Phase Of HfO2 Research Articles

Effect of oxygen partial pressure on phase, local structure and photoluminescence properties of Hf(1-x)YxO2 thin films prepared by pulsed laser deposition

Hf(1-x)YxO2 (x = 0, 0.10, 0.15, 0.20) epitaxial thin films on YSZ(100) substrate were prepared at different oxygen (O2) partial pressures (100, 70, 30, and 10 sccm) by pulsed laser deposition. The effect of oxygen partial pressure on crystalline and local structure was investigated using X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) and X-ray absorption spectroscopy (XAS). The surface composition was determined by X-ray photoelectron spectroscopy. The XRD and FTIR studies reveal that undoped HfO2 thin films remain in monoclinic phase, and the Hf0.85Y0.15O2 & Hf0.80Y0.20O2 films stabilize in cubic phase at all oxygen pressures, whereas the structure of Hf0.90Y0.10O2 films changes from orthorhombic to cubic as oxygen pressure reduces from 100 sccm. The XAS at Hf-L3 edge and O K-edge shows the changes in bond distances, disorder and hybridization. In monoclinic HfO2 films the Hf–O bond distance and disorder increases and Hf–Hf bond decreases as the O2 partial pressure decreases, whereas disorder in doped films shows anomalous behaviour. The oxygen vacancies are evident from both XAS and photoluminescence studies. The results enlighten a deeper understanding of phase change, local structure, optical properties and pave a way to stabilize and identify different technologically important HfO2 phases for dielectric and ferroelectric applications.

Formation Mechanism of Al<sub>2</sub>O<sub>3</sub>/HfO<sub>2</sub> Eutectic Coating on SiC Substrate Using Light Focusing Method

In this paper, we discuss our previous experimental results and recent considerations on the preparation of solidified oxide eutectic coating on non-oxide ceramics by unidirectional solidification method using light focusing apparatus. As an example, an explanation is given here with regard to the formation mechanism of Al2O3-HfO2 eutectic coatings on SiC substrate. The Al2O3 component in oxide melt easily reacts with SiC and primary HfO2 phase solidified on SiC substrate. However, the solidified HfO2 phase also react with SiC substrate and HfC-HfO2 graded functional layer is formed on SiC substrate and Al2O3-HfO2 eutectic microstructure is formed on the graded functional layer

Structural stability and polarization analysis of rhombohedral phases of HfO2

Since the experimental observation of a rhombohedral phase of hafnium oxide, there remains controversy over whether this phase belongs to the R3 or R3m space group. Moreover, the origin of polarization in these two rhombohedral phases has not been comparatively elucidated. Here, we present a theoretical study comparing the relative stability and ferroelectricity of the R3 and R3m phases of HfO2, representing two potential forms of heavily Zr-doped ferroelectric thin films of hafnia found recently. We comprehensively investigate their structural stability and polarization response under in-plane compressive strain. A phase transition from R3 to R3m is discovered under biaxial compressive strain. The direction and magnitude of polarization in both phases can be tuned by strain. Through symmetry mode analysis, we elucidate the improper nature of ferroelectricity. These findings may advance understanding of ferroelectricity in hafnia thin films.

Unraveling the effect of pressure on structural phase transition, electronic, and optical properties of Hf1-xSixO2 (x = 0, 0.03, 0.06, 0.09): A first-principles investigation

The non-centrosymmetric phase of HfO2 with excellent Si compatibility has great importance in integrated photonic circuits, non-volatile memory, and ferroelectric field effect transistors. We report a theoretical study of pressure-induced phase transition from a centrosymmetric m-HfO2 to non-centrosymmetric o-HfO2 with Si doping. The reported phase transition pressures, i.e. 15, 14, 8, and 8 GPa for x = 0, 0.03, 0.06, and 0.09, respectively, for Hf1-xSixO2 are in excellent agreement with available experimental results. With increasing Si concentrations, the transition pressures reduce significantly, which is explained in terms of bond length and charge transfer. The thermal stability of the obtained o-HfO2 phase is examined via ab-initio molecular dynamics up to its synthesis temperature. The negative formation energy also confirms its thermodynamical stability. Density of states indicates the noticeable appearance of Si states in the lower conduction band and an increase in the extent of hybridization between Hf-5d, O-2p, and Si-2p atomic states. The doping changes the nature of band gap from indirect to direct, concurrently reducing the magnitude. The o-HfO2 shows a low value of static dielectric constant, which results in a smaller refractive index (1.82) and lower reflectivity in the infrared and visible regions.

