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Crystal structures of borides as viewed along the c axis: (a) AlB2 (M = Al), in which boron atoms form honeycomb layers, and Al atoms are located at the centers of hexagonal prisms formed by boron sheets. (b) α-REAlB4, in which boron atoms form the sheets consisting of pentagonal and heptagonal rings and the rare earth (Tm) and Al atoms sit between boron sheets. (c) Structural difference between the α- and β-REAlB4: the pairs of condensed pentagonal rings are not parallel in α-REAlB4, but are parallel in β-REAlB4. In (c), those two smaller black boxes (left) depict the unit cells of α- and β-REAlB4, and the bigger one (right) indicates the insertion of a slice of β-TmAlB4 into the crystal structure of α-TmAlB4 composed of fully ordered models. The crystallographic directions are labeled with red arrows for each structure with the c axis being perpendicular to the a-b plane. The small inset in the middle region of (c) represents the common structural unit existing in both phases.
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Rare earth metal borides have attracted great interest due to their unusual properties, such as superconductivity and f-electron magnetism. A recent discovery attributes the tunability of magnetism in rare earth aluminoborides to the effect of so-called “building defects.” In this paper, we report data for the effect of building defects on the ther...
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As many nations continue to develop and industrialize, the global energy demands are rising rapidly. With the threat of climate change disaster looming, the search for sustainable, “green” energy has become of higher priority. Thermoelectric materials add an important facet to the mosaic of future energy plans by allowing the scavenging of (“low-qu...
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... A. Atomic structure and phonons and covalent behaviour. [15,43,44] With such bonding, MB 2 has a compact unitcell with three atoms (one metal and two borons) and only slight differences in the ⊥ and lattice parameters (c/a 1). The obtained relaxed cell parameters for ZrB 2 are a= 3.17Å, c=3.54Å and HfB 2 are a = 3.13Å, c=3.47Å. ...
Similar effects of metal and boron vacancies on phonon scattering and lattice thermal conductivity ($\kappa_l$) of ZrB$_2$ and HfB$_2$ are reported. These defects challenge the conventional understanding that associates larger impacts to bigger defects. We find the underlying reason to be a strong local perturbation caused by the boron vacancy that substantially changes the interatomic force constants. In contrast, a long ranged but weaker perturbation is seen in the case metal vacancies. We show that these behaviours originate from a mixed metallic and covalent bonding nature in the metal diborides. The thermal transport calculations are performed in a complete \textit{ab initio} framework based on Boltzmann transport equation and density functional theory. Phonon-vacancy scattering is calculated using \textit{ab initio} Green's function approach. Effects of natural isotopes and grain boundaries on $\kappa_l$ are also systematically investigated, however we find an influential role of vacancies to explain large variations seen in the experiments. We further report a two-order of magnitude difference between the amorphous and pure-crystal limits. Our results outline significant material design aspects for these multi-functional high temperature ceramics.
... As temperature rises up to 673 K, the κ l values converge gradually to 0.3 ~ 0.4 W m −1 K −1 in the x = 0 sample. The ultralow κ l in CTS has been reported frequently, and can be caused by the complex microstructure with disordered cation arrangement, which can also be referred to other materials like TmAlB 4 [42]. The convergence of κ l at high temperatures indicates the predominant role of the cation-disordered feature on phonon transport interruption. ...
Heavily acceptor-doped Cu2SnS3 (CTS) shows promisingly large power factor (PF) due to its rather high electrical conductivity (σ) which causes a modest ZT with a high electronic thermal conductivity (κe). In the present work, a strategy of carrier compensation through Sb-doping at the Sn site in Cu2Sn0.8Co0.2S3 was investigated, aiming at tailoring electrical and phonon transport properties simultaneously. Rietveld analysis suggested a complex polymorphic microstructure in which the cation-(semi)ordered tetragonal phase becomes dominant over the coherently bonded cation-disordered cubic phase, as is preliminarily revealed using TEM observation, upon Sb-doping and Sb would substitute Sn preferentially in the tetragonal structure. With increasing content of Sb, the σ was lowered and the Seebeck coefficient (S) was enhanced effectively, which gave rise to high PFs maintained at ~10.4 μWcm⁻¹K⁻² at 773 K together with an optimal reduction in κe by 60–70% in the whole temperature range. The lattice thermal conductivity was effectively suppressed from 1.75 Wm⁻¹K⁻¹ to ~1.2 Wm⁻¹K⁻¹ at 323 K while maintained very low at 0.3–0.4 Wm⁻¹K⁻¹ at 773 K. As a result, a peak ZT of ~0.88 at 773 K has been achieved for Cu2Sn0.74Sb0.06Co0.2S3, which stands among the tops so far of the CTS-based diamond-like ternary sulfides. These findings demonstrate that polymorphic microstructures with cation-disordered interfaces as an approach to achieve effective phonon-blocking and low lattice thermal conductivity, of which further crystal chemistry, microstructural and electrical tailoring are possible by appropriate doping.
