TEM analysis of boride-based ultra-high temperature ceramics

Hafnium and zirconium borides are leading candidate materials for use in ultra-high-temperature applications thanks to their excellent combination of physical, mechanical and oxidation resistance properties. It has been shown that the addition of MoSi2 allows the densification without the application of pressure, improves the oxidation resistance and the mechanical properties at high temperatures. Despite the use of this sintering additive for several ultra high temperature ceramics, the densification mechanisms are still unclear and matter of debate. Transmission electron microscopy (TEM) is a powerful tool to explore microstructure at small length scale. A careful literature analysis reveals that neither detailed TEM work nor reports on densification mechanisms are available for this class of materials. In the present work, the microstructure of pressureless sintered ZrB2-MoSi2 and HfB2-MoSi2 composites was analyzed by scanning and transmission electron microscopy in order to disclose the mechanisms leading to densification and to understand the role of MoSi2 during sintering. The formation of solid solutions was observed in ZrB2-MoSi2 system, whilst the solubility of Mo into HfB2 lattice seems to be more limited. For both composites the presence of (TM,Mo)5SiB2, where TM=Hf or Zr, was detected. The formation of secondary phases is analysed and discussed in accordance with thermodynamical calculations and phase diagram

Microstructure characterization of boride-based ultra-high-temperature ceramics

Hafnium and zirconium borides are leading candidate materials for use in ultra-high-temperature applications thanks to their excellent combination of physical, mechanical and oxidation resistance properties. It has been shown that the addition of MoSi2 allows the densification without the application of pressure, improves the oxidation resistance and the mechanical properties at high temperatures. Despite the use of this sintering additive for several ultra high temperature ceramics, the densification mechanisms are still unclear and matter of debate. Transmission electron microscopy (TEM) is a powerful tool to explore microstructure at small length scale. A careful literature analysis reveals that neither detailed TEM work nor any reports on densification mechanism are available for this class of materials. In the present work, the microstructure of HfB2-MoSi2 and ZrB2-MoSi2 composites was analyzed in detail in order to gain an insight into the densification mechanism during pressureless sintering. The formation of solid solutions was observed in ZrB2-MoSi2 system, whilst the solubility of Mo into HfB2 lattice seems to be more limited. For both composites the presence of (TM,Mo)5SiB2, where TM=Hf or Zr, was detected. The formation of secondary phases is analysed and discussed in accordance with thermodynamical calculations and the phase diagram

Growth of Boron-Rich Nanowires by Chemical Vapor Deposition (CVD)

B-rich nanowires are grown on Ni coated oxidized Si(111) substrate using diborane as the gas precursor in a CVD process at 20 torr and 900°C. These nanowires have diameters around 20–100 nanometers and lengths up to microns. Icosahedron B12 is shown to be the basic building unit forming the amorphous B-rich nanowires as characterized by EDAX, XRD, XPS, and Raman spectroscopies. The gas chemistry at low [B₂H₆]/ [N₂] ratio is monitored by the in situ mass spectroscopy, which identified N₂ as an inert carrier gas leading to formation of the B-rich compounds. A nucleation controlled growth mechanism is proposed to explain the rugged nanowire growth of boron. The role of the Ni catalyst in the synthesis of the B-rich nanostructures is also discussed

Controlled oxygen vacancy induced p-type conductivity in HfO{2-x} thin films

We have synthesized highly oxygen deficient HfO$_{2-x}$ thin films by controlled oxygen engineering using reactive molecular beam epitaxy. Above a threshold value of oxygen vacancies, p-type conductivity sets in with up to 6 times 10^{21} charge carriers per cm3. At the same time, the band-gap is reduced continuously by more than 1 eV. We suggest an oxygen vacancy induced p-type defect band as origin of the observed behavior.Comment: 4 pages, 3 figure

Super-strong materials for temperatures exceeding 2000 °C

Ceramics based on group IV-V transition metal borides and carbides possess melting points above 3000 °C, are ablation resistant and are, therefore, candidates for the design of components of next generation space vehicles, rocket nozzle inserts, and nose cones or leading edges for hypersonic aerospace vehicles. As such, they will have to bear high thermo-mechanical loads, which makes strength at high temperature of great importance. While testing of these materials above 2000 °C is necessary to prove their capabilities at anticipated operating temperatures, literature reports are quite limited. Reported strength values for zirconium diboride (ZrB₂) ceramics can exceed 1 GPa at room temperature, but these values rapidly decrease, with all previously reported strengths being less than 340 MPa at 1500 °C or above. Here, we show how the strength of ZrB₂ ceramics can be increased to more than 800 MPa at temperