New discovery in whisker growth on Cr-Al-B MAB phase,which was formed during hot-dip aluminizing and subsequent thermal diffusion treatment of Fe-Cr-B cast steel,after cultivation under 400℃,was found.Specifically,al...New discovery in whisker growth on Cr-Al-B MAB phase,which was formed during hot-dip aluminizing and subsequent thermal diffusion treatment of Fe-Cr-B cast steel,after cultivation under 400℃,was found.Specifically,along with the growth of Sn whiskers on the whitened Cr-Al-B MAB phase,Al whiskers also formed,mainly attributed to the thermal activation of whisker cultivation at 400℃and promotion effect of Sn in the Cr-(Al,Sn)-B MAB phase solid solution.Partial Al whiskers,interspersed with small amounts of Sn,exhibited transparency under the scanning electron microscopy image.These whiskers underwent random growth and sucking back into the matrix simultaneously.The temperature during whisker cultivation had a significant effect on the whisker growth.展开更多
In this communication, the possibility for the preparation of two-dimensional MBene CrB from MAB phase Cr2AlB2 is demonstrated for the first time. Herein M is a transition metal, A is a group IIIAor IVA element and B ...In this communication, the possibility for the preparation of two-dimensional MBene CrB from MAB phase Cr2AlB2 is demonstrated for the first time. Herein M is a transition metal, A is a group IIIAor IVA element and B is boron; MAB phases are layered transition metal ternary borides, MBene is used to emphasize the loss of A group element from the parent MAB phases and to highlight the 2 D nature. The possible 2 D CrB nano sheets are prepared by selectively etching out Al layers from Cr2AlB-2 by immersing the Cr2AlB2 powders in dilute HCl solution at room temperature. The as-prepared 2 D CrB nano sheets are examined using X-ray diffraction and scanning electron microscopy and energy dispersive X-ray spectroscopy（EDS）. In addition, different structure models are built to explain the observed phenomena.The discovery in this work opens a door to the synthesis of a large number of new 2 D MBenes from MAB phases.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
Through a combination of electronic structure, chemical bonding and mechanical property investigations, anisotropic electrical and mechanical properties, and damage tolerant ability of MAB phases CrsSi3 B and HfsSi3B ...Through a combination of electronic structure, chemical bonding and mechanical property investigations, anisotropic electrical and mechanical properties, and damage tolerant ability of MAB phases CrsSi3 B and HfsSi3B are predicted. The anisotropic electrical conductivity is due to the anisotropic distribution of Cr in CrsSiaB and Hf in HfsSi3B, which mainly contribute to the electrical conductivity. The anisotropic mechanical properties are underpinned by the anisotropic chemical bonding within the crystal structures of CrsSi3B and HfsSi3B. The high stiffness is determined by the strong covalent-ionic Crl--B--Crl and Crl--Si bonds in CrsSi3B and the ionic-covalent Hfl--B--Hfl and Si--B bonds in HfsSi3B; while the low shear deformation resistance is attributed to the presence of metallic Cr--Cr, Hf--Hf and Si--Si bond. Based on the low Pugh＇s ratio, CrsSi3B and Hfs Si3B are predicted tolerant to damage. The possible cleavage plane is （0001） and the possible slip systems are 〈1 100〉1{11 20} and 〈11 20〉1{0001} for both CrsSi3B and HfsSi3B.展开更多
In this communication, the crystal structure of Cr_4AlB_4, a new MAB phase compound(where M is a transition metal, A is Al or Si, B is boron) discovered in Cr-Al-B system is reported. This new MAB phase was synthesize...In this communication, the crystal structure of Cr_4AlB_4, a new MAB phase compound(where M is a transition metal, A is Al or Si, B is boron) discovered in Cr-Al-B system is reported. This new MAB phase was synthesized from a mixture of CrB and Al powders at 1000?C and its crystal structure was determined by a combination of X-ray diffraction, first-principles calculations and energy dispersive X-ray spectroscopy(EDS). Cr_4AlB_4 crystallizes in an orthorhombic structure with Immm space group. The lattice constants are a = 2.9343(6) ?, b = 18.8911(0) ?, c = 2.9733(7) ?, and the atomic positions are Cr1 at 4 g(0, 0.2936(5),0), Cr2 at 4 h(0.5, 0.5859(7), 0), Al at 2 b(0, 0.5, 0.5), B1 at 4 h(0, 0.3839(8), 0.5) and B2 at 4 g(0.5, 0.6646(2),0.5).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
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 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
