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, 57.50μΩ·cm, 11.65 GPa and 1.46 × 10 −4 mm/N·m, respectively.

Traditionally, CeB 6 is synthesized by high temperature reaction processes such as direct solid-phase reaction of cerium or its oxides with elemental boron and carbothermal synthesis using carbon and boron carbide [7,13,14,[16][17][18].However, the disadvantages of these methods, such as high content of carbon or high processing temperatures, limit the possibility of preparing CeB 6 powders with high purity and fine particle sizes [14].In the literature, CeB 6 was produced by borothermal reduction of CeO 2 with a boron source, under formation of volatile boron suboxides at 1200-1800 °C in vacuum [17].Similarly, CeB 6 was prepared by the reaction of CeO 2 with B 4 C at 1600 °C after 60 min under vacuum and it was stated that 0.02 wt.% of C remained in the product due to the formation of gaseous reaction products [18].CeB 6 was also prepared from its oxide by reacting with boron (10 wt.% stoichiometric excess) and carbon at 1500 °C for 15 h under Ar atmosphere [19].
Recently, CeB 6 powders were prepared via different synthesis techniques, such as self-propagating high temperature synthesis (SHS -also known as combustion synthesis), mechanochemical, electrochemical and gas phase synthesis methods.Accordingly, nanoscale CeB 6 powders were prepared by SHS using CeO 2 , B 2 O 3 and Mg as raw materials [14,20].After leaching of the SHS products, CeB 6 powders were obtained with purity higher than 99% and an average particle size of 30-70 nm [14].Akgun et al. [21] reported the mechanochemical and combustion synthesis of CeB 6 starting from CeO 2 , B 2 O 3 and Mg powder mixtures.It was stated that after the leaching processes, the respective reaction yields were 68.6% and 84.4%, and average particle sizes were 290 and 240 nm for combustion and mechanochemical synthesis methods, respectively [21].Furthermore, CeB 6 nanowires with a diameter of around 20-100 nm were synthesized by self-catalyst metal-gas reaction method at 1125 °C on a Si substrate using Ce powders and BCl 3 /H 2 /Ar gas mixtures [7].Furthermore, several investigations relate the synthesis of CeB 6 films or coatings and their field emission properties [13,22,23].Bluish-violet CeB 6 ranging in size between 4 and 40 µm was produced at 1250 and 1300 °C for 2 h through reactions of CeCl 3 with circulating H 2 /Ar gas mixture (after dehydration of CeCl 3 • 7 H 2 O at 100 °C for 24 h under a reduced pressure of 10 −3 Pa) or CeF 3 with amorphous or crystalline boron in the presence and absence of Al reducing agent, using conventional apparatus such as graphite crucible or alumina boat placed into a tubular furnace [24,25].Additionally, electrochemical synthesis (at 900 °C) was performed to obtain CeB 6 crystals by using the molten salt technique from the electrolyte consisting of LiCl/Li 2 B 4 O 7 /CeCl 3 [11] and LiF/B 2 O 3 /CeCl 3 [12] raw materials.
It has been reported that bulk rare-earth hexaborides with high density cannot be prepared easily via conventional hot-pressing method due to their high plastic yield stress [26].Although fully dense rare-earth bulk CeB 6 can be prepared by sintering, the process of sintering the powders has not been comprehensively investigated in the literature.Recently, ball milled CeB 6 nanopowders were fully densified at 1000-1550 °C and a pressure of 50 MPa by using spark plasma sintering (SPS) method [26].Polycrystalline bulk CeB 6 cathodes were fabricated via SPS method using the mixed powders of nano-CeH x and micron-sized boron.High density CeB 6 sintered body was obtained at 1500 °C under a pressure of 50 MPa and holding time of 5 min [27].Apart from the above-mentioned synthesis techniques, mechanochemistry is a novel, simple and roomtemperature process which enables control of the product microstructure (phase, shape, size, etc.) and obtaining homogeneous materials [28].In the present study, nanostructured CeB 6 powders were fabricated via mechanochemical route using low-cost raw materials of CeO 2 , B 2 O 3 and Mg powders and simple equipment.The effect of milling duration on the obtained phases was investigated as a process parameter in detail.A purification treatment was applied after the synthesis process to achieve high-purity CeB 6 powders.Consolidation of the synthesized powders was conducted by pressureless sintering and the bulk CeB 6 specimen was studied for its microstructural and some physical and mechanical properties.We report here the first study of the synthesis and consolidation of homogeneous nanocrystalline bulk CeB 6 .

