ZrB2 Surface Research Articles

Deuterium plasma induced preferential erosion in ultra-high temperature ceramics TiB2 and ZrB2*

Abstract Steady-state deuterium plasma exposures were performed on ultra-high temperature ceramics titanium diboride (TiB2) and zirconium diboride (ZrB2) using the PISCES-RF linear plasma device (LPD) as early screening for first wall, plasma-facing applications. Deuterium plasma exposures were performed using 40 eV ion energies at 240, 525, and 800 °C sample temperatures and 90 eV ion energies at 240 °C sample temperatures to analyze TiB2 and ZrB2 surface morphology and chemistry evolution behavior. Post-plasma exposure chemistry characterization of the near surface ( < 50 nm) region of the samples all show transition metal enrichment, indicating boron preferential erosion. Transition metal to boron fractions vary with plasma exposure temperature under the 40 eV ion energy; metal enrichment is maximized at 800 °C and then minimized at 525 °C. SEM micrographs of all plasma exposed sample surfaces show no significant or noticeable plasma induced damage from cracking or blistering.

Heating samples to 2000 °C and above for scanning tunneling microscopy studies in ultrahigh vacuum

A simple device for heating single-crystal samples to temperatures ≥2000 °C in ultrahigh vacuum that is compatible with the standard sample plates used in a common commercial scanning tunneling microscope (STM) is described. Heating high melting point samples to higher temperatures than is possible with many existing STM sample holders is necessary to obtain clean, well-ordered surfaces. Results are demonstrated for the (0001) surface of ZrB2, which has a melting point of 3050 °C.

Effect of transition metal on the structure and oxidation behavior of ZrB2 (0 0 0 1): Experimental and theoretical calculations

ZrB2 can withstand extreme environments, but the high-temperature oxidation prevented them from being applied as possible thermal protection system materials. The effect of transition metal (Tm = Ta, Hf, Mo, and W) on oxidation resistance of ZrB2 was systematically investigated by the first principles calculations, including ab initio molecular dynamics, and experimental tests. The results showed that the doped atoms, especially Hf atom, increased the adsorption energy of O atoms, which prohibited the O adsorb on the surface of ZrB2. Besides, the energy barrier of O atom migration on the Hf-doped surface and inward migration is 0.60 eV and 6.68 eV, which is higher than the un-doped surface. This showed that Hf performs well in inhibiting the diffusion of O. The reason was that the doped atoms increased the strength of B-Zr and O-Zr bonds, respectively. Finally, to verify the theoretical calculations, ZrB2 and (Zr, Tm) B2 solid solutions were prepared by using pressure-less sintering and the thermal gravimetric tests were carried out. The experimental results showed that the (Zr, Tm) B2 solid solutions have better oxidation resistance than ZrB2, which is consistent with the results from our theoretical calculations. In this work, the modification mechanism of doping atoms on the oxidation resistance of ZrB2 was explained from the point of view of adsorption and diffusion. And it provides guidance for promoting the application of ZrB2 at high temperature.

Microstructural and nanoindentation study of TaN incorporated ZrB2 and ZrB2–SiC ceramics

This study assessed the sinterability and microstructure of ZrB2-SiC-TaN and ZrB2-TaN ceramics. Spark plasma sintering at 2000 °C and 30 MPa for 5 min produced both ceramics. The relative density of ZrB2 ceramic containing TaN was 95.3%; the addition of SiC increased this value to 98.1%. SiC’s contribution to the elimination of ZrB2 surface oxides was the primary factor in the advancement of densification. The in situ formation of hexagonal boron nitride at the interface of TaN and ZrB2 was confirmed by high-resolution transmission electron microscopy, field emission-electron probe microanalyzer, X-ray diffractometry, and field emission scanning electron microscopy. Moreover, the in situ graphite might be produced as a byproduct of the SiC-SiO2 process, hence boosting the reduction of oxide compounds in the ternary system. The SiC compound had the highest hardness (29 ± 3 GPa), while the ZrB2/TaN interface exhibited the greatest values of elastic modulus (473 ± 26 GPa) and stiffness (0.76 ± 0.13 mN/nm).

Atomistic simulations of surface reactions in ultra-high-temperature ceramics: O[formula omitted], H[formula omitted]O and CO adsorption and dissociation on ZrB[formula omitted](0001) surfaces

Understanding surface reactivity is crucial in many fields, going from heterogeneous catalysis to materials oxidation and corrosion. In order to decipher the surface reactions of ZrB2 exposed to the harsh environment of aerospace components, the chemical activity of both Zr- and B-surfaces is predicted and compared by using density functional theory and nudged elastic band methods. In particular the adsorption, dissociation and diffusion of O2, CO and H2O are extensively examined through the calculation of surface adsorption energies and reaction pathways. We find the dissociative adsorption of O2 dominating the reactivity of ZrB2 surfaces, while the dissociation of H2O and CO is weakly active on Zr-surfaces, and even less activated on B-terminated ones. Importantly, we discover that the reaction of O2 and CO can trigger strong surface reconstruction at B-surfaces. Our work thus provides significant insights into the diverse adsorption and reaction mechanisms on ZrB2 surfaces.

High temperature interaction between molten Ni50Al50 alloy and ZrB2 ultra-high temperature ceramics

In this work, Ni50Al50 alloy is taken into consideration as potential brazing material for joining ZrB2 ultra-high temperature ceramic. The results of experimental study on high temperature interfacial phenomena between molten binary Ni50Al50 alloy and polycrystalline ZrB2, are shown. A sessile drop method combined with a capillary purification procedure was applied to investigate the wetting behavior of Ni50Al50/ZrB2 couple during holding for 400 s at temperature of 1688 °C (i.e. at T = 1.02Tm). It was found that the molten Ni50Al50 rapidly wets and spreads over the surface of ZrB2, while involved reactive infiltration into the solid substrate allowed reaching a final contact angle of ~0° in 250 s. The wetting kinetics was much faster than that reported in the literature for Cu, Ag or Au tested at T = 1.05Tm. The solidified couple was subjected to SEM/EDS microstructural characterization in order to reveal a course of interfacial phenomena. The results point towards (I) an existence of Ni-enriched Ni-Al/ZrB2 surface interfacial layer; (II) a formation of infiltration zone assisted by reactively formed Al2O3 due to a reaction with Al-rich melt and (III) a partial transfer of ZrB2 phase to Ni50Al50 alloy by a dissolution/precipitation mechanism.

Rhombohedral boron nitride epitaxy on ZrB2

Epitaxial rhombohedral boron nitride (r-BN) films were deposited on ZrB2(0001)/4H-SiC(0001) by chemical vapor deposition at 1485 °C from the reaction of triethylboron and ammonia and with a minute amount of silane (SiH4). X-ray diffraction (XRD) φ-scans yield the epitaxial relationships of r-BN(0001)∥ZrB2(0001)out-of-plane and r-BN[112¯0]∥ZrB2[112¯0] in-plane. Cross-sectional transmission electron microscopy (TEM) micrographs showed that epitaxial growth of r-BN films prevails to ∼10 nm. Both XRD and TEM demonstrate the formation of carbon- and nitrogen-containing cubic inclusions at the ZrB2 surface. Quantitative analysis from x-ray photoelectron spectroscopy of the r-BN films shows B/N ratios between 1.30 and 1.20 and an O content of 3–4 at. %. Plan-view scanning electron microscopy images reveal a surface morphology where an amorphous material comprising B, C, and N is surrounding the epitaxial twinned r-BN crystals.

