CeB6 Research Articles

Revealing the Nature of Antiferroquadrupolar Ordering in Cerium Hexaboride: CeB_{6}.

The cerium hexaboride (CeB_{6}) f-electron compound displays a rich array of low-temperature magnetic phenomena, including a "magnetically hidden" order, identified as multipolar in origin via advanced x-ray scattering. From first-principles electronic-structure results, we find that the antiferroquadrupolar (AFQ) ordering in CeB_{6} arises from crystal-field splitting and yields a band structure in agreement with experiments. With interactions of p electrons between Ce and B_{6} being small, the electronic state of CeB_{6} is suitably described as Ce(4f^{1})^{3+}(e^{-})(B_{6})^{2-}. The AFQ state of orbital spins is caused by an exchange interaction induced through spin-orbit interaction, which also splits the J=5/2 state into a Γ_{8} ground state and a Γ_{7} excited state. Within the smallest antiferromagnetic (AFM) (111) configuration, an orbital-ordered AFQ state appears during charge self-consistency, and it supports the appearance of a "hidden" order. Hydrostatic pressure (either applied or chemically induced) stabilizes the AFM (AFQ) states over a ferromagnetic one, as observed at low temperatures.

Высокоэффективный термоэлектрический однофотонный детектор на основе гексаборидов лантана и церия

AbstractA design for a high-efficiency single-photon detector based on lanthanum and cerium hexaborides, which operates from the infrared to ultraviolet spectral ranges, is suggested. The results of computer simulations of heat transfer in the sensitive element of the detector upon the absorption of photons with energies of 0.5–4.13 eV are presented. To attain a high efficiency of the system of photon detection in the wavelength range from near infrared to ultraviolet, lanthanum hexaboride is proposed as the absorber and heat-sink material in the sensitive element. It is shown that a sensitive element of both single- and three-layer design made entirely of hexaborides will possess a gigahertz counting rate and a detection efficiency exceeding 90%.

Mechanochemical synthesis and consolidation of nanostructured cerium hexaboride

This study reports on the mechanochemical synthesis (MCS) and consolidation of nanostructured CeB6 powders of high purity. CeB6 powders were prepared via MCS by milling CeO2, B2O3 and Mg powders in a high-energy ball mill for different milling times. The effects of milling time on the formation, microstructure and thermal behaviour of the synthesized powders were investigated and the optimum MCS duration was determined. Purified powders were obtained after HCl leaching by removing MgO by-product. The prepared powders were characterized by a number of techniques including X-ray diffraction, stereomicroscopy, scanning and transmission electron microscopy coupled with energy dispersive spectrometry, differential scanning calorimetry, atomic absorption spectrometry, particle size analysis, surface area analysis and vibrating sample magnetometry. The high-purity CeB6 powders having an average particle size of 86 nm were consolidated by cold-pressing followed by pressureless sintering at 1700 ?C for 5 h. The bulk CeB6 specimen was investigated for its microstructure, density, electrical resistivity, surface roughness and some mechanical properties (microhardness and wear behaviour). The relative density, electrical resistivity, microhardness and wear rate of the bulk CeB6 were determined as 95.2%TD, 57.50 ?W?cm, 11.65GPa and 1.46 ? 10?4 mm3/N?m, respectively.

Influence of the Cathode Material Properties in Reducing the Back-Bombardment Effect in Thermionic RF Gun

The effect of back-bombardment (BB) electrons is considered one of the main obstacles for extensive use of thermionic RF guns (T-RF). The 10 hexaboride materials named (Ca, Sr, Ba, La, Ce, Pr, Nd, Sm, Eu, and Gd) B6 are investigated in this paper to survey the effect of the cathode material on reducing BB electrons and compare them with LaB6 (which is widely used as a thermionic cathode). A numerical model was used to conduct this paper. Besides the numerical calculations, an experiment has been performed to determine the work functions of CeB6 and LaB6. The results from the numerical calculations revealed that (Ba, Ca, and Nd) B6 are less affected by BB electrons: 42%, 50%, and 59%, respectively, compared with LaB6, for low beam current applications. In contrast, for high beam current duties, (Nd, Ce, and Sm) B6 have minimum influence by BB electrons compared with other hexaborides. The study concluded that BB electrons are strongly affected by the properties of the cathode material, especially thermionic emission and material density. Moreover, the study suggests that it is worthwhile to prepare BaB6, CaB6, NdB6, and SmB6 as cathode materials, then to subject them to a real experimental test using T-RF gun to compare their performance and BB effect against LaB6.