Effects of La-Doped HfO2 Films on Dielectric Properties by Sol-Gel Method

HfO2 and La-doped HfO2 with 20, 40, 50, and 60 mol% were deposited on silicon wafers by spin-coating technique. The crystallization process was annealed temperature at 600 °C. Then, characterization techniques of XRD, AFM, SEM, Full-field XRF imaging, and XAS were applied to reveal structural morphology, local structure, grain, distribution, etc. This study aims to investigate the effect of doping La into the HfO2 system on surface formation, microstructure, and dielectric properties. According to the results, RMS surface roughness, and grain size increased relative to La concentration levels. These films demonstrated homogeneous, low surface roughness, and crack-free. The XRD confirms the existence of the monoclinic phase in pure HfO2 and the mixture phase of the monoclinic and orthorhombic phases in La-doped HfO2 films. Increasing La concentration caused the structural transformation, the XAS of La L3 -edge indicating that lanthanum remained in the HfO2 system. Dielectric response based on interface-phase polarization was improved with La content from 0 mol% to 60 mol%. Compared with HfO2 film, the dielectric constant of La-doped HfO2 films was increased and dielectric loss was decreased.

Preparation and Study of Ca/Tb Co-Doped HfO2 Infrared Coatings with Different Atomic Ratios

In this study, HfO2 coatings co-doped with different Ca/Tb atomic ratios were prepared via the atmospheric plasma spraying (APS) method. The microstructure, infrared radiation properties, and high-temperature stability of the coatings were investigated. All of the doped coatings possessed a porous surface and were composed of two phases, namely the monoclinic HfO2 phase and the cubic HfO2 phase. In addition, the content of the cubic phases increased when raising the doping atomic ratio of Ca/Tb, suggesting that Ca could stabilize the cubic HfO2 phase more effectively. The results also show that the coating with a Ca/Tb atomic ratio of 1/0 (CT1 coating) had more excellent infrared radiative properties, whose total emissivity was 0.844 in the 0.75~6.5 μm band and 0.900 in the 6.5~15 μm band, respectively. The improvement in emissivity in the 0.75~6.5 μm band was mainly due to the impurity energy levels introduced via oxygen vacancy, which promoted the absorption of free carriers. And, in 6.5~15 μm, because the approximate masses of the Ca-O and Tb-O bonds were smaller than that of the Hf-O bonds, the infrared absorption of the lattice vibration shifted, favoring absorption below 10 μm. Moreover, Ca had a more significant strengthening effect than Tb in the whole band. In terms of high-temperature infrared radiation performance, the total emissivity of the CT1 coating at 2.5~25 μm increased as the temperature increased from 500 °C to 1100 °C, which might be attributed to the thermal-enhanced lattice vibration absorption. However, the emissivity of the CT1 coating at 3~5 μm was kept around 0.9 from 1100 °C to 2000 °C, owing to the fact that infrared absorption was more determined by the intrinsic width of the energy levels because of the weakening of the doping effect at high temperatures. In terms of thermal stability, the surface morphology and chemical composition of the CT1 coating were barely changed within 4 h of heat treatment at 2000 °C. The total infrared emissivity of the CT1 coating after 4 h of heat treatment was 0.826 in the 0.75~6.5 μm band and 0.895 in the 6.5~15 μm band, slightly lower than that before heat treatment, suggesting good thermal stability and good application prospects as a high-temperature infrared material.

Ferroelectricity in the Al doped HfO2

Recently, hafnia (HfO2)-based ferroelectrics have attracted much attention due to their unique features such as superior ferroelectricity at an ultra-thin thickness, large coercive electric field, and compatibility with the standard silicon process platform. In this work, the growth of ferroelectric HfO2 thin films through Al doping have been demonstrated. Structural analysis showed that non-centrosymmetric polar orthorhombic phase HfO2 have been observed after doping Al. Interestingly, the ferroelectricity was shown to be dependent on the Al composition, where the remnant polarization as high as 2Pr = 22.9 μC/cm2 was obtained in HfO2 thin film with a 5.0 mol% Al doping. The mechanisms of structural transition to ferroelectric orthorhombic-phase owing to oxygen vacancies and generation of oxygen vacancies benefiting from Al doping were further studied by density functional theory calculations. The results shown in this work provide insights into the formation of ferroelectric HfO2 due to Al doping and further offer a simple way for controlled fabrication of ferroelectric HfO2 thin films.