Influence of Y2O3 addition on the mechanical and oxidation behaviour of carbon fibre reinforced ZrB2/SiC composites

The influence of Y2O3 addition on the microstructure, thermo-mechanical properties and oxidation resistance of carbon fibre reinforced ZrB2/SiC composites was investigated. Y2O3 reacted with oxide impurities present on the surface of ZrB2 and SiC grains and formed a liquid phase, effectively lowering the sintering temperature and allowing to reach full density at 1900 °C. The presence of a carbon source (fibres) led to additional reactions which resulted in the formation of new secondary phases such as yttrium boro-carbides. Mechanical properties were significantly enhanced compared to the un-doped composite. Further tests at high temperatures resulted in strength increase up to 700 MPa at 1500 °C which was attributed to stress relaxation. Oxidation tests carried out at 1500 °C and 1650 °C in air showed that the presence of the Y-based secondary phases enhanced the growth of ZrO2 grains, but offered limited protection to oxygen due to the lower availability of surficial SiO2 formed from SiC.

Influence of Al addition on microstructures of Cu−B alloys and Cu−ZrB2 composites

Abstract The influence of Al addition on the microstructure of Cu−B alloys and Cu−ZrB2 composites was investigated using scanning electron microscopy, X-ray diffraction and first-principles calculation. The results show that the eutectic B in Cu−B alloys can be modified by Al from coarse needles to fine fibrous structure and primary B will form in hypoeutectic Cu−B alloys. As for Cu−ZrB2 composites, Al can significantly refine and modify the morphology of ZrB2 as well as improve its distribution, which should be due to its selective adsorption on ZrB2 surfaces. The first-principles calculation results indicate that Al is preferentially adsobed on ZrB2 (12(__)10), then on ZrB2 (101(__)0), and finally on ZrB2 (0001). As a result, smaller sized ZrB2 with a polyhedron-like, even nearly sphere-like morphology, can form. Due to Al addition, the hardness of Cu−ZrB2 composites is greatly enhanced, but the electrical conductivity of the composites is seriously reduced.

Pursuing low infrared emissivity materials with wider coverage band in ZrB2–CeO2 compounds and their reaction mechanisms

Low infrared emissivity in 3–5 μm or 8–14 μm band has been realized in different host materials. Realizing the low infrared emissivity in single material covering the entire band is rarely achieved. This research explores a binary system of ultra-high temperature ceramic ZrB2–CeO2 composites prepared by solid state sintering. their phase composition, microstructure evolution, reaction mechanism and infrared emissivity performance were discussed in detail. It was found that CeO2, ZrB2, ZrO2, and ZrnCe1-nO2 existed in the sample in addition to the coated glass phase of CeO2 on the surface of the sample. In-situ HRTEM results clearly revealed a series of reaction mechanism of CeO2 and ZrB2. With the increase of sintering temperature, the CeO2 particles gradually transformed into a glass phase, and were distributed on the surface of ZrB2 particles, and ZrO2 was formed on the surface of ZrB2, simultaneously. Consequently, ZrnCe1-nO2 is formed as the reaction of ZrO2 and CeO2. Compared with the infrared emissivity spectra of pure CeO2 and ZrB2, a low infrared emissivity less than 0.2 in a wider coverage band of 3–5 and 8–14 μm in ZrB2–CeO2 compounds was achieved as a result of synergistic interaction between CeO2 and ZrB2. The obtained information about the phase composition and microstructure evolution helps to realize low infrared emissivity in a wider coverage band in compounds via composition and microstructure modification.

