracture toughness were 96.2%,22±0.3 GPa and 5.78±0.5 MPa·m^(1/2),respectively.
