Metal Hexaborides with Sc, Ti or Mn

Comparison of well-determined single crystal data for stoichiometric, or near-stoichiometric, metal hexaborides confirms previously identified lattice parameter trends using powder diffraction. Trends for both divalent and trivalent forms suggest that potential new forms for synthesis include Sc and Mn hexaborides. Density Functional Theory (DFT) calculations for KB6, CaB6, YB6, LaB6, boron octahedral clusters and Sc and Mn forms show that the shapes of bonding orbitals are defined by the boron framework. Inclusion of metal into the boron framework induces a reduction in energy ranging from 1 eV to 6 eV increasing with ionic charge. For metals with d1 character, such a shift in energy brings a doubly degenerate band section along with the G-M reciprocal space direction within the conduction bands tangential to the Fermi surface. ScB6 band structure and density of states calculations show directions and gap characteristics similar to those of YB6 and LaB6. These calculations for ScB6 suggest that it may be possible to realize superconductivity in this compound if synthesized.

One-step synthesis and electromagnetic absorption properties of high entropy rare earth hexaborides(HE REB6)and high entropy rare earth hexaborides/borates(HE REB6/HE REB03)composite powders

Considering the emergence of severe electromagnetic interference problems,it is vital to develop electromagnetic(EM)wave absorbing materials with high dielectric,magnetic loss and optimized impedance matching.However,realizing the synergistic dielectric and magnetic losses in a single phase material is still a challenge.Herein,high entropy(HE)rare earth hexaborides(REB6)powders with coupling of dielectric and magnetic losses were designed and successfully synthesized through a facial one-step boron carbide reduction method,and the effects of high entropy borates intermedia phases on the EM wave absorption properties were investigated.Five HE REB6 ceramics including(Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6,(Ce0.2Hu0.2Sm0.2Er0.2Yb0.2)B6,(Ce0.2Y0.2Eu0.2Er0.2Yb0.2)B6,(Ce0.2Ya2Sm0.2Eu0.2Yb0.2)B6,and(Nd0.2Y0.2Sm0.2Eu0.2Yb0.2)B6 possess CsCl-type cubic crystal structure,and their theoretical densities range from 4.84 to 5.25 g/cm^(3).(Ce02Y0.2Sm0.2Er0.2Yb02)B6 powders with the average particle size of 1.86 jim were found to possess the best EM wave absorption properties among these hexaborides.The RLmin value of(Ce0.2Y0.2Sm0.2Er0.2Yb0.2)B6 reaches-33.4 dB at 11.5 GHz at thickness of 2 mm;meanwhile,the optimized effective absorption bandwidth(EAB)is 3.9 GHz from 13.6 to 17.5 GHz with a thickness of 1.5 mm.The introduction of HE REB03(RE=Ce,Y,Sm,Eu,Er,Yb)as intermediate phase will give rise to the mismatching impedance,which will further lead to the reduction of reflection loss.Intriguingly,the HEREB6/HEREB03 still possess wide effective absorption bandwidth of 4.1 GHz with the relative low thickness of 1.7 mm.Considering the better stability,low density,and good EM wave absorption properties,HE REB6 ceramics are promising as a new type of EM wave absorbing materials.

High entropy rare earth hexaborides/tetraborides(HE REB_(6)/HE REB_(4)) composite powders with enhanced electromagnetic wave absorption performance