Raw materials and precursor preparation
CeO 2 (ABCR ™ , 99.99% pure), B 2 O 3 (ETI Mine, 98% pure) and Mg powders (MME, 99.7% pure) were used as the sources of cerium and boron, and reducing agent, respectively.The average particle sizes of the CeO 2 , B 2 O 3 and Mg starting powders were measured by using a Malvern ™ Mastersizer 2000 particle size analyser and determined as 9.5 µm, 466.9 µm and 142.7 µm, respectively.
Stoichiometry for the synthesis of CeB 6 from the CeO 2 , B 2 O 3 and Mg powders is given by the reduction reaction in Eq. 1.The stoichiometric amounts of the initial reactants CeO 2 : B 2 O 3 : Mg were 1.593 : 1.933 : 2.474 g for powders as per Eq. 1.Thus, the total powder batches of 6 g were weighed under Ar atmosphere (Linde ™ , 99.999% pure) in a Plaslabs ™ glove box.
Powder batches were mixed in a WAB ™ T2C Turbula blender for 1 h with the aim of homogenization and named as-blended powders hereafter.

Mechanochemical synthesis and purification
Mechanochemical synthesis (MCS) was conducted by milling the as-blended powders in a high-energy Spex ™ 8000D Mixer/Mill at 1200 rpm for different durations up to 5 h.The required energy needed to trigger the reaction was supplied by the high-energy collisions, induced into powder particles via mechanical deformation.The ball-to-powder weight ratio was kept constant at 10 : 1. Hardened steel vial (50 ml) and hardened steel balls (6 mm diameter) were used as milling media.Milling vials were sealed under Ar atmosphere in the glove-box.
After MCS, selective acid leaching was applied for removal of MgO by-product with 4 M HCl solution under effect of ultrasonic stirring and heating using a Bandelin Sonorex ™ RK-100H ultrasonic bath.The leaching treatment was conducted at 80 °C for 15 min with a solid-to-liquid ratio of 1 g/10 cm 3 .The residue was then separated from the leaching solution by repeated centrifugation (Hettich ™ Rotofix 32 A, 3500 rpm, 30 min), decantation and rinsing steps.The residue was dried in an FN 500 stove at 120 °C for 24 h in air.The residue is hereafter referred to as leached powders.

Consolidation
Mechanochemically synthesized and leached CeB 6 powders were compacted under a uniaxial pressure of 800 MPa in an MSE ™ MP-0710 hydraulic press which resulted in cylindrical green compacts with a diameter of 12.7 mm.They were then sintered at 1700 °C for 5 h in a Linn ™ HT-1800 high-temperature controlled atmosphere furnace operated at a heating and cooling rate of 10 °C/min under Ar gas at a flow rate of 1000 ml/min.The sintering temperature was selected as 1700 °C in regard to preliminary experiments carried out at temperatures lower than the utilized temperature and reported literature [26,27,30].According to previous studies, high density CeB 6 was obtained after SPS at 1500 °C [26,27].Since the applied sintering method is pressureless sintering, the samples should be exposed to higher temperature to reach an adequate densification as compared to SPS method.