Finite temperature effects on the structural stability of Si-doped HfO2 using first-principles calculations

The structural stabilities of the monoclinic and tetragonal phases of Si-doped HfO2 at finite temperatures were analyzed using a computational scheme to assess the effects of impurity doping. We proposed a method that the finite temperature effects, i.e., lattice vibration and impurity configuration effects, are considered. The results show that 6% Si doping stabilizes the tetragonal phase at room temperature, although a higher concentration of Si is required to stabilize the tetragonal phase at zero temperature. These data indicate that lattice vibration and impurity configuration effects are important factors determining structural stability at finite temperatures.

Ferroelectricity and pseudo-coherent growth in HfO2/SrHfO3 nanolaminates

Ferroelectricity in thin films of HfO2 has been the subject of extensive studies in materials science as well as device applications. The emergence of ferroelectricity is attributable to the orthorhombic phase (Pca21) of HfO2, stabilized in the films by metal-element doping, strains from substrates and electrode films, and oxygen deficiency. Recently, ferroelectricity has been reported in nanolaminates of HfO2 with other oxides such as ZrO2 and Al2O3, implying that nanolaminates are another effective way to bring about ferroelectricity in HfO2. However, the mechanism of orthorhombic phase stabilization in nanolaminates is not fully understood. In this study, we demonstrated that ferroelectricity emerges in nanolaminates consisting of undoped HfO2 and perovskite SrHfO3 deposited on Sn-doped In2O3 bottom electrodes, when the thickness of HfO2 layers was ≥6 nm. For nanolaminates in which the thickness of the HfO2 layers was ≤5 nm, ferroelectricity was remarkably suppressed due to Sr-incorporation into the HfO2 layers at the interface. In those nanolaminates, the crystal orientations of HfO2 grains were well aligned throughout the HfO2 layers, indicating that the HfO2 layers grew in a pseudo-coherent manner. This study aids to understand the stabilization of the ferroelectric orthorhombic phase in nanolaminates in terms of their structural properties.

Comprehensive study on the origin of orthorhombic phase stabilization in Gd-doped HfO2 and DFT calculations.

In recent times, ultra-thin films of hafnium oxide (HfO2) have shown ferroelectricity (FE) attributed to the orthorhombic (o) phase of HfO2 with space group Pca21. This polar o-phase could be stabilized in the doped thin film of the oxide. In the present work, both polar and non-polar o-phases of HfO2 could be stabilized in Gd-doped bulk polycrystalline HfO2. Rietveld analysis of XRD data shows that the relative population of o-phases in the presence of the monoclinic (m) phase of HfO2 increases with increasing Gd-content. The local environment around the host atom has been investigated by time differential perturbed angular correlation (TDPAC) spectroscopy, synchrotron based X-ray near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) measurements showed a reduction in grain size with increasing Gd-dopant indicating a solute drag effect. It could be established that the segregation of the Gd-dopant in the grain boundary is a thermodynamically favorable process and the solute drag effect plays an important role in nucleation of the o-phase in bulk HfO2. Stabilization of Gd in both Pbca and Pca21 phases of HfO2 was supported by defect formation energy calculations using density functional theory (DFT). The present study has important implications in future applications of HfO2 in ferroelectric devices and in understanding the role of dopants in stabilizing the o-phase of HfO2 in the bulk.

Ferroelectric Hf0.5Zr0.5O2 films with improved endurance obtained through low temperature epitaxial growth on seed layers.

Crystallization temperature is a critical parameter in the stabilization of the metastable ferroelectric phase of HfO2. The optimal crystallization temperature used for polycrystalline films is too low to grow epitaxial films. We have developed a new growth strategy, based on the use of an ultrathin seed layer, to obtain high-quality epitaxial films of orthorhombic Hf0.5Zr0.5O2 at a lower temperature. The threshold temperature for epitaxy is reduced from about 750 °C to about 550 °C using a seed layer. Epitaxial films deposited at low temperatures exhibit highly enhanced endurance, and films grown at 550-600 °C show high polarization, no wake-up effect, and greatly reduced fatigue and improved endurance in comparison with the films deposited at high temperatures without a seed layer. We propose that the endurance enhancement is due to a positive effect of the defects, which limits the propagation of pinned ferroelectric domains.