Simulation of Heat Propagation Processes in the Detection Pixel with Superconducting Layers of Single-Photon Thermoelectric Detector

The results of computer simulation of heat propagation processes in the three-layer detection pixel with the superconducting layers of thermoelectric detector after the absorption of single photons energy of 1–1000 eV are presented. We consider the different geometries of the detection pixel consisting of CeB6 or (La,Ce)B6 thermoelectric sensor, absorber and heat sink of Nb, Pb or YBCO superconductors. The computations based on the heat conduction equation from the limited volume are carried out by the three-dimensional matrix method for differential equations. It is shown that by changing the materials and dimension of the detection pixel elements, as well as the operating temperature of the detector enables one to obtain the detector to register the photons within the given spectral range, required energy resolution, and counting rate. Such a detector has a number of advantages that allow one to consider the thermoelectric detector as a real alternative to the most promising single photon detectors.

Flexible three-dimensional CeB6 nanowire arrays and excellent field emission emitters

We have developed an effective chemical-vapor-deposition method to fabricate large-scale cerium hexaboride nanowire arrays on flexible carbon substates. The nanowires show diameters of 80–200 nm and lengths of 5–30 μm. The high resolution transmission electron microcopy (HRTEM) indicates that the nanowire is of high crystalline with the growth orientation of [001]. The field emission of these nanowires shows favorable electron emission properties, including the turn-on fields of 1.76 V μm−1, threshold fields of 3.60 V μm−1 and high field emission enhancement factors of 2203 ± 27, respectively. The field emission properties depending on the gap between the anode and cathode are also investigated. The results show that the turn-on fields and threshold fields decrease from 1.76 and 3.60 V μm−1 to 1.28 and 2.77 V μm−1, respectively; whereas the field emission enhancement factors increase from 2203 ± 27 to 4012 ± 66 with the anode-cathode distance increasing from 500 to 700 μm. The excellent field emission performances indicate the promising application prospects of this material in flexible electronics and optoelectronics, such as high light electron sources, flat panel displays and thermal electrical power converters.

CeB6 thin films produced on different substrates by electron-beam deposition

Films of cerium hexaboride, a material promising for use in thermoelectric devices at liquidhelium temperatures, are produced by electron-beam deposition. Deposition is carried out from ceramic targets onto insulator, semiconductor, and metal substrates at different temperatures. The microstructure, the elemental and phase compositions, the temperature dependences of the resistivity and the Seebeck coefficient are thoroughly studied. CaB6-structured films, for which the structure is characteristic of cerium hexaboride and the elemental composition is close to the stoichiometric composition, are obtained. At low temperatures, the resistivity of the films is somewhat higher than that of single-crystal samples, and the Seebeck coefficient is close to the corresponding coefficient for single-crystal samples. The main cause of the difference between the resistance values is a high concentration of oxygen impurity detected in the films.

Prospects of using rare-earth hexaborides in thermoelectric single-photon detectors

The results of the numerical simulation of heat propagation processes occurring after the absorption of single photons with energies of 1 eV–1 keV in a three-layer sensor of a thermoelectric detector are analyzed. Different configurations of the sensor with a tungsten absorber, a thermoelectric layer of cerium hexaboride, and a tungsten heat sink are considered. It is shown that sensors for detecting photons of a specific spectral range with the required energy resolution and counting rate can be developed by varying the geometric sizes of the sensor layers. It is concluded that a three-layer sensor has a number of advantages in comparison with a single-layer sensor and has characteristics allowing consideration of the thermoelectric detector as a realistic alternative to superconducting single-photon detectors.