Oxygen adsorption on ideal ZrB2 and ZrC surfaces

To understand the initial oxidation of ZrB2 and ZrC, the adsorption behaviors of an O2 molecule on ideal ZrC (1 1 1) and ZrB2 (0 0 0 1) surfaces were investigated by first-principle calculation, and thermal gravimetric and differential scanning calorimetry test was executed. The results reveal that the initial oxidation temperature of ZrC was 210 °C, while that of ZrB2 was up to 500 °C since the adsorption energies of O2 molecule on Zr–ZrC surface was higher than that on Zr–ZrB2 surface. Meanwhile, the adsorbed O2 molecule affected the C atom bonding with the oxidized Zr atom in O–Zr–ZrC, while the B atom bonding with oxidized Zr atom in O–Zr–ZrB2 underwent little influence. Besides, the higher adsorption energy was obtained in O–C–ZrC than in O–B–ZrB2, implying that the C atom in C–ZrC might be oxidized more easily than the B atom in B–ZrB2. In addition, big holes (C vacancies) were formed when O2 molecule adsorbed on the C–ZrC surface because of the generation of C–C dimer and C–O bond, which led to that the next O2 molecule might penetrate the Zr atomic layer and react with the Zr atoms. While O2 molecule doped into the B atomic layer in B–ZrB2, and more O2 molecules might be adsorbed on the B–ZrB2 surface to generate a B2O3 structure to protect the inner material.

Decomposition of ammonia on ZrB2(0001)

Zirconium diboride has been recently identified as a promising substrate for the growth of Group-III nitride semiconductors using reactive vapors that include ammonia as the nitrogen source. Adsorption energies and dissociation pathways of NH3 on the (0001) surface of ZrB2 were investigated using density functional theory calculations. Our results indicate that NH3 readily adsorbs onto the ZrB2 surface terminated with Zr and decomposes to atomic N and H with relatively small activation barriers. The resulting atomic species are found to be mobile with the computed diffusion barriers between 0.11 and 0.78 eV.

Atomic-scale investigation on the growth behavior of rod shape ZrB2

The microstructure evolution in the growth of zirconium diboride (ZrB2) rods was directly observed with the help of high resolution-transmission electron microscopy. Key features of layered structure and step contours are proposed to interpret the growth mechanism of the new layer on the surface of ZrB2. Under electron beam irradiation, on the surface of the ZrB2 rod amorphous layer first nucleated and grew into two-dimensional (2D) islands. Then with the continuous migration and rearrangement of atoms, the 2D islands began to interconnect and formed a continuous new layer. The energies that govern migration and diffusion of atoms in new ZrB2 layer comes from a high density of islands, interface defects and the distortion energy caused by atomic mismatches in the initial growth stage of each layer.

Evolution of microstructure and anti-oxidation ability of ZrB2 improved by a unique inert glass phase

Abstract Zirconium Boride is widely used as thermal protection materials. However, the characteristics of poor oxidation resistance of ZrB2 at high temperatures limits its applications. In order to improve the anti-oxidation performance of ZrB2, a novel method of doping LaF3 is adopted in present study. In the synthesis process of ZrB2 via carbothermal reduction method, LaF3 was introduced to the initial materials. Motivated by the slightly higher formation temperature of ZrB2 than the melting point of LaF3, a liquid glass phase composed of F and La was deposited on the surface of ZrB2, which naturally brought in oxygen starvation and contributed to improved oxidation resistance performance. The crystal lattice parameters and the anti-oxidation ability of ZrB2 are closely dependent on the added LaF3 content, which can ascribe to La3+/Zr2+ substitution and the amount of glass phase on the surface of ZrB2.

Catalytic properties of ZrB2–SiC–W ultra–high temperature ceramics at moderate temperatures

Catalytic behaviors related to recombination reactions of dissociated oxygen atoms on the surface of ZrB2–SiC–W ultra–high temperature ceramic was investigated quantitatively in the temperature ranges from room temperature to 1300 K, and recombination coefficient can be obtained by the photochemical method based on spectral diagnostic techniques using microwave–discharge plasma apparatus. Oxidation of specimens even at relatively low temperature during the experiment had a significant influence on the catalytic properties of ZrB2–SiC–W ultra–high temperature ceramics, and the results revealed the detectable dependence of the catalytic properties upon environment–induced surface modification. The relationship between catalytic properties of ZrB2–SiC–W ultra–high temperature ceramics and surface microstructure was also discussed in detail.