Ferroelectricity in HfO2 from a Coordination Number Perspective

Ferroelectricity observed in thin-film HfO2, either doped with Si, Al, and so forth or in the Hf0.5Zr0.5O2 form, has gained great technical significance. While a trilinear coupling between phonon modes could explain its ferroelectric distortion, from a practical perspective, one may be concerned with a theory that is more straightforward to predict similar ferroelectric candidates through some apparent features of HfO2 and ZrO2. In this work, we propose that the 7 cation coordination number of HfO2/ZrO2 lies at the heart of this ferroelectricity, which stems from the proper ionic radii of Hf/Zr compared with O. Among the numerous compounds with a non-centrosymmetric nature, for example, the mm2 point group, HfO2 and ZrO2 are special in that they are close to the border of 7 and 8 cation coordination, such that the 8-coordination tetragonal intermediate phase could greatly reduce the switching barrier. Other 7-coordination candidates, including SrI2, TaON, YSBr, and YOF, are also studied in comparison to HfO2/ZrO2, and six switching paths are analyzed in detail for the Pca21 phase. A rule of the preferred switching path in terms of the ionic radii ratio and coordination number has been established. We also show the possible route from the ferroelectric Pca21 phase to the monoclinic P21/c phase in HfO2, which is relevant to the fatigue phenomenon.

Microstructure, mechanical and tribological properties of oxide dispersion strengthened CoCrFeMnNi high-entropy alloys fabricated by powder metallurgy

Developing materials with superior strength and wear resistance has always drawn challenging research for advanced engineering applications. Herein, considerable efforts have been devoted to enhancing the strength and wear resistance of face-centered cubic (FCC)-structured CoCrFeMnNi high entropy alloy (HEA) by incorporating HfO2 nanoparticles (NPs) through a powder metallurgy approach. The phase composition, microstructure, mechanical, and tribological properties of HEA composites were investigated. The XRD results reveal that the composite powders consisted of the FCC solid solution along with minor HfO2 phases. In addition, sintered HEAs showed the major FCC phase along with minor Cr-rich and HfO2 phases. Microscopic results confirmed the uniform distribution of HfO2 NPs throughout the matrix during the milling process. With increasing HfO2 contents, the hardness of the HEAs increased from 270 ± 10 to 520 ± 10 HV, while the compressive yield strength increased from 370 to 1500 MPa, due to dispersion and grain boundary strengthening effects. Furthermore, composite HEAs showed a decrease in coefficient of friction and wear rates with increasing HfO2 content due to increased surface hardness and transition in wear mechanism from severe wear to mild abrasive wear. The present study demonstrates the feasibility of producing high-performance oxide dispersion-strengthened HEAs for advanced structural applications.

The CMAS corrosion behavior of high-entropy (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 hafnate material prepared by ultrafast high-temperature sintering (UHS)

High-temperature molten calcium-magnesium-alumina-silicate (CMAS) corrosion has become a fatal factor for the failure of aero-engine thermal barrier coatings. In this study, a promising entropy-stabilized (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 (5YH) hafnate was prepared by the emerging ultrafast high-temperature sintering (UHS), and its corrosion and wetting behavior of molten CMAS were investigated. For the corrosion mechanism, the precipitation of the high-entropy apatite phase promotes the formation of the HfO2 phase, and it can improve the density and stability of the slow-growing reaction layer, hindering the further penetration of molten CMAS. At 1300 ℃, a reaction layer with a three-layered morphology is generated, resulting from the decreased viscosity of the molten CMAS. Moreover, computational analysis shows that molten CMAS on the 5YH surface has a larger contact angle (17°) than traditional YSZ (13°), and the spreading area is about 90 % of traditional YSZ, which benefits for its good CMAS corrosion resistance.