The increasing electromagnetic hazards including electromagnetic interference and electromagnetic pollution,which were stemmed from massive usage of electromagnetic technology,have triggered widespread concerns.To cope with this challenge,electromagnetic wave absorbing materials with high performance are greatly needed.Composite construction has been widely applied in electromagnetic(EM)wave absorbing materials to achieve high permittivity,high permeability and impedance matching.However,high-temperature stability,oxidation and corrosion resistance are still unignorable issues.Herein,high entropy hexaborides/tetraborides(HE REB_(6)/HE REB_(4))composites with synergistic dielectric and magnetic losses were designed and successfully synthesized through a one-step boron carbide reduction method.The five as-prepared(Y_(0.2) Nd_(0.2) Sm_(0.2) Eu_(0.2) Er_(0.2))B_(6)/(Y_(0.2) Nd_(0.2) Sm_(0.2) Eu_(0.2) Er_(0.2))B_(4),(Y_(0.2) Nd_(0.2) Sm_(0.2) Er_(0.2) Yb_(0.2))B_(6)/(Y_(0.2) Nd_(0.2) Sm_(0.2) Er_(0.2) Yb_(0.2))B_(4),(Y_(0.2) Nd_(0.2) Eu_(0.2) Er_(0.2) Yb_(0.2))B_(6)/(Y_(0.2) Nd_(0.2) Eu_(0.2) Er_(0.2) Yb_(0.2))B_(4),(Nd_(0.2) Sm_(0.2) Eu_(0.2) Er_(0.2) Yb_(0.2))B_(6)/(Nd_(0.2) Sm_(0.2) Eu_(0.2) Er_(0.2) Yb_(0.2))B_(4) and(Y_(0.2) Sm_(0.2) Eu_(0.2) Er_(0.2) Yb_(0.2))B_(6)/(Y_(0.2) Sm_(0.2) Eu_(0.2) Er_(0.2) Yb_(0.2))B_(4) contain two phases of HE REB_(6) and HE REB_(4).Among them(Y_(0.2) Nd_(0.2) Sm_(0.2) Eu_(0.2) Er_(0.2))B_(6)/(Y_(0.2) Nd_(0.2) Sm_(0.2) Eu_(0.2) Er_(0.2))B_(4)(HE REB_(6)/HE REB_(4)-1)and(Y_(0.2) Nd_(0.2) Sm_(0.2) Er_(0.2) Yb_(0.2))B_(6)/(Y_(0.2) Nd_(0.2) Sm_(0.2) Er_(0.2) Yb_(0.2))B_(4)(HE REB_(6)/HE REB_(4)-2)exhibit excellent EM wave absorption properties.The optimal minimum reflection loss(RL_(m in))and effective absorption bandwidth(E_(AB))of HE REB_(6)/HE REB_(4)-1 and HE REB_(6)/HE REB_(4)-2 are–53.3 dB(at 1.7 mm),4.2 GHz(at 1.5 mm)and–43.5 dB(1.3 mm),4.2 GHz(1.5 mm),respectively.The combination of conducting HE REB_(4) with magnetism into HE REB_(6) as a second phase enhances dielectric and magnetic losses,which lead to enhanced EM wave absorption performance.Considering superior high-temperature stability,oxidation and corrosion resistance of HE REB_(6) and HE REB_(4),HE REB_(6)/HE REB_(4) composite ceramics are promising as a new type of high-performance EM wave absorbing materials.

Band topologies of hexaborides CaB6 and EuB6

In last decades, topological materials [1-3] have attracted lots of research interest. Topological Kondo insulator （TKI） [4], as an exotic quantum state, has been proposed theoretically in two hexaborides StuB6 and YbB6, in which there is a d/f（p） band inversion at momentum point X [5]. There are lots of experimental work supporting the TKI state and metallic sur- face state in typical Kondo insulator SmB6 [6-10]. However, more and more experimental results demonstrated that YbB6 is trivial insulator at ambient pressure [11]. Under high pres- sure, the band gap of YbB6 will close and the dip band inver- sion can happen under pressure about 15 GPa [12].

Preparation and characterization of LaB_6 ultra fine powder by combustion synthesis

High-purity,homogeneous and ultra fine LaB6 powders were prepared by combustion synthesis.The effects of reactant ratio and molding pressure on the phase and morphology of the combustion products were studied.The combustion products and leached products were analyzed by XRD,SEM and EDS.The results indicate that the combustion product consists of LaB6,MgO and a little Mg3B2O6.The combustion product becomes denser and harder when the molding pressure increases.The purity of LaB6 is higher than 99.0%.The LaB6 particle size is in range of 1.92-3.00 μm and the lattice constant of LaB6 is a=0.414 8 nm.

Synthesis,thermionic emission and magnetic properties of(Nd_xGd_(1-x))B_6

Polycrystalline rare-earth hexaborides （NdxGdl-x）B6 （x = 0, 0.2, 0.6, 0.8, 1） were prepared by the reactive spark plasma sintering （SPS） method using mixed powder of GdH2, NdH2 and B. The effects of Nd doping on the crystal structure, the grain orientation, the thermionie emission and the magnetic properties of the hexaboride were investigated by X-ray diffraction, electron backscattered diffraction and magnetic measurements. It is found that all the samples sintered by the SPS method exhibit high densities （〉95%） and high values of Vickers hardness （2319 kg/mm2）. The values are much higher than those obtained in the traditional method. With the increase of Nd content,the thermionic emission current density increases from 11 to 16.30 A/cm2 and the magnetic phase transition temperature increases from 5.85 to 7.95 K. Thus, the SPS technique is a suitable method to synthesize the dense rare-earth hexaborides with excellent properties.