Characterization
X-ray diffraction (XRD) investigations of the asblended, mechanochemically synthesized and leached powders and sintered samples were performed using a Bruker ™ D8 Advanced Series X-ray powder diffractometer, with CuK α (λ = 0.154 nm, 40 kV and 40 mA) radiation in the 2θ range of 10-90°incremented at a step size of 0.02°at a rate of 2°/min.The International Center for Diffraction Data ® (ICDD) powder diffraction files were utilized to determine the crystal structure and lattice parameter(s) of the crystalline phases.
The average crystallite sizes and lattice strains of the mechanochemically synthesized CeB 6 powders were measured by utilizing Bruker-AXS ™ TOPAS V3.0 software based on the modified Scherrer's formula [29].The amounts of the different phases in the mechanochemically synthesized CeB 6 powders were identified by the semi-quantitative Rietveld method based on the related XRD patterns.General images of the as-blended, mechanochemically synthesized and leached powders were captured by using a Zeiss ™ Discovery.V12 stereomicroscope (SM) coupled with a Zeiss ™ Axiocam ERc5s high resolution digital camera.Particle size measurements were conducted for the leached powders using a Microtrac ™ Nano-flex particle size analyzer (PSA) equipped with a Bandelin Sonopuls ™ ultrasonic homogenizer.Thermal properties of the as-blended and mechanochemically synthesized powders were examined using a TA ™ Instruments SDT Q600 differential scanning calorimeter (DSC): for each run, 15 mg sample was placed in an alumina crucible and heated up to 1150 °C at a heating rate of 10 °C/min under Ar atmosphere.After DSC analyses, the mechanochemically synthesized and heated powders were subjected to additional XRD analyses to detect the resulting phases.Microstructural characterizations and energy dispersive spectroscopy (EDS) analyses of the mechanochemically synthesized and leached powders were carried out using a Hitachi ™ TM-1000 and a FEI ™ Quanta FEG 250 scanning electron microscopes (SEM) operated at 15 kV and also using a JEOL ™ JEM-2000EX transmission electron microscope (TEM) operated at 160 kV.EDS results were reported as the arithmetic means of three different measurements taken from the same regions in the samples.The powder densities of the final powder products after leaching were measured by a Micromeritics ™ AccuPyc II 1340 gas pycnometer in a 1 cm 3 sample chamber at room temperature using He gas (Linde ™ , 99.996% pure) as the displacement medium.Density results of each powder product include the arithmetic mean of ten measurements and standard deviations.The surface areas of the leached powders were determined using a Quantachrome ™ Autosorb-IO/MP-XR surface area analyser (SAA) based on the Brunauer-Emmett-Teller (BET) theory.The samples were degassed at 150 °C under N 2 gas (Linde ™ , 99.999% pure) and analysed by N 2 absorption at −196 °C using He (for reference) gas.Magnetic properties of the leached powders were measured using a vibrating sample magnetometer (VSM) at room temperature under DC-biased magnetization between 0 and 8000 Oe.Furthermore, the amounts of the elements (B, Mg and Fe) in the supernatant liquid after purification by the leaching process were analysed using a Perkin Elmer ™ 1100B atomic absorption spectrometer (AAS).
The density of the sintered sample was measured in ethanol using the Archimedes' method and the results were reported as the arithmetic mean of three different measurements taken from the same sample.To obtain scratch-free mirror finish for electrical resistivity measurements, SEM analyses, Vickers microhardness measurements and sliding wear tests, the sintered samples were subjected to a series of standard metallographic sample preparation treatments.Electrical resistivity of the polished specimen was measured at room temperature using a four-point probe method in a Signatone ™ four-point probe system equipped with a high impedance current source and a voltmeter.The expression for the calculation of electrical resistivity of the bulk structure (ρ) is given elsewhere [30].Electrical resistivity of the sample includes the arithmetic mean of five measurements carried out at 100-300 mA and the standard deviations.
Vickers microhardness value of the sintered sample was measured using a Shimadzu ™ HMV microhardness tester at 100 g load (9.807 N) for 15 s.Microhardness test results for each sample include the arithmetic mean of fifteen successive indentations and standard deviations.Surface roughness value of the sintered sample was measured by using a Veeco ™ Dektak 6 M Stylus profilometer under 5 mg applied force for 30 s and with 2 mm length.The sintered sample was subjected to sliding wear test at room temperature in a Tribotechnic ™ oscillating tribotester using a 6 mm alumina ball (Tribotechnic ™ , 99.5% pure) under 4 N applied force, with 2 mm/s sliding speed and 2 mm stroke length for a total sliding distance of 30 000 mm. Wear track of the sintered sample was imaged by the SM and examined by the profilometer.Wear test result in terms of wear volume loss value is the arithmetic mean of five different measurements for each sample.