The Effect of Y Doping on Monoclinic, Orthorhombic, and Cubic Polymorphs of HfO2: A First Principles Study

HfO2 can assume different crystalline structures, such as monoclinic, orthorhombic, and cubic polymorphs, each one characterized by unical properties. The peculiarities of this material are also strongly related to the presence of doping elements in the unit cell. Thus, the present paper has the main purpose of studying and comparing twelve different systems characterized by diverse polymorphs and doping percentages. In particular, three different crystalline structures were considered: the monoclinic P21/c, the orthorhombic Pca21, and the cubic Fm3¯m phases of HfO2. Each one has been studied by using Y as a doping agent with three different contents: 0% Y:HfO2, 8% Y:HfO2, 12% Y:HfO2, and 16% Y:HfO2. For all the systems, density functional theory (DFT) methods based on PBE/GGA, and on the HSE hybrid functionals were used to optimize the geometry as well as to study their optical properties. Depending on the polymorphs, Y affects the formation energy in different ways and causes changes in the optical properties. When the percentage of Y did not exceed 12%, a stabilization of the cubic phase fraction and an increase of the dielectric constant was observed. Additionally, the calculated optical bandgap energies and the refractive index are examined to provide an overview of the systems and are compared with experimental data. The bandgaps obtained are in perfect agreement with the experimental values and show a slight increase as the doping percentage grows, while only minor differences are found between the three polymorphs in terms of both refractive index and optical band gap. The adopted first principles study generates a reasonable prediction of the physical-chemical properties of all the systems, thus identifying the effects of doping phenomena.

Simulation of XRD, Raman and IR spectrum for phase identification in doped HfO2 and ZrO2

Fluorite-structured hafnium and zirconia require different, complementary characterization methods to identify the numerous metastable phases. This is because of the many possible positions of the oxygen ions, which are difficult to observe directly. Ab initio simulations are useful to probe the corresponding XRD, Raman, and infrared spectra for fingerprints. However, the predictive power of theoretical methods is limited both by model errors and by boundary conditions such as defects, stresses, and morphology that are difficult to detect. We first consider the calculation of Raman and infrared spectra of the most interesting undoped phases of HfO2 and ZrO2, compare the results with known results, and discuss the uncertainties. Next, we consider the possibilities of classifying the phases using X-ray diffraction. To this end, we introduce the effects of doping, which increases the uncertainty due to structural disorder. For illustration, we examine a large data set of doped structures obtained with ab initio calculations. To make an unbiased assignment of phases, we use machine learning methods with clusters. The limits of X-ray diffraction spectroscopy are reached when phase mixtures are present. Resolution of single-phase polycrystalline samples may only be possible here if these three characterization methods are used.

Carrier concentration mediated enhancement in thermoelectric performance of various polymorphs of hafnium oxide: a plausible material for high temperature thermoelectric energy harvesting application

The optimization of figure of merit by tuning carrier concentrations is an effective way to realize efficient thermoelectrics (TEs). Recently, the feasibility of high p-type carrier concentration (order of ∼1022cm−3) is experimentally demonstrated in various polymorphs of hafnium oxide (HfO2). In light of these studies, using the first-principles calculation combined with the semi-classical Boltzmann transport theory and phonon dynamics, we realized high TE performance in various polymorphs of HfO2 in a range of carrier concentrations at high temperatures. The phonon dispersion calculations confirm the dynamical stability of all polymorphs. The observed values of the Seebeck coefficient are 945.27 mV K−1, 922.62 mV K−1, 867.44 mV K−1, and 830.81 mV K−1 for tetragonal (t), orthorhombic (o), monoclinic (m), and cubic (c) phases of HfO2, respectively, at 300 K. These values remain positive at all studied temperatures which ensures the p-type behaviour of HfO2 polymorphs. The highest value of electrical conductivity (2.34 × 1020 Ω−1m−1s−1) observed in c-HfO2 at 1200 K, and the lowest value of electronic thermal conductivity (0.37 × 1015 W mK s−1) observed in o-HfO2 at 300 K. The lattice thermal conductivities at room temperature are 5.56 W mK−1, 2.87 W mK−1, 4.32 W mK−1, and 1.75 W mK−1 for c-, m-, o- and t- HfO2, respectively which decrease to 1.58 W mK−1, 0.92 W mK−1, 1.12 W mK−1, 0.53 W mK−1 at 1200 K for respective phases. The low lattice thermal conductivities lead to the high values of the figure of merit, i.e. 0.97, 0.87, 0.83, and 0.77 at 1200 K for the m-, o-, t-, and c- HfO2, respectively, at the optimized carrier concentrations (∼1021 cm−3). The predicted optimized carrier concentrations for various phases are in close agreement with the experimental reports. The estimated high figure of merit can make HfO2 a potential material for TE energy harvesting applications at elevated temperatures.