Two-orbital model for CeB6

We describe a two-orbital tight-binding model with bases belonging to the Γ8 quartet. It captures several characteristics of the Fermiology unravelled by the recent angle-resolved photoemission spectroscopic (ARPES) measurements on cerium hexaboride CeB6 samples cleaved along different high-symmetry crystallographic directions, which includes the ellipsoid-like Fermi surfaces (FSs) with major axes directed along Γ-X. We calculate various multipolar susceptibilities within the model and identify the susceptibility that shows the strongest divergence in the presence of standard onsite Coulomb interactions and discuss its possible implication and relevance with regard to the signature of strong ferromagnetic correlations existent in various phases as shown by the recent experiments.

How do specimen preparation and crystal perfection affect structure factor measurements by quantitative convergent-beam electron diffraction?

The effectiveness of tripod polishing and crushing as methods of mechanically preparing transmission electron microscopy specimens of hard brittle inorganic crystalline materials is investigatedviathe example of cerium hexaboride (CeB6). It is shown that tripod polishing produces very large electron-transparent regions of very high crystal perfection compared to the more rapid technique of crushing, which produces crystallites with a high density of imperfections and significant mosaicity in the case studied here where the main crystallite facets are not along the natural {001} cleavage planes of CeB6. The role of specimen quality in limiting the accuracy of structure factor measurements by quantitative convergent-beam electron diffraction (QCBED) is investigated. It is found that the bonding component of structure factors refined from CBED patterns obtained from crushed and tripod-polished specimens varies very significantly. It is shown that tripod-polished specimens yield CBED patterns of much greater integrity than crushed specimens and that the mismatch error that remains in QCBED pattern matching of data from tripod-polished specimens is essentially nonsystematic in nature. This stands in contrast to QCBED using crushed specimens and lends much greater confidence to the accuracy and precision of bonding measurements by QCBED from tripod-polished specimens.

Anisotropy study on thermionic emission and magnetoresistivity of single crystal CeB6

Cerium hexaboride (CeB6) as a heavy fermion compound displays a number of interesting low-temperature physical properties such as dense Kondo behavior and a complex magnetic phase diagram due to the interaction between itinerant and local electrons. Recently, the electron emission property of CeB6 has received much attention because it has potential applications in replacing the commercial LaB6 cathode and serving as new-generation thermal cathodes. In addition, by comparison with other metal cathodes, it also possesses some advantages, such as a low work function, low volatility, high brightness, thermal stability and high mechanical strength. However, so far the thermionic emission properties of CeB6 single crystal surfaces except for the (100) surfaces have been rarely reported. Whether the different crystal surfaces of CeB6 contribute to the various interesting emission properties is main research purpose of the present work. In this paper, the (110), (111), (210) and (310) crystal surfaces of single crystal CeB6 are determined by the X-ray Laue diffraction method, and their thermionic emission current densities are measured at different temperatures and applied voltages. As a result, the maximum emission current densities of the (110), (111), (210) and (310) crystal surfaces at 1873 K are 38.4, 11.54, 50.4 and 20.8 A/cm2, respectively. When their cathode temperatures are all 1773 K, their maximum emission current densities are 15.2, 5.43, 28.0 and 11.44 A/cm2. In addition, when the cathode temperature decreases to 1673 K, their maximum emission current densities are 4.24, 0.9, 6.2 and 2.43 A/cm2. It means that the thermionic emissions are strongly anisotropic for the different crystal surfaces. In general, the maximum emission current density of (100) crystal surface of LaB6 single crystal is about 10 A/cm2 at 1700 K. By comparing the emission current density of CeB6 single crystal at 1773 K with that of LaB6 at 1700 K, it is found that the emission properties of (210) crystal surface are maybe close to those of LaB6. The work function values of the (110), (111), (210) crystal surfaces calculated by the Richardson-Dushman formula are 2.64, 2.71 and 2.40 eV, respectively. Among these, the (210) crystal surface possesses the smallest value of the work function, which is hopeful for being used as an electron source of scanning electron microscopy. Zero-field magnetoresistivity measurments confirm the transition temperatures of TQ=3.3 K and TN=2.4 K. Field-angle dependent magnetoresistivity measurments show that the electrical resistivity varies between 69 μupΩ·cm and 73 μupΩ·cm when the crystal rotates from the[001] to the[011] direction. This indicates that the electrical resistivity in a magnetic field is also anisotropic.