III. Thermodynamic calculations
Thermodynamic calculations for CeO 2 , B 2 O 3 and Mg powders were performed using the FactSage ™ 6.3 program.The standard Gibbs free energy and enthalpy changes as a function of temperature for Eq. 1 are presented in Fig. 1a and 1b, respectively.According to the plots, Eq. 1 has a large negative standard free energy change (between −2100 and −300 kJ) and standard enthalpy (between −3800 and −2000 kJ) change, which indicates that this reaction is feasible and it can occur spontaneously in the temperature range of 0-2000 °C.As seen in Fig. 1b, the slope of the standard enthalpy-temperature curve changes at certain temperatures, arising from the melting of B 2 O 3 at 450 °C, respective melting and boiling of Mg at 650 °C and 1095 °C and formation of magnesium borate phases until 1100 °C [30].Additionally, these thermodynamic calculations have revealed that the proposed reduction reaction in Eq. 1 can be spontaneously carried out at room temperature.
Figure 2a shows probable reaction products and their molar amounts as a function of increasing temperature for the CeO  Therefore, 2 h milling duration can be considered as the initial point for the formation of the borate and boride phases.At the end of 3 h milling, the only detected phases were CeB 6 , CeBO 3 and MgO (Fig. 4f).
Further milling for 1 h (Fig. 4g) showed the disappearance of CeBO 3 which was a transition phase during the formation of CeB 6 .Therefore, the reaction in Eq. 1 completely took place after mechanochemical synthesis conducted by milling for 4 h (Fig. 4g).Extending the milling duration to 5 h did not change the nature of the resulting products: CeB 6 and MgO phases were still present in the 5 h milled powders (Fig. 4h) and no new phases were formed.XRD patterns of the CeB 6 phase contained twelve peaks which were consistent with the standard CeB 6 pattern (Fig. 4e-h).The peak intensities of the CeB 6 and MgO phases gradually decreased from 3 to 5 h milling as seen from Fig. 4f-h.On the basis of the related XRD patterns, the average crystallite sizes of the CeB 6 phase in the powders milled for 3, 4 and 5 h were calculated as 178, 149 and 90 nm, respectively.Stereomicroscope (SM) images of the as-blended and mechanically milled CeO 2 , B 2 O 3 and Mg powders are shown in Fig. 5. Particle size heterogeneity is clearly observed in Fig. 5a.After milling for 1 h, a homogenous distribution of the particles throughout the microstruc-ture is enabled (Fig. 5b).The CeO 2 , B 2 O 3 and Mg powders have a granular and finer morphology after 1.5 h and 1 h 45 min of milling (Figs.5c,d).When the reaction partially takes place after 2 h of milling, the morphology is converted into an agglomerated form (Fig. 5e) and remains almost unchanged after 3, 4 and 5 h of milling except for reduction in particle size and disappearance of shiny points, probably corresponding to Mg particles (Figs.5f,g,h).
Figure 6 displays the DSC thermograms of the asblended CeO 2 , B 2 O 3 and Mg powders and those mechanically milled for different durations up to 5 h.The DSC curve of the as-blended powders (Fig. 6a) has a very broad endotherm with an onset temperature of 450 °C and a peak point at around 920 °C.This is an overlapping endotherm probably representing the melting of B 2 O 3 , the melting of Mg and their reaction with CeO 2 .DSC curves of the powders milled for 1 h, 1.5 h and 1 h 45 min have the same characteristics with that of the as-blended powder.However, they have three small and shallow exothermic peaks approximately at 600, 650 and 700 °C, probably corresponding to the formation of CeBO 3 phase, oxidation of Mg and formation of Mg 3 B 2 O 6 phase, (Fig. 6b-d).The latter two exotherms shift and get smaller in the DSC curve of the 2 h of milled powders (Fig. 6e) because they already contain CeO 2 , Mg, CeBO 3 , CeB 6 and MgO phases after milling.On the other hand, no exothermic peaks are observed in the DSC curves of the powders milled for 3, 4 and 5 h, since they already contain the reaction products of CeB 6 and MgO (Fig. 6f-h).