Finite-size effects on ferroelectricity in rhombohedral HfO2

In this work we analyze the finite-size effects on the structural properties and on the polarization of the rhombohedral phase of HfO$_2$ subjected to a biaxial compressive strain. We show how the presence of surface charges affects the polarization, leading to a strong reduction with respect to its bulk value. This reduction can be ascribed to two mechanisms: the coupling between compressive strain and the phase-transition order parameter; the changes in the ferroelectric distortion. We give two alternative explanations of this phenomenon: from an atomistic point of view, analyzing the evolution of the bond lengths, and from a symmetry-analysis point of view, considering the changes in the amplitude of the symmetry-allowed distortions, when a slab configuration is considered. These results are independent on the slab-thickness in the considered range, suggesting the absence of a critical thickness for ferroelectricity in HfO$_2$, in agreement with the proposed improper nature of hafnia ferroelectricity.

Different polymorphs of Y doped HfO2 epitaxial thin films: Insights into structural, electronic and optical properties

We report the investigation of local structure, electronic and optical properties of different polymorphs of (YO1.5)x(HfO2)1-x (x = 0, 0.05, 0.10, 0.15, and 0.20) epitaxial thin films on YSZ(100) substrate prepared by pulsed laser deposition technique. The X-ray diffraction results confirm that the deposited films are epitaxial and crystal structure changes from monoclinic to orthorhombic to cubic/tetragonal as Y concentration increases in HfO2 thin films. Spectroscopic identification of phase transformation in these thin films was investigated using Fourier transform infrared and X-ray absorption spectroscopy (XAS). Effect of crystal structure on Hf-O & Hf-Hf bond distances and disorder were investigated by extended X-ray absorption fine structure. O K-edge XAS spectra show profound changes in eg and t2g peaks for different HfO2 polymorphs. X-ray photoelectron spectroscopic studies confirm that different chemical environments are present in different polymorphs of HfO2 thin film. The photoluminescence measurements validate the wide band spectral emission in wavelength region of 400–550 nm for all polymorphs and photoluminescence areal intensity shows strong dependence on phase and local structure disorder. This work will be helpful in understanding the basic material properties and spectroscopic identification of different HfO2 phases of high application relevance in semiconductor industry.

Equiatomic 9-cation high-entropy carbide ceramics of the IVB, VB, and VIB groups and thermodynamic analysis of the sintering process

The preparation of high-entropy (HE) ceramics with designed composition is essential for verifying the formability models and evaluating the properties of the ceramics. However, inevitable oxygen contamination in non-oxide ceramics will result in the formation of metal oxide impurity phases remaining in the specimen or even escaping from the specimen during the sintering process, making the elemental compositions of the HE phase deviated from the designed ones. In this work, the preparation and thermodynamic analysis during the processing of equiatomic 9-cation HE carbide (HEC9) ceramics of the IVB, VB, and VIB groups were studied focusing on the removing of the inevitable oxygen impurity existed in the starting carbide powders and the oxygen contamination during the powder mixing processing. The results demonstrate that densification by spark plasma sintering (SPS) by directly using the mixed powders of the corresponding single-component carbides will inhibit the oxygen-removing carbothermal reduction reactions, and most of the oxide impurities will remain in the sample as (Zr,Hf)O2 phase. Pretreatment of the mixed powders at high temperatures in vacuum will remove most part of the oxygen impurity but result in a remarkable escape of gaseous Cr owing to the oxygen-removing reaction between Cr3C2 and various oxide impurities. It is found that graphite addition enhances the oxygen-removing effect and simultaneously prevents the escape of gaseous Cr. On the other hand, although WC, VC, and Mo2C can also act as oxygen-removing agents, there is no metal-containing gaseous substance formation in the temperature range of this study. By using the heat-treated powders with added graphite, equiatomic HEC9 ceramics were successfully prepared by SPS.