The features of thermal properties and CEF-influence in intermediate valence compound CeB4 at the temperatures of 2–300 K

Heat capacity and lattice parameters of cerium tetraboride were experimentally determined at 2 – 300K. An anomalously large contribution of free electrons to the heat capacity was influenced by Fermi liquid state in the cerium boride. The parameters of Einstein and Debye contributions to CeB4 heat capacity, as well as free electron gas contribution and Schottky heat capacity have been determined. A negative thermal expansion (NTE) of CeB4 was found (5–25K). We were not able to explain the NTE by the influence of intermediate valence of cerium ions. The NTE was attributed to the influence of the crystal electric field (CEF) on Ce3+ ions.

Computer simulation of heat distribution processes in W/(La,Ce)B6/W sensor of thermoelectric detector

The results are presented of the computer simulation of heat distribution in the W/(La,Ce)B6/W multi-layer sensor of the thermoelectric detector after the absorption of single photons with the energies 1–100 eV. The influence of the choice of computer simulation parameters on the revealing of the peculiarities of heat transmission processes arising in the sensor of detector depending on the photon energy, the sensor geometry, the absorption area of the absorber surface and the depth of photon thermalization is investigated in details. The energy resolution and the count rate of sensor are evaluated. It is shown that the multi-layer sensor with the thermoelectric (La,Ce)B6 is capable to register a single photon in a wide range of the electromagnetic spectrum at 0.5 K; it has an advantages as compared with the sensor based on the CeB6 with the operating temperature 9 K, and has perspectives to be used in the science and technology.

Investigation of processes of heat propagation in multilayer sensor of thermoelectric single-photon detector

The results of computer simulation of the processes of heat propagation inside the multilayer sensor of thermoelectric detector after absorbing the single photons with the energies 1 keV and 100 eV are presented. The variants are considered for different geometries of the sensor which consists of the tungsten absorber and of the thermoelectric layer of the cerium hexaboride. The differences of temporal dependence of the signal arising in the thermoelectric layer when the photon is absorbed in the various areas of absorber are studied in details. The energy resolution and the count rate of the sensor are estimated. It is shown that the multilayer sensors have a number of advantages as compared to the single-layered, and their use is promising in the various fields of science and technology.

Mechanochemical Synthesis of High Crystalline Cerium Hexaboride Nanoparticles from CeO2‐B2O3‐Mg Ternary System

AbstractHigh crystalline cerium hexaboride (CeB6) nanoparticles (NPs) were synthesized using mixture of mag‐ nesium (Mg), cerium oxide (CeO2) and boron oxide (B2O3) via the mechanochemical process at room tem‐ perature. Based on the results, magnesiothermic reduction of B2O3 occurred after about 2 h of milling in a mechanically induced self‐sustaining reaction (MSR). The significant amount of heat produced by the reduction reaction resulted in CeO2 reduction to elemental Ce which finally reacted with elemental B and formed CeB6 compound. According to XRD analyses, the degree of crystallinity and lattice parameter of the product was calculated about 93 % and 4.1458 Å, respectively. The morphology observations revealed that the synthesized CeB6 had semi‐cubic shape with the range of size 25–60 nm. The synthesis of CeB6 during the thermal treatment was studied by simultaneous thermal analysis (STA) technique. It was found that the reduction of B2O3 took place after melting of Mg meanwhile, no CeB6 phase achieved even up to 1100 °C.