The XRD patterns of the 1 h, 1 h 45 min, 2 h and 5 h mechanically milled CeO 2 , B 2 O 3 and Mg powders after DSC analyses up to 1150 °C are shown in Fig. 7.The milled and heated powders contain the same phases: CeB 6 , MgO, Mg 3 B 2 O 6 and CeBO 3 .However, the powder milled for 1 h and 1 h 45 min and heated (Fig. 7a,b) have the dominant CeBO 3 phase rather than CeB 6 .Thus, additional CeBO 3 and Mg 3 B 2 O 6 phases occur instead of yielding only CeB 6 and MgO phases, when a subsequent heat treatment is conducted after milling without the complete reduction reaction.On the other hand, MCS is a one-step process for the formation of CeB 6 phase.Even if 2 h milled powder has the phases of CeO 2 , Mg, CeB 6 , CeBO 3 and MgO together in its structure and 5 h milled powder has the phases of CeB 6 and MgO (Fig. 4e,h), they have similar phases after DSC heating (Fig. 7c,d).CeB 6 is partially converted to CeBO 3 and Mg 3 B 2 O 6 phases during the heating of the 5 h milled powders due to the reaction of some amount of CeB 6 and MgO with each other (Fig. 7d).Similar DSC and following XRD observations were previously reported by Agaogulları et al. [30] for the Sm 2 O 3 -B 2 O 3 -Mg system.On the basis of the XRD and DSC analyses, it is understood that milling duration of 5 h yielded CeB 6 and MgO phases and there exists no exothermic peak corresponding to the oxidation of residual Mg phase.Thus, 5 h was selected as the sufficient MCS time for the latter investigations.
Figure 8 illustrates the XRD pattern of the CeB 6 powders mechanically milled for 5 h and leached with 4 M HCl.As clearly seen, the acid concentration is sufficient to completely remove the MgO phase from the powders.On the other hand, no impurity phases were observed in the XRD pattern within the detection limit of the used diffractometer (>2 wt.%).Moreover, based on the XRD pattern, the average crystallite size of the leached powders was determined as 98 nm.It was reported in a related study that leaching of the CeB 6 , MgO and Fe containing MCS products in a 1 M HCl solution for 30 min was sufficient to remove the MgO phase [21].Although leaching the CeB 6 , MgO and Mg 3 B 2 O 6 containing combustion synthesis (CS) products in a 1 M HCl solution for 15 h enabled to eliminate the MgO phase, it was not adequate for the dissolution of Mg 3 B 2 O 6 [21].In another study, however, MgO and Mg 3 B 2 O 6 phases in CS products were completely removed from CeB 6 powders by leaching with 6 M HCl for more than 4 h under mechanical agitation of 200 rpm at 50 °C [6,31].Furthermore, HCl concentration of 3 M was earlier accepted as a sufficient leaching concentration to obtain pure (>99%) CeB 6 powders without MgO and Mg 3 B 2 O 6 contaminations [5].In another related study, the as-synthesized powders containing LaB 6 , SmB 6 and CeB 6 phases were purified by applying post treatments such as stirring in 1-6 M HCl so-  lution at 95 °C, filtering, washing and drying [32].In the present study, AAS analysis was also conducted on the supernatant liquid decanted from leaching treatment of the 5 h milled powders using 4 M HCl; 198 ppm B, 6 ppm Fe and 6563 ppm Mg were determined in the solution.The detection of B and Fe is related with the dissolution of CeB 6 because of the highly concentrated HCl acid and dissolution of Fe impurity released from the milling media, respectively.Repeated HCl leaching was also carried out, and Fe and Mg were found as trace amounts in the supernatant liquid.Thus, it can be stated that the leached powders have sufficient purity without the presence of any impurity soluble in HCl acid.Figure 9a displays the SM image of the CeB 6 powders obtained after milling for 5 h and leaching with 4 M HCl.As compared to the SM image of the 5 h milled powders in Fig. 5h, it can be seen that MgO was completely removed and the bluish-violet colour of the CeB 6 powders appeared after the leaching treatment.The leached CeB 6 powders have round-shaped particles (SEM image, Fig. 9b).The general EDS measurement (Fig. 9c) taken from the area in this image reveals the presence of Ce and Au (from the coating) elements, indicating that only CeB 6 particles are present in the microstructure without any detection of impurity element.The SEM image at a higher magnification in Fig. 9d shows the CeB 6 particles ranging in size between 50 and 150 nm.In addition, the PSA graph (Fig. 9e) indicates that the average particle size of the CeB 6 is about 86 nm which is in the range observed in the SEM image in Fig. 9d.
Figure 10 illustrates the TEM/EDS images of the CeB 6 powders mechanically milled for 5 h and leached with 4 M HCl. Figure 10a is a BF image showing equiaxed particles varying in sizes between 75 and 100 nm. Figure 10b is the corresponding SADP image of the white-circled region marked in this BF image; the sharp and continuous rings suggest the presence of fine CeB 6 particles.Figure 10c is another BF image representing the irregular-shaped particles in size range of 50-150 nm.The general EDS measurement (Fig. 10d) taken on this BF image reveals that the pow- The density value of the synthesized CeB 6 powders after milling and leaching was determined by He gas pycnometer as 4.77 ± 0.009 g/cm 3 , which is close to its theoretical density value of 4.79 g/cm 3 .Additionally, the surface area value of the synthesized CeB 6 powders was determined as 13.45 m 2 /g.This indicates that the particle size of CeB 6 is in nanoscale (86 nm).Additionally, Fig. 11 represents the magnetic measurement of the synthesized CeB 6 powders at room temperature and confirms its tendency to be paramagnetic.As previously reported in the literature, rare-earth hexaborides display a remarkable variation in their magnetic properties [33].There are diamagnetic, Pauli-paramagnetic, ferromagnetic, anti-ferromagnetic and more complex spin-ordered hexaborides reported in the literature [33].Most of the magnetic properties are considered very sensitive to the number of valence electrons available in the valence band.In other words, the magnetic properties of the rare-earth hexaborides strongly depend on stoichiometry (rare-earth metal/B ratio), impurity and homogeneity ranges.Thus, there are inconsistencies among the numerous reported magnetic investigations of rare-earth hexaborides due to the differences in composition arising from the method of preparation [33].Consequently, it can be inferred from the magnetic measurements that the synthesized CeB 6 powders do not include any Fe contamination since they do not exhibit a ferromagnetic tendency (Fig. 11).