Comparison of Functionals for Metal Hexaboride Band Structure Calculations

Density functional calculations of the electronic band structure for superconducting and semiconducting metal hexaborides are compared using a consistent suite of assumptions and with emphasis on the physical implications of computed models. Spin polarization enhances mathematical accuracy of the functional approximations and adds significant physical meaning to model interpretation. For YB6 and LaB6, differences in alpha and beta projections occur near the Fermi energy. These differences are pronounced for superconducting hexaborides but do not occur for other metal hexaborides.

Gas solid techniques for preparation of pure lanthanum hexaboride

The processes reported for the preparation of lanthanum hexaboride (LaB6) from lanthanum oxide involve the use of carbon either elemental or in the form of boron carbide or elemental boron itself as reducing agents, fused salts at high temperatures or reactions which require the product boride to be leached out. Each of these techniques either increases the process costs and/or increases chances of contamination in fi-nal product. Pure LaB6 can best be prepared by a reaction which produces a gaseous byproduct. In the present study, such a reaction was successfully used to yield pure lanthanum hexaboride. The process involved mixing of anhydrous lanthanum chloride with aluminium and boron and heating the charges under dynamic argon flow. Lanthanum chloride is known to be highly hygroscopic; hence the process using improperly dehydrated LaCl3 led to the formation of lanthanum oxychloride which does not convert to LaB6 under conditions wherein LaCl3 converts. Not only the formation of AlCl3 but also its continuous removal from the reaction zone is necessary for the success of the process.

Preparation of CaB6 powder via calciothermic reduction of boron carbide

The method of calciothermic reduction of B4C was proposed for preparing CaB6.The phase transition and morphology evolution during the reaction were investigated in detail.The experimental results reveal that Ca first reacts with B4C to generate CaB2C2 and CaB6 at a low temperature and that the CaB2C2 subsequently reacts with Ca to produce CaB6 and CaC2 at a high temperature.After the products were leached to remove the byproduct CaC2,pure CaB6 was obtained.The grain size of the prepared CaB6 was 2–3μm,whereas its particle size was 4–13μm;it inherited the particle size of B4C.The residual C content of the product was decreased to 1.03 wt%after the first reaction at 1173 K for 4 h and the second reaction at 1623 K for 4 h.

Oxidation process of lanthanum hexaboride ceramics

Oxidation process of lanthanum hexaboride (LaB_6) ceramic powderwas investigated. The laB_6 powder sma- ples were heated continuallyfrom room temperature to 1473 K at a heating rate of 10 k/min bydifferential scanning calorimetry. The oxidation tests were conductedat different exposure temeratures. The phases and morphologies of thesamples before and after exposure were analyzed by XRD and SEM. Itwas pointed out that before 1273 K, LaB_6 has high oxidationresistant ability, which was due to that the oxide layer hinders theoxygen diffusion from outer to the sur- face of Lab_6 grains.

Characteristics and Controlling of LaB_6-ZrB_2 Eutectic Microstructure

LaB 6 ZrB 2 eutectic composites were fabricated using electric arc furnace and EBZM(electron beam zone melting) technique Orientated ZrB 2 fibers yielded in a LaB 6 matrix The average diameter of ZrB 2 fibers is about 0 2～1 2 μm, and the length can be up to 100 μm For the ingot solidified with planar growth, the orientation relation of the phases is: Growth direction: LaB 6 ∥ ZrB 2 , Interfacial plane: (110) LaB 6 ∥(110) ZrB 2

Fabrication and characterization of high density LaB_(6) polycrystalline with(100) preferred orientation

High density lanthanum hexaboride(LaB_(6))polycrystalline with(100)preferred orientation was prepared by spark plasma sintering(SPS)using LaB_(6) nanocubes as raw materials in this work.Microstructure and thermionic electron emission property of LaB_(6) polycrystalline were investigated detailedly.The results show that the LaB_(6) polycrystalline had a relative density of 95.8%,and there was a(100)preferred orientation on its surface normal to SPS pressing direction.The work function of LaB_(6) polycrystalline normal surface was only 2.73 eV,which was almost close to the theoretical work function of LaB_(6)(100)single crystal surface.The reasons for preferential orientation of LaB_(6) polycrystalline were analyzed.

Synthesis of Al-SrB_6 composite via powder metallurgy processing

The potential of powder metallurgy processing for the manufacture of Al?SrB6 composites was explored. Al4Sr particles fractured extensively during the ball milling of Al?15Sr/Al?4B powder mixtures. There was no interaction between the Al4Sr and AlB2 compounds across the section of the aluminium grains in the as-milled state. SrB6 formed, when the ball milled powder blends were subsequently annealed at sufficiently high temperatures. Ball milling for 1 h was sufficient for SrB6 to become the major constituent in powder blends annealed at 700 °C while it took 2 h of ball milling for powder blends annealed at 600 °C. Higher annealing temperatures and longer ball milling time encouraged the formation of the SrB6 compound while the latter made a great impact on the microstructural features of the Al?SrB6 composite. The SrB6 compound particles were much smaller and more uniformly distributed across the aluminium matrix grains in powder grains ball milled for 2 h before the annealing treatments at 600 °C and 700 °C.

STUDY ON THE MICROSTRUCTURE OF LaB_6-ZrB_2 IN-SITU COMPOSITE

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The Mechanical and Electronic Properties of Ternary Rare-Earth Hexaboride LaxNd8-xB6 (x = 0, 1, 7, 8) Materials

We have carried out density functional theory to study the lattice constants and electronic properties of LaB6, NdB6, Nd-doped LaB6, and La-doped NdB6. The lattice constant, intra-octahedral bond, inter-octahedral boron bond, and positional parameter (z) were calculated for LaB6, La7Nd1B6, La1Nd7B6, and NdB6. Our results show that the doped Nd increases the lattice constant of La7Nd1B6. Likewise, La-doping leads to an increase in the lattice constant of the La1Nd7B6. The PDOSs of LaB6, B of LaB6, La7Nd1B6, B of La7Nd1B6, La1Nd7B6, B of La1Nd7B6, NdB6, and B of NdB6 were calculated. La d-electron bands cross the Fermi energy, showing classical conductor behavior. The charge density results indicate that light and dark colors show high and low-intensity zones, respectively. La1Nd7B6 has a low-density region and LaB6 has a high-density region. The LaB6 midpoint has strong charge density peaks. Weak peaks are also observed for La1Nd7B6. Thus, ternary REB6 has good potential for many applications. This article reports an investigation of the electronic features and structural parameters of binary and ternary hexaborides.

Toughened (Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)–SiC composites fabricated by one-step reactive sintering with a unique SiB_(6) additive

High-entropy diboride has been arousing considerable interest in recent years.However,the low toughness and damage tolerance limit its applications as ultra-high-temperature structural materials.Here we report that a unique SiB_(6) additive has been first incorporated as boron and silicon sources to fabricate a high-entropy boride(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)–SiC composite though one-step boro/carbothermal reduction reactive sintering.A synergetic effect of high-entropy sluggish diffusion and SiC secondary phase retarded the grain growth of the(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2)–51SiC composites.The small grain size was beneficial to shorten the diffusion path for mass transport,thereby enhancing the relative density to~99.3%.These results in an increase of fracture toughness from~5.2 in HEBS-1900 to~7.7 MPa·m^(1/2) in HEBS-2000,which corresponded to a large improvement of 48%.The improvement was attributed to a mixed mode of intergranular and transgranular cracking for offering effective pinning in crack propagation,resulting from balanced grain boundary strength collectively affected by improved densification,solid solution strengthening,and incorporation of SiC secondary phase.