... This material contains graphite-like boron layers separated by aluminum layers formed by the atoms in hexagonal prismatic voids (space group P6/mmm) [1]. AlB 2 possesses metal conductivity of ≈ 7 µΩ cm along the c axis and ≈ 14 µΩ cm parallel to the a − b plane at room temperature [2]. The anisotropy of conductivity is due to the direction-dependent scattering events caused by the existence of the layered hexagonal structure. ...
First-principles molecular dynamics simulations were used to generate the samples of amorphous a-AlB2, a-AlBC and a-AlBN alloys. All alloys exhibit boron clustering. The Al–C network of a-AlBC shows some similarity to the structure of the Al4C3 phase however noticeable segregation of this phase is not identified. In a-AlBN, a small segregation of nanoscale w-AlN-like domains was revealed. The phonon spectra of the amorphous samples consist of one broad band spreading up to 39 THz. Hardness, ideal tensile strength, Debye temperature, and fracture toughness of the alloys are in the range 7.1–9.1 GPa, 9.2–20.1 GPa, 745–789 K and 1.43–1.46 mPa m1/2, respectively. Crystalline AlB2 is predicted to be closer to a brittle material, whereas the amorphous alloys will exhibit ductile behavior. All the amorphous samples should exhibit semiconducting properties with a predicted mobility gap in the range of 1.52–3.03 eV.
... The addition of boron to the aluminum matrix is accompanied by the formation of stable metal boride clusters AlB 2 , AlB 10 , and AlB 12 with the hardness typical of amorphous Al-B coatings, which reaches 6 GPa if there are no crystalline inclusions in these coatings and 28 GPa in crystalline bulk materials [1,10,11]. Materials of the Al-B system, as well as alloys and composites based on them, such as Al-B-C and Al-B-Si [7,12], have a high hardness value in the bulk form (H Al-B-C = 34 GPa [11]), a low weight, good thermal conductivity and corrosion and chemical resistance, and excellent wear resistance [13][14][15][16], owing to which these new modern materials are intensively studied as unique structural materials for use in the aircraft, aerospace, and arms industries. ...
... And new phenomenon, such as thermoelectric enhancement in the samarium phase of REB 66 (RE = rare-earth) [23,24] and excellent p-n control in Zr-doped elemental beta-boron [25], have been discovered. Relatively novel borides for boron cluster compounds like REB 44 Si 2 and layered compounds like the AlB 2 -analogues have also been discovered with promising thermoelectric properties [26][27][28][29][30][31]. The topics that particularly relate to single crystals and crystal growth will be reviewed. ...
... In regard to thermoelectrics, there are two interesting aspects of these compounds discovered so far. The first is that the thermal conductivity of small single crystals of TmAlB 4 were measured by the thermoreflectance method, and it was found that a couple percent of β-type fragments in α-TmAlB 4 led to a ∼30 % decrease in the thermal conductivity, indicating the strong phonon-scattering effect of the building defects [31]. Since formation of building defects is now found to be controllable by crystalgrowth conditions, this represents an interesting effective control over the thermal conductivity. ...
Considering that more than half of all primary energy, i.e. fossil fuels, that mankind consumes is lost in the form of waste heat, the solid state conversion of heat to electricity that thermoelectric materials represent through the Seebeck effect can have a huge benefit to society (Koumoto and Mori 2013, Mori and Nolas 2016). However, it is not easy to achieve a high thermoelectric conversion efficiency. The conversion efficiency is a function of the figure of merit ZT which is composed of contradictory requirements for the physical properties. Namely, ZT = T, where is the Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, and T is temperature. Therefore, to achieve very high thermoelectric performance ZT, the material should have a high electrical conductivity like a metal but low thermal conductivity, and large Seebeck coefficient like an insulator but high electrical conductivity. These requirements have made straightforward enhancement of the thermoelectric performance difficult. However, recently, novel principles have been proposed to try to enhance the power factor (Heremans et al. 2008, Mori 2017, Ang et al. 2015, Ahmed et al. 2017, Zebarjadi et al. 2011, Shakouri et al. 1999, Mori and Hara 2016, Liu et al. 2016). In regards to the paradox of conducting electricity but blocking heat, nanostructuring has been shown to be effective to achieve phonon selective scattering and thereby enhance ZT (Wang et al. 2008, Biswas et al. 2012, Koumoto and Mori 2013, Grasso et al. 2013, Nethravathi et al. 2014, Rogl et al. 2015, Khan et al. 2017).