Consolidation and bulk properties of CeB 6
The XRD pattern of the sintered specimen obtained from the pure CeB 6 powders prepared via milling for 5 h and leaching with 4 M HCl is shown in Fig. 12.The XRD pattern shows that the bulk sample has only the CeB 6 phase.There is no new phase formation at high- temperature or contamination resulting from the sintering process by the effect of high temperature.Thus, XRD pattern of the sintered sample (Fig. 12) is very similar to that of the leached powder (Fig. 8).However, the average crystallite size of the bulk sample calculated from the XRD is slightly higher (115 nm) as compared to the leached powder (98 nm) due to the effect of temperature.Table 1 lists some physical and mechanical properties of this bulk CeB 6 sample.As seen from Table 1, the density of the sintered sample was determined as 4.56 g/cm 3 , accordingly the relative density was calculated as 95.20% TD.Bao et al. [16] reported that nano (ball-milled) and coarse powders of CeB 6 spark plasma sintered at 1550 °C at a pressure of 50 MPa had the relative density of 99.1 and 76.7% TD, respectively.In an earlier study related to the conventional hot pressing of rare-earth hexaborides, the sintering temperature and the relative density were respectively reported as 1900 °C and 90% TD, indicating that this method is not sufficient to obtain full densification [16].A higher relative densification rate was achieved in this study when compared with the reported study.
It is most likely due to the nanocrystalline structure and high purity of the synthesized powders which contribute to accelerated densification.Furthermore, the electrical resistivity of bulk CeB 6  ).Lundström [33] reported that the rare-earth hexaborides exhibit significant variations in terms of electrical properties.Most of the electrical properties are very sensitive to the number of valence electrons available in the valence band and depend on the stoichiometry, impurity and homogeneity ranges of the borides.Moreover, the electrical resistivities of the rare-earth hexaborides may differ because of the differences in composition arising from the synthesis method [32][33][34].
For example, the resistivity of the CeB 6 crystals obtained by the flux method was higher than that obtained by the floating zone method due to the deviation from the stoichiometry, presence of uncontrolled Al impurity, and higher oxygen content [35,36].Also, the electrical resistivity of bulk CeB 6 prepared by the SPS method increased from 36.34 to 60.54 µΩ•cm with increasing temperature from 25 to 500 °C, indicating a metallic conducting behaviour [16].Furthermore, CeB 6 has approximately 20 µΩ•cm higher electrical resistivity than that of LaB 6 in the temperature range of 25-500 °C pointing out its suitability as a more appropriate direct heating cathode [16].
As seen from Table 1, the microhardness value of the bulk CeB 6 was determined as 11.65 ± 0.50 GPa.In a related study carried out on rare-earth hexaborides, the hardness of CeB 6 crystals prepared by travelling solvent floating zone method using oxides and boron powders was found to be 1090 kg/mm 2 [36].Polycrystalline bulk CeB 6 cathode fabricated by the SPS method using optimum process parameters (1500 °C, 50 MPa and 5 min) had a relative density of 99.61% TD and a hardness value of 2051 kg/mm 2 [27].In another study, the hardness value of bulk CeB 6 sample was reported as 2550 HV 50 [22].Vickers hardness values of the CeB 6 bulks sintered from nanoscale and coarse powders by SPS (at 1550 °C and 50 MPa) were respectively found to be 2160 and 1435 kg/mm 2 and the difference was attributed to the finer particle size and more homogeneous microstructure provided by the usage of nanopowders [16].Thus, the hardness values for CeB 6 can differ significantly from one sample to another since the hardness strongly depends on the powder properties (influenced by the synthesis method) and consolidation techniques.In addition, the surface roughness and friction coefficient of CeB 6 were determined as 0.605 µm and 0.33, respectively.It was reported that the surfaces of the CeB 6 crystals grown by flux and floating zone methods are considerably different.The roughness amplitudes of the samples obtained by the floating zone method are higher than those obtained by the flux method [35].
After dry sliding wear test of bulk CeB 6 , wear width and depth of 709 and 15.75 µm, wear area of 8765.89µm 2 and wear volume loss of 0.0175 mm 3 were recorded.Accordingly, the wear rate of bulk CeB 6 was calculated as 1.46 × 10 −4 mm 3 /N•m indicating a high wear resistance.A similar wear rate value was reported previously by Agaogulları et al. [30] for the bulk SmB 6 as 1.61 × 10 −4 mm 3 /N•m.Figure 13a shows the SM image taken from the surface of the bulk CeB 6 after sliding wear test, and Fig. 13b illustrates the OM image of the contact surface of the sliding Al 2 O 3 ball used during the wear test.The general image in Fig. 13a shows a shallow wear track on the surface which conforms well with the measured wear width and area values.Therefore, it can be said that CeB 6 shows a high wear resistance under the utilized test conditions by virtue of its high densification rate and high hardness.Furthermore, the worn and flattened surface of the Al 2 O 3 ball in Fig. 13b indicates a mutual wear between the bulk CeB 6 and Al 2 O 3 ball although Al 2 O 3 has a higher hardness.This wear behaviour can be accepted as an indication of high hardness and wear resistance of CeB 6 (Table 1).Thus, nanostructured CeB 6 powders of high-purity were prepared via a facile synthesis route consisting of room temperature mechanochemical synthesis in a high-energy ball mill (5 h of milling) and a subsequent purification process (leaching with 4 M HCl).Additionally, the bulk CeB 6 sample prepared by cold pressing and pressureless sintering showed high densification rate.Its physical and mechanical properties were improved thanks to the characteristics of CeB 6 powders.Thus, the processing techniques described in this study can be proposed as simple and economical routes for preparing nanostructured rare-earth hexaborides.