Nanocrystalline high-entropy hexaboride ceramics enable remarkable performance as thermionic emission cathodes

The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential,yet relevant studies have been rarely reported.In this work,we report nanocrystalline(La_(0.25)Ce_(0.25)Nd_(0.25)Eu_(0.25))B6 high-entropy rare-earth hexaboride(HEReB6-1)ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time.The as-fabricated samples exhibited a highly dense(96.3%)nanocrystalline(94 nm)microstructure with major(001)fiber textures and good grain boundaries without any impurities,resulting in a remarkable mechanical,electrical,and thermionic emission performance.The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures;these were greater than the average values of corresponding binary rare-earth hexaborides,such as a Vickers hardness of 23.4±0.6 GPa and a fracture toughness of 3.0±0.4 MPa•m^(1/2)at room temperature.More importantly,they showed high emission current densities at elevated temperatures,which were higher than the average values of the corresponding binary rare-earth hexaborides.For instance,the maximum emission current density reached 48.3 A•cm^(−2)at 1873 K.Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes.

Synthesis of substitutional hexaboride of lanthanum and cerium La_(1-x)Ce_(x)B_(6) via aluminothermic reduction

Rare-earth hexaborides(REB_(6))are vital raw materials for cathode materials and high temperature structural ceramics that are widely applied as high-frequency electron tubes and ceramics adaptive for extreme environment,respectively.In this work,single phase substitutional solid solution REB_(6)(LaB_(6),La_(0.75)Ce_(0.25)B_(6).La_(0.5)Ce_(0.5)B_(6),La_(0.25)Ce_(0.75)B_(6) and CeB_(6))powders were prepared with the raw materials of La_(2)O_(3),CeO_(2),B_(4)C and Al powders,after calcining at 1773 K for 4 h and the following alkaline leaching.All substitutional solid solution products have homogeneous distributions of La and Ce in particles.Through microscopic morphology analysis,it is discovered that the formation of solid solution is beneficial for reducing the particle size of product,relative to LaB_(6) and CeB_(6).Moreover,Al flux plays an important role in decarbonizing reaction,and C contents of all products are below 0.4 wt%.

Effect of particle size on the polycrystalline CeB_6 cathode prepared by spark plasma sintering

The full densification polycrystalline cerium hexaboride （CeB6） cathode material was prepared by using the spark plasma sintering （SPS） method in an oxygen free system. The starting precursor nanopowders with an average grain size of 50 nm were prepared by high-energy ball milling. The ball-milled nanopowder was fully densified at 1550 °C under 50 MPa, which was about 350 °C lower than the conventional hot-pressing method and it was also lower than that of coarse powder under the same sintering condition. The mechanical properties of nanopowder sintered samples were significantly better than that of coarse powder, e.g., the flexural strength and Vickers hardness were 211% and 51% higher than that of coarse powder, respectively. The electron backscattered diffraction （EBSD） result showed that the （100） fiber texture could be fabricated by the ball-milled nanopowder sintered at 1550 °C and the thermionic emission current density was measured to be 16.04 A/cm2 at a cathode temperature of 1873 K.

A new method of preparing NdB_(6)ultra-fine powders

Combustion synthesis method was used to prepare NdB_(6)ultra-fine powders with B_(2)O_(3),Nd_(2)O_(3)and Mg powders as the raw materials.The basic thermody-namic data of NdB_(6) were estimated.The standard forma-tion heat of NdB_(6) is-357.48 kJ·mol^(-1).The values of the heat capacity and the standard entropy are 96.87 and 86.60 J·K^(-1)·mol^(-1),respectively.The adiabatic tempera-ture of the reaction is 2726 K,which is higher than the thermodynamic criterion of 1800 K.This indicates that the combustion synthesis reaction of the B_(2)O_(3)-Nd_(2)O_(3)-Mg system could spontaneously take place by itself to generate NdB_(6).The NdB_(6) powders were characterized by X-ray diffractometer(XRD),scanning electron microscopy(SEM)and differential scanning calorimetry-thermo-gravimetry(DSC-TG).The results indicate that the com-bustion products consist of NdB_(6),MgO,and a few Mg_(3)B_(2)O_(6)and Nd_(2)B_(2)O_(6).The leached products consist of single NdB_(6) phase,and its purity is 98.6 wt%.When the sample preparation pressure is 20 MPa,the average parti-cle size of NdB_(6) powders is less than 500 nm.The antioxidant ability of NdB_(6) is very strong,which is oxi-dized step by step.The apparent activation energies of the oxidation reactions are 986.14 and 313.83 kJ·mol^(-1),respectively.In addition,the reaction orders are 4.10 and 3.75,respectively.