Therefore, in the thermoelectrics fields, single crystals are usually not sought after so much, because the grain boundaries in polycrystalline sintered bodies, especially nanostructured designed ones can lead to selective scattering of the phonons and thereby to higher thermoelectric performance. But there are several notable examples of thermoelectric materials where the crystal growth is very important and this will be reviewed in this chapter, which contains sections on borides: cluster compounds and layered compounds, chalcogenides like SnSe, TiS2, and FeSb2, and clathrates.
... 20,21 Figure 2 Multi-frequency through-plane TDTR measurements have been conducted on all three samples. 22,23 A heat capacity of 1.27 MJ m 3 K 1 is used for ZrTe5 crystals in the data analysis. 24 The obtained through-plane (along b axis) thermal conductivities (b) are 0.30 ± 0.02, 0.33 ± 0.03, and 0.35 ± 0.03 W m 1 K 1 for S-1, S-2, and S-3, respectively. ...
Zirconium pentatelluride (ZrTe5 ) has recently attracted renewed interest owing to many of its newly discovered extraordinary physical properties, such as 2D and 3D topological-insulator behavior, pressure-induced superconductivity, Weyl semimetal behavior, Zeeman splitting, and resistivity anomaly. The quasi-1D structure of single-crystal ZrTe5 also promises large anisotropy in its thermal properties which have not yet been studied. In this work, via time-domain thermoreflectance measurements, ZrTe5 single crystals are discovered to possess a record-low thermal conductivity along the b-axis (through-plane), as small as 0.33 ± 0.03 W m⁻¹ K⁻¹ at room temperature. This ultralow b-axis thermal conductivity is twelve times smaller than its a-axis thermal conductivity (4 ± 1 W m⁻¹ K⁻¹) owing to the material’s asymmetrical crystalline structure. First-principles calculations are further conducted to reveal the physical origins of the ultralow b-axis thermal conductivity, which can be attributed to: 1) the resonant bonding and strong lattice anharmonicity induced by electron lone pairs, 2) the weak interlayer van der Waals interactions, and 3) the heavy mass of Te atoms, which results in low phonon group velocity. This work sheds light on the design and engineering of high-efficiency thermal insulators for applications such as thermal barrier coatings, thermoelectrics, thermal energy storage, and thermal management.
... For example, AlB 2 is reported to have thermal conductivity of around ∼100 Wm 1 K 1 along the c-axis. 30 TmAlB 4 was reported to have 24 ± 3 Wm 1 K 1 along the c-axis. Intrinsic building defects regarding the tiling patterns in-plane 27 were found to reduce the thermal conductivity to 18 ± 3 Wm 1 K 1 . ...
... Intrinsic building defects regarding the tiling patterns in-plane 27 were found to reduce the thermal conductivity to 18 ± 3 Wm 1 K 1 . 30 layered structure, intuitively a lower thermal conductivity would be expected cross-plane, but an interesting feature of the layered borides is that in the case of AlB 2 crystals where anisotropic measurements were possible, the thermal conductivity was discovered to be lower in-plane (∼60 Wm 1 K 1 ) parallel to the graphitic (graphene-like) structure, compared to cross-plane (∼100 Wm 1 K 1 ). 30 This was attributed to the different bondings of borides compared to carbon, i.e., graphite-related material. ...
... 30 layered structure, intuitively a lower thermal conductivity would be expected cross-plane, but an interesting feature of the layered borides is that in the case of AlB 2 crystals where anisotropic measurements were possible, the thermal conductivity was discovered to be lower in-plane (∼60 Wm 1 K 1 ) parallel to the graphitic (graphene-like) structure, compared to cross-plane (∼100 Wm 1 K 1 ). 30 This was attributed to the different bondings of borides compared to carbon, i.e., graphite-related material. ...
Transmission electron microscopy (TEM), specific heat, and thermal conductivity measurements were carried out for PrRh4.8B2 single crystals. PrRh4.8B2 takes an interesting layered crystal structure, where PrRh3B2 blocks are separated by metal Rh honeycomb layers. The existence of the Rh layers enhances the ferrimagnetic transition temperature. From area-selective picosecond thermoreflectance, the cross-plane thermal conductivity of PrRh4.8B2 is estimated to be 1.39 Wm⁻¹ K⁻¹, much lower than other layered borides like AlB2. Judging from the fine crystal structures obtained by TEM, phonon conduction is considered to be depressed due to random dispersion of Rh vacancy sites and two-dimensional nature of PrRh4.8B2.