V. Conclusions
In the present study, nanostructured bulk CeB 6 was successfully prepared through three steps including mechanochemical synthesis, purification by HCl leaching, and cold-pressing/pressureless sintering.The results of this study can be summarized as follows:

1 Figure 2 . 1 Figure 3 .
Figure 1.Standard Gibbs free energy change (a) and enthalpy change (b) versus temperature of the reaction in Eq. 1

Figure 8 .
Figure 8. XRD pattern of the CeO 2 , B 2 O 3 and Mg powders mechanically milled for 5 h and leached with 4 M HCl

Figure 9 .
Figure 9. SM (a), low-magnification SEM image (b), general EDS taken from SEM image (c), high-magnification SEM image (d) and PSA image (e) of the CeB 6 powders mechanically milled for 5 h and leached with 4 M HCl

Figure 10 .
Figure 10.TEM images of the CeB 6 powders mechanically milled for 5 h and leached with 4 M HCl: a) BF image, b) its SADP taken from the white-circled region revealing CeB 6 phase, c) another BF image and d) its general EDS analysis (objective aperture is on (001) and camera length is 100 cm)

Figure 11 .
Figure 11.Magnetic field versus magnetization curve of the CeB 6 powders mechanically milled for 5 h and leached with 4 M HCl

Figure 12 .
Figure 12.XRD pattern of the sintered CeB 6 sample

Figure 13 .
Figure 13.Wear track SM image (a) of the bulk CeB 6 sample, and worn surface OM image (b) of the alumina ball sliding on the surface of bulk CeB 6

Table 1 .
Some properties of the sintered CeB 6 bulk derived from the CeB 6 powders mechanically milled for 5 h and leached with 4 M HCl was measured as 57.50 ± 3.80 µΩ•cm.Weimer [4] reported the electrical resistivity values of some borides: CrB (45.5 µΩ•cm), CrB 2 (30 µΩ•cm), HfB 2 (10.6 µΩ•cm), NbB 2 (25.7 µΩ•cm), TaB 2 (32.5 µΩ•cm), TiB 2 (9 µΩ•cm), VB (35 µΩ•cm), VB 2 (22.7 µΩ•cm) and ZrB 2 (9.7 µΩ•cm).Additionally, the electrical resistivity of bulk SmB 6 was previously reported as ∼211 µΩ•cm [30].Therefore, it can be stated that CeB 6 exhibits an electrical resistivity value similar to other boride compounds excluding IVB diborides (TiB 2 , ZrB 2 and HfB 2 • The formation mechanism of CeB 6 was explained by thermodynamic calculations using FactSage software.Based on the thermodynamical investigations, the formation reaction of CeB 6 included different steps as follows: The reduction of B 2 O 3 by Mg and the reaction of CeO and Mg in the presence of B and the formation of CeB 6 and MgO.• During the mechanochemical processing of CeO 2 , B 2 O 3 and Mg starting powders, formation of the CeB 6 phase started only after 2 h of milling together with CeO 2 , Mg, MgO and CeBO 3 phases.• Extending the milling duration to 5 h resulted in the disappearance of the borate phases yielding only CeB 6 and MgO.• Powders milled for 5 h were successfully purified by leaching in 4 M HCl resulting in the formation of high-purity CeB 6 powders free from undesired MgO.• The purified CeB 6 powders exhibited rounded-shape morphology and had an average particle size of 86 nm.• CeB 6 powders were consolidated by cold pressing and pressureless sintering at 1700 °C for 5 h and densified up to 95.2% TD which is a comparatively high relative density, attributed mainly to the nanostructured and highly pure powder characteristics.• Bulk CeB 6 had paramagnetic tendency and electrical resistivity value of ∼57.50 µΩ•cm.• The bulk CeB 6 had a hardness value of ∼11.65 GPa and a wear rate of 1.46 × 10 −4 mm 3 /N•m indicating a relatively high wear resistance.