MAB phases are layered ternary compounds with alternative stacking of transition metal boride layers and group A element layers.Until now,most of the investigated MAB phases are concentrated on compounds with Al as th...MAB phases are layered ternary compounds with alternative stacking of transition metal boride layers and group A element layers.Until now,most of the investigated MAB phases are concentrated on compounds with Al as the A element layers.In this work,the family of M_(5)SiB_(2)(M=IVB-VIB transition metals)compounds with silicon as interlayers were investigated by density functional theory(DFT)methods as potential MAB phases for high-temperature applications.Starting from the known Mo5SiB2,the electronic structure,bonding characteristics,and mechanical behaviors were systematically investigated and discussed.Although the composition of M_(5)SiB_(2) does not follow the general formula of experimentally reported(MB)_(2z)A_(x)(MB_(2))_(y)(z=1,2;x=1,2;y=0,1,2),their layered structure and anisotropic bonding characteristics are similar to other known MAB phases,which justifies their classification as new members of this material class.As a result of the higher bulk modulus and lower shear modulus,Mo_(5)SiB_(2) has a Pugh’s ratio of 0.53,which is much lower than the common MAB phases.It was found that the stability and mechanical properties of M_(5)SiB_(2) compounds depend on their valence electron concentrations(VECs),and an optimum VEC exists as the criteria for stability.The hypothesized Zr and Hf containing compounds,i.e.,Zr_(5)SiB_(2) and Hf_(5)SiB_(2),which are more interesting in terms of high-temperature oxidation/ablation resistance,were found to be unfortunately unstable.To cope with this problem,a new stable solid solution(Zr_(0.6)Mo_(0.4))_(5)SiB_(2) was designed based on VEC tuning to demonstrate a promising approach for developing new MAB phases with desirable compositions.展开更多
The emergence of MAX borides as well as MAB phases attracted great attention because of the renewable developments of ternary ceramics and offering great opportunities in potential applications.However,the number of b...The emergence of MAX borides as well as MAB phases attracted great attention because of the renewable developments of ternary ceramics and offering great opportunities in potential applications.However,the number of borides remains limited,and further fundamental descriptions and detailed investigations on various properties are still lacking.In this report,we employ an integrated computational scheme that combines density functional theory with the evolutional algorithm to search for the favorable structures of P-and S-glued ternary borides terminated by Nb metal.We discover that the structures of 212-type,as e.g.Nb_(2)PB_(2)and Nb_(2)SB_(2),belong to the P6m2 space group,while those of 211-type,as e.g.Nb_(2)PB and Nb_(2)SB,prefer to crystallize in the P6_(3)/mmc space group,and the corresponding carbides Nb_(2)PC and Nb_(2)SC are also considered for the sake of completeness and comparative analsys.The predicted Nb_(2)PB_(2),Nb_(2)PB,Nb_(2)SB,Nb_(2)PC and Nb_(2)SC are energetically stable,as revealed by the negative formation energies and by the proposed reaction paths with respect to the most competing phases,as well as dynamically stable,as suggested by the non-imaginary phonon spectra.The thermal conductivities of the six materials show unusual behaviors,particularly for the acoustic and optical contributions,and are accompanied by a strong anisotropy.Most importantly,Nb_(2)PB_(2) is found to be an excellent thermal conductor with a total thermal conductivity of~65 W/(m K),while Nb_(2)SC is found to be an ultra-low thermal conductor,with a total thermal conductivity of~5 W/(m K).These values are clearly outside the currently reported range of thermal conductivities,which makes Nb_(2)PB_(2)and Nb_(2)SC extremely interesting for fundamental research as well as prospective applications with the aid of artificial tunings on the almost independent MB block and the A layer.The discovery of these novel materials is expected to contribute substantially to the rapid development of ternary ceramics and to accelerate attempts in the applicability of MAX phases for heat conduction.展开更多
Phase pure and well crystalline Cr2AlB2 powders are synthesized by heating the mixtures of CrB and Al powders at 900~℃.Cr2AlB2 exhibits nanolaminated morphology which transforms from flake-like crystallite to needle-...Phase pure and well crystalline Cr2AlB2 powders are synthesized by heating the mixtures of CrB and Al powders at 900~℃.Cr2AlB2 exhibits nanolaminated morphology which transforms from flake-like crystallite to needle-like grain with the increase of holding time.The morphology-structure relationships of Cr2AlB2 are delicately discussed.Meanwhile,as the precursor for fabrication of Cr2AlB2,high purity CrB powders are also prepared by high-temperature reaction of B and Cr elemental powders at 1800℃.CrB grains grow into regular plate-like morphology.Through Rietveld structure refinement,new sets of diffraction data are presented for both CrB and Cr2AlB2 and overlapped peak positions and intensities are revealed which make up for the deficiency of the existing data in ICDD PDF#32-0277(CrB)and ICDD PDF#72-1847(Cr2AlB2).Moreover,since MAB phases are precursors for preparing MBenes,2D-CrBnanosheets are successfully prepared by completely etching out Al atomic layers from Cr2AlB2.2D-CrB crystalizes in CrB structure with two-dimensional lamellar morphology.Simultaneously the formation mechanism of 2D-CrB is vividly depicted.A system of materials preparation from CrB to Cr2AlB2 and then to 2D-CrB is well established.展开更多
Two-dimensional(2D)transition metal borides(MBenes)have emerged as a rising star and hold great potential promise for catalysis and metal ion batteries owing to a well-defined layered structure and ex-cellent electric...Two-dimensional(2D)transition metal borides(MBenes)have emerged as a rising star and hold great potential promise for catalysis and metal ion batteries owing to a well-defined layered structure and ex-cellent electrical conductivity.Unlike well-studied graphene,perovskite and MXene materials in various fields,the research about MBene is still in its infancy.The inadequate exploration of efficient etching methods impedes their further study.Herein,we put forward an efficient microwave-assisted hydrother-mal alkaline solution etching strategy for exfoliating MoAlB MAB phase into 2D MoB MBenes with a well accordion-like structure,which displays a remarkable electrochemical performance in sodium ion batter-ies(SIBs)with a reversible specific capacity of 196.5 mAh g^(-1)at the current density of 50 mA g^(-1),and 138.6 mAh g^(-1)after 500 cycles at the current density of 0.5 A g^(-1).The underlying mechanism toward excellent electrochemical performance are revealed by comprehensive theoretical simulations.This work proves that MBene is a competitive candidate as the next generation anode of sodium ion batteries.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
基金financial support from the National Natural Science Foundation of China(Grant Nos.42066003 and 51701053)Hainan Province Science and Technology Special Fund(Grant No.ZDYF2023GXJS148)+1 种基金General Program of Natural Science Foundation of Hainan Province(Grant Nos.420RC522 and 517076)Evaluation Project of Guangdong Provincial Key Laboratory(Grant No.2023B1212060043).
文摘New discovery in whisker growth on Cr-Al-B MAB phase,which was formed during hot-dip aluminizing and subsequent thermal diffusion treatment of Fe-Cr-B cast steel,after cultivation under 400℃,was found.Specifically,along with the growth of Sn whiskers on the whitened Cr-Al-B MAB phase,Al whiskers also formed,mainly attributed to the thermal activation of whisker cultivation at 400℃and promotion effect of Sn in the Cr-(Al,Sn)-B MAB phase solid solution.Partial Al whiskers,interspersed with small amounts of Sn,exhibited transparency under the scanning electron microscopy image.These whiskers underwent random growth and sucking back into the matrix simultaneously.The temperature during whisker cultivation had a significant effect on the whisker growth.
基金supports from the National Natural Science Foundation of China under grant No. 51672064 and No. U1435206the Beijing Municipal Science & Technology Commission under grant No. Z151100003315012 and No. D161100002416001
文摘In this communication, the possibility for the preparation of two-dimensional MBene CrB from MAB phase Cr2AlB2 is demonstrated for the first time. Herein M is a transition metal, A is a group IIIAor IVA element and B is boron; MAB phases are layered transition metal ternary borides, MBene is used to emphasize the loss of A group element from the parent MAB phases and to highlight the 2 D nature. The possible 2 D CrB nano sheets are prepared by selectively etching out Al layers from Cr2AlB-2 by immersing the Cr2AlB2 powders in dilute HCl solution at room temperature. The as-prepared 2 D CrB nano sheets are examined using X-ray diffraction and scanning electron microscopy and energy dispersive X-ray spectroscopy（EDS）. In addition, different structure models are built to explain the observed phenomena.The discovery in this work opens a door to the synthesis of a large number of new 2 D MBenes from MAB phases.
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.
基金supported by the Natural Sciences Foundation of China under Grant No.51672064 and No.U1435206Beijing Municipal Science&Technology Commission under Grant number Z151100003315012 and D161100002416001
文摘Through a combination of electronic structure, chemical bonding and mechanical property investigations, anisotropic electrical and mechanical properties, and damage tolerant ability of MAB phases CrsSi3 B and HfsSi3B are predicted. The anisotropic electrical conductivity is due to the anisotropic distribution of Cr in CrsSiaB and Hf in HfsSi3B, which mainly contribute to the electrical conductivity. The anisotropic mechanical properties are underpinned by the anisotropic chemical bonding within the crystal structures of CrsSi3B and HfsSi3B. The high stiffness is determined by the strong covalent-ionic Crl--B--Crl and Crl--Si bonds in CrsSi3B and the ionic-covalent Hfl--B--Hfl and Si--B bonds in HfsSi3B; while the low shear deformation resistance is attributed to the presence of metallic Cr--Cr, Hf--Hf and Si--Si bond. Based on the low Pugh＇s ratio, CrsSi3B and Hfs Si3B are predicted tolerant to damage. The possible cleavage plane is （0001） and the possible slip systems are 〈1 100〉1{11 20} and 〈11 20〉1{0001} for both CrsSi3B and HfsSi3B.
基金financial supports from the National Natural Science Foundation of China under grant No.51672064 and No.U1435206
文摘In this communication, the crystal structure of Cr_4AlB_4, a new MAB phase compound(where M is a transition metal, A is Al or Si, B is boron) discovered in Cr-Al-B system is reported. This new MAB phase was synthesized from a mixture of CrB and Al powders at 1000?C and its crystal structure was determined by a combination of X-ray diffraction, first-principles calculations and energy dispersive X-ray spectroscopy(EDS). Cr_4AlB_4 crystallizes in an orthorhombic structure with Immm space group. The lattice constants are a = 2.9343(6) ?, b = 18.8911(0) ?, c = 2.9733(7) ?, and the atomic positions are Cr1 at 4 g(0, 0.2936(5),0), Cr2 at 4 h(0.5, 0.5859(7), 0), Al at 2 b(0, 0.5, 0.5), B1 at 4 h(0, 0.3839(8), 0.5) and B2 at 4 g(0.5, 0.6646(2),0.5).
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 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 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financially supported by the National Natural Science Foundation of China(No.52072238).
文摘MAB phases are layered ternary compounds with alternative stacking of transition metal boride layers and group A element layers.Until now,most of the investigated MAB phases are concentrated on compounds with Al as the A element layers.In this work,the family of M_(5)SiB_(2)(M=IVB-VIB transition metals)compounds with silicon as interlayers were investigated by density functional theory(DFT)methods as potential MAB phases for high-temperature applications.Starting from the known Mo5SiB2,the electronic structure,bonding characteristics,and mechanical behaviors were systematically investigated and discussed.Although the composition of M_(5)SiB_(2) does not follow the general formula of experimentally reported(MB)_(2z)A_(x)(MB_(2))_(y)(z=1,2;x=1,2;y=0,1,2),their layered structure and anisotropic bonding characteristics are similar to other known MAB phases,which justifies their classification as new members of this material class.As a result of the higher bulk modulus and lower shear modulus,Mo_(5)SiB_(2) has a Pugh’s ratio of 0.53,which is much lower than the common MAB phases.It was found that the stability and mechanical properties of M_(5)SiB_(2) compounds depend on their valence electron concentrations(VECs),and an optimum VEC exists as the criteria for stability.The hypothesized Zr and Hf containing compounds,i.e.,Zr_(5)SiB_(2) and Hf_(5)SiB_(2),which are more interesting in terms of high-temperature oxidation/ablation resistance,were found to be unfortunately unstable.To cope with this problem,a new stable solid solution(Zr_(0.6)Mo_(0.4))_(5)SiB_(2) was designed based on VEC tuning to demonstrate a promising approach for developing new MAB phases with desirable compositions.
基金the National Natural Science Foundation of China(No.51902052)partially supported by“the Fundamental Research Funds for the Central Universities”+1 种基金supported by appointments to the JRG program at the APCTP through the Science and Technology Promotion Fund and Lottery Fund of the Korean Government,the Korean Local Governments,Gyeongsangbuk-do Province and Pohang Citypartially funded by the Swedish Research Council through grant agreement no.2018-05973 for providing the facility support on the numerical calculations in this paper。
文摘The emergence of MAX borides as well as MAB phases attracted great attention because of the renewable developments of ternary ceramics and offering great opportunities in potential applications.However,the number of borides remains limited,and further fundamental descriptions and detailed investigations on various properties are still lacking.In this report,we employ an integrated computational scheme that combines density functional theory with the evolutional algorithm to search for the favorable structures of P-and S-glued ternary borides terminated by Nb metal.We discover that the structures of 212-type,as e.g.Nb_(2)PB_(2)and Nb_(2)SB_(2),belong to the P6m2 space group,while those of 211-type,as e.g.Nb_(2)PB and Nb_(2)SB,prefer to crystallize in the P6_(3)/mmc space group,and the corresponding carbides Nb_(2)PC and Nb_(2)SC are also considered for the sake of completeness and comparative analsys.The predicted Nb_(2)PB_(2),Nb_(2)PB,Nb_(2)SB,Nb_(2)PC and Nb_(2)SC are energetically stable,as revealed by the negative formation energies and by the proposed reaction paths with respect to the most competing phases,as well as dynamically stable,as suggested by the non-imaginary phonon spectra.The thermal conductivities of the six materials show unusual behaviors,particularly for the acoustic and optical contributions,and are accompanied by a strong anisotropy.Most importantly,Nb_(2)PB_(2) is found to be an excellent thermal conductor with a total thermal conductivity of~65 W/(m K),while Nb_(2)SC is found to be an ultra-low thermal conductor,with a total thermal conductivity of~5 W/(m K).These values are clearly outside the currently reported range of thermal conductivities,which makes Nb_(2)PB_(2)and Nb_(2)SC extremely interesting for fundamental research as well as prospective applications with the aid of artificial tunings on the almost independent MB block and the A layer.The discovery of these novel materials is expected to contribute substantially to the rapid development of ternary ceramics and to accelerate attempts in the applicability of MAX phases for heat conduction.
基金financial supports from the National Natural Science Foundation of China under grant No. 51672064, No. U1435206 and No. 61271049
文摘Phase pure and well crystalline Cr2AlB2 powders are synthesized by heating the mixtures of CrB and Al powders at 900~℃.Cr2AlB2 exhibits nanolaminated morphology which transforms from flake-like crystallite to needle-like grain with the increase of holding time.The morphology-structure relationships of Cr2AlB2 are delicately discussed.Meanwhile,as the precursor for fabrication of Cr2AlB2,high purity CrB powders are also prepared by high-temperature reaction of B and Cr elemental powders at 1800℃.CrB grains grow into regular plate-like morphology.Through Rietveld structure refinement,new sets of diffraction data are presented for both CrB and Cr2AlB2 and overlapped peak positions and intensities are revealed which make up for the deficiency of the existing data in ICDD PDF#32-0277(CrB)and ICDD PDF#72-1847(Cr2AlB2).Moreover,since MAB phases are precursors for preparing MBenes,2D-CrBnanosheets are successfully prepared by completely etching out Al atomic layers from Cr2AlB2.2D-CrB crystalizes in CrB structure with two-dimensional lamellar morphology.Simultaneously the formation mechanism of 2D-CrB is vividly depicted.A system of materials preparation from CrB to Cr2AlB2 and then to 2D-CrB is well established.
基金supported by the National Key Re-search and Development Program of China(No.2020YFC1909604)SZIIT Startup Fund(No.SZIIT2022KJ072)+1 种基金Shenzhen Peacock Project Startup Fund(No.RC2023-002)Shenzhen Steady General Projects(No.KJ2024C010).
文摘Two-dimensional(2D)transition metal borides(MBenes)have emerged as a rising star and hold great potential promise for catalysis and metal ion batteries owing to a well-defined layered structure and ex-cellent electrical conductivity.Unlike well-studied graphene,perovskite and MXene materials in various fields,the research about MBene is still in its infancy.The inadequate exploration of efficient etching methods impedes their further study.Herein,we put forward an efficient microwave-assisted hydrother-mal alkaline solution etching strategy for exfoliating MoAlB MAB phase into 2D MoB MBenes with a well accordion-like structure,which displays a remarkable electrochemical performance in sodium ion batter-ies(SIBs)with a reversible specific capacity of 196.5 mAh g^(-1)at the current density of 50 mA g^(-1),and 138.6 mAh g^(-1)after 500 cycles at the current density of 0.5 A g^(-1).The underlying mechanism toward excellent electrochemical performance are revealed by comprehensive theoretical simulations.This work proves that MBene is a competitive candidate as the next generation anode of sodium ion batteries.
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.