... Thermal characterization of 2D materials, such as graphene, hexagonal boron nitride (h-BN), black phosphorus (BP), and MoS 2 [104][105][106][107], with anisotropic thermal properties is urgently required. Recently, an improved approach for measuring in-plane heat transport using spatially offset pump and probe beams, named "beam-offset TDTR," has been developed by Feser et al. [64,108,109]. In this novel method, the full-width at half-maximum (FWHM) of the out-of-phase signal V out at negative delay times is measured as a function of the in-plane offset distance between the pump and probe beams. ...
Advances in nano-electronics, nano-optics, energy harvesting materials, and nanoparticle-based photothermal therapies are motivating studies of the thermal properties of micro/nanostructures. Thus, the demands for highly sensitive and accurate thermal measurement techniques are encouraged for both fundamental studies and industrial applications. The time-domain thermoreflectance (TDTR) method, based on an ultrafast pump-probe technique , enables high-fidelity thermal measurements at the micro/nanoscale and the observation of dynamic processes with sub-picosecond time resolution. TDTR is an optical technique, capable of measuring the thermal properties of micro/nanostructures, including thermal conductivity and interfacial thermal conductance of bulk substrates, thin films, and nanopar-ticles, among others. Here we review some recent developments in the state-of-the-art ultrafast pump-probe method applied to study the thermal and magnetic properties of materials at the micro-and nanometer scales. We also discuss in detail improvements to this technique by presenting several example extensions to its capabilities.
... In this work, we applied the frequency-dependent time domain thermoreflectance (TDTR) method to quantify the thermal properties of SiO 2 NR arrays that may possess inhomogeneity along the depth direction. [19][20][21][22][23] The DSG (a.k.a. glancing angle deposition, GLAD) [24][25][26] method was applied to synthesize thermal insulating thin films with different thicknesses constructed by slanted or vertically aligned SiO 2 NR arrays. ...
Quantitative characterization of thermal properties of nanorod (NR) arrays appears to be challenging due to the complex combination of high volume of air voids, anisotropy, and structural non-uniformity. This work investigates the structure-thermal property correlation of arrays consisting of either vertically aligned or slanted silicon dioxide (SiO2) NRs, fabricated by the dynamic shadowing growth technique. We apply the frequency-dependent time-domain thermoreflectance method to quantify the thermal properties of SiO2 NR arrays that may possess inhomogeneity along the depth direction. The effective thermal conductivities of four SiO2 NR array films and one reference capping layer for the SiO2 NR array are obtained. The impact of the structure on the effective thermal conductivities of the SiO2 NR array is discussed. The lowest effective thermal conductivity among all samples in this work is found to be 0.13 W m–1 K−1 for the slanted NR array. We attribute the reduction in the effective thermal conductivity of the NR array to the discontinuous nature of SiO2 NRs, which reduces the density of the thermal transport channels and thus prevents heat flux from propagating downwards along the through-plane direction. The results from this work facilitate the potential applications of NR-array-based thermal insulators for micro-thermal devices.
... This property is quite interesting since the boron cluster compounds are strongly covalently bonded solids with high sound velocities. Several mechanisms, such as symmetry mismatch effect or degrees of freedom of boron dumbbells in the B 80 cluster, in addition to the well-known crystal complexity first demonstrated by Slack [8], have been proposed to be the origin of this intrinsic low thermal conductivity [5,9,10]. Many of the boron cluster compounds follow the variable range hopping (VRH) transport mechanism [11] which is advantageous at high temperatures, since the electrical conductivity and Seebeck coefficient have a positive temperature coefficient [3,4]. ...
SmB62 single crystals were successfully grown by the floating zone (FZ) method. The high-temperature thermoelectric properties were investigated, together with magnetic properties and specific heat at low-temperature. The electrical resistivity, ρ, shows variable-range-hopping (VRH) behavior with significantly lower values than other rare-earth RB62 (RB66) compounds. An effective magnetic moment, μeff, of 0.42 μB/Sm was estimated, which if straightforwardly taken indicates a mixed valency for SmB62 with Sm2+:Sm3+ = 1:1, which is the first ever indicated for RB66-type compounds. Localization length of the VRH at the Fermi level, ξ, was estimated to be 3.33 Å indicating that carriers in SmB62 are much less localized than in YB66 which has 0.56 Å. The thermoelectric behavior of SmB62 is striking, with ρ reduced by two orders of magnitude while maintaining large Seebeck coefficients, and as a result the power factor is ∼30 times higher than other rare-earth phases. Overall the figure of merit ZT amounts to ∼0.13 at 1050 K, with an extrapolated value of ∼0.4 at 1500 K, an expected working temperature for topping cycles in thermal power plants; that gives a ∼40 times enhancement for Sm. Since there are few thermoelectric materials applicable for very-high temperature applications, this discovery gives new interest in the samarium higher borides.