Insights into the spontaneous formation of silicene sheet on diboride thin films

The realization of silicene-free ZrB2(0001) thin films grown on Si(111) by Ar+ ion bombardment allowed for studying the spontaneous formation of silicene on their surfaces. Imaging the bare ZrB2(0001) surface by STM revealed the structures of Zr-terminated and B-terminated ZrB2(0001) created by the bombardment. The spontaneous formation of a continuous silicene sheet on a sputtering-induced disordered ZrB2 surface demonstrates that silicene does not require an atomically-flat crystalline template to be stabilized. This opens the way to the fabrication of large scale single-crystal sheets and points out the potential of silicene to be used in the next generation silicon-based technologies.

A Theoretical Investigation on the Anisotropic Surface Stability and Oxygen Adsorption Behavior of ZrB2

Surfaces play pivotal roles during the oxidation and interfacial bonding of ZrB2. To understand the surface properties, the anisotropic stability and oxygen adsorption behavior of ZrB2 surfaces, including (), two types of (0001) and three types of (), were investigated by first‐principles calculations. Using a series of two‐region models, the surface energies were calculated and the (0001) surfaces were found to be less stable than the prismatic ones. The hexagonal rod‐like ZrB2 grain morphology was predicted during the crystal growth under equilibrium conditions. The adsorption energies, electronic structure, and bonding feature of the adsorbed surfaces were also investigated. The Zr‐terminated surfaces were predicted to be more favorable to adsorb oxygen, and the (0001) surfaces should have better oxidation resistance than other surfaces in the equilibrium ZrB2 grains. The Zr‐terminated (0001) surface was also speculated to be stable in the oxygen‐rich environment.

A nitride-based epitaxial surface layer formed by ammonia treatment of silicene-terminated ZrB2.

We present a method for the formation of an epitaxial surface layer involving B, N, and Si atoms on a ZrB2(0001) thin film on Si(111). It has the potential to be an insulating growth template for 2D semiconductors. The chemical reaction of NH3 molecules with the silicene-terminated ZrB2 surface was characterized by synchrotron-based, high-resolution core-level photoelectron spectroscopy and low-energy electron diffraction. In particular, the dissociative chemisorption of NH3 at 400 °C leads to surface nitridation, and subsequent annealing up to 830 °C results in a solid phase reaction with the ZrB2 subsurface layers. In this way, a new nitride-based epitaxial surface layer is formed with hexagonal symmetry and a single in-plane crystal orientation.

Oxidation of ZrB2 Nanoparticles at High Temperature under Low Oxygen Pressure

The oxidation of ZrB2 nanoparticles was observed at high temperature of 1500°C under low oxygen partial pressure of 5 × 10−2 Pa by an environmental transmission electron microscope. The results demonstrate that the oxidation starts on the surface of ZrB2 nanoparticles with decomposition of ZrB2 into ZrO2 and B2O3. The nucleation and growth of ZrO2 on the surface of ZrB2 proceed with B2O3 being evaporated.

Dissociation of trimethylgallium on the ZrB2(0001) surface

X-ray photoelectron spectroscopy and reflection absorption infrared spectroscopy (RAIRS) have been used to study the dissociative adsorption of trimethylgallium (TMG) on the ZrB2(0001) surface. Spectra were obtained as a function of annealing temperature following TMG exposure at temperatures of 95 and 300 K, and also as a function of TMG exposure for a surface temperature of 300 K. After annealing above 220 K, a significant decrease in the relative concentration of carbon and gallium occurred accompanied by a shift of ∼0.2 eV in the Ga 2p3/2 binding energy. The RAIR spectra show that after annealing to ∼220 K, only one CH3 deformation band at 1196 cm−1 remains, the intensity of which is considerably decreased indicating loss of at least one methyl group from TMG. Further annealing leads to the sequential loss of the other methyl groups. The first methyl desorbs while the last two dissociate to deposit two C atoms per TMG molecule onto the ZrB2 surface.