Ultrafast TSPD on the basis of CeB6 sensor

The results of computer simulation of heat distribution on the sensor of Thermoelectric Single Photon Detector after photon absorption are presented in this work. The kinetic processes emerging after the absorption of 100 eV (hard UV) and 1 KeV (X-ray) energy photons in different areas of the absorber for different sensor and absorber geometries are investigated. The calculations were performed for the tungsten absorber and the cerium hexaboride sensor. It is shown, that waveform of time dependence of response doesn't strongly depend from the depth of photon thermalization area in absorber and the height on which the temperature gradient on the edges of thermoelectric sensor is measured.

Effect of TiSi2 addition on densification of Cerium hexaboride

This paper reports the result of investigation carried out on the synthesis and densification of Cerium hexaboride. CeB6 was synthesized by boron carbide reduction of CeO2 in presence of carbon. Effect of temperature on product quality was investigated. Pure CeB6 powder was obtained at 1600°C, in vacuum. It was found that CeBO3 is formed as an intermediate phase during CeB6 synthesis. Spark plasma sintering of CeB6 powder at 1900°C and 70MPa resulted in a density of only 93.4% of theoretical value. Addition of 5% TiSi2 resulted in 97.4% of theoretical density at the lower temperature of 1650°C. Hardness and fracture toughness of the composite was measured as 21.08GPa and 3.8MPa·m1/2 respectively. Fractured surface of the composite reveals the mode of fracture to be transgranular. SEM and EDS analysis indicate the presence of TiSi2 in CeB6 matrix.

A resistively heated CeB6 emissive probe

The plasma potential, V(p), is a key quantity in experimental plasma physics. Its spatial gradients directly yield the electrostatic field present. Emissive probes operating under space-charge limited emission conditions float close to V(p) even under time-varying conditions. Throughout their long history in plasma physics, they have mostly been constructed with resistively heated tungsten wire filaments. In high density plasmas (>10(12) cm(-3)), hexaboride emitters are required because tungsten filaments cannot be heated to sufficient emission without component failure. A resistively heated emissive probe with a cerium hexaboride, CeB6, emitter has been developed to work in plasma densities up to 10(13) cm(-3). To show functionality, three spatial profiles of V(p) are compared using the emissive probe, a cold floating probe, and a swept probe inside a plasma containing regions with and without current. The swept probe and emissive probe agree well across the profile while the floating cold probe fails in the current carrying region.

Microstructure and Reinforcing Mechanisms of Boron Carbide–Cerium Boride Porous Composites

boron carbide–cerium boride porous composites are prepared by hot pressed sintering, and mechanical properties and microstructure of boron carbide–cerium boride porous composites were tested. The results show that Flexibility strength of B4C-CeB6 porous composites is greatly improved compared with that of monolithic porous boron carbide. B4C react with CeO2 to completely form CeB6 in porous composites. CeB6 particles in B4C grain boundary are produced by in-situ reaction. The presence of CeB6 reinforcing particles could also suppress growth of B4C grains which normally leads to improved strength.

Enhanced field emission from cerium hexaboride coated multiwalled carbon nanotube composite films: A potential material for next generation electron sources

Intensified field emission (FE) current from temporally stable cerium hexaboride (CeB6) coated carbon nanotubes (CNTs) on Si substrate is reported aiming to propose the new composite material as a potential candidate for future generation electron sources. The film was synthesized by a combination of chemical and physical deposition processes. A remarkable increase in maximum current density, field enhancement factor, and a reduction in turn-on field and threshold field with comparable temporal current stability are observed in CeB6-coated CNT film when compared to pristine CeB6 film. The elemental composition and surface morphology of the films, as examined by scanning electron microscopy, transmission electron microscopy, and energy dispersive X-ray measurements, show decoration of CeB6 nanoparticles on top and walls of CNTs. Chemical functionalization of CNTs by the incorporation of CeB6 nanoparticles is evident by a remarkable increase in intensity of the 2D band in Raman spectrum of coated films as compared to pristine CeB6 films. The enhanced FE properties of the CeB6 coated CNT films are correlated to the microstructure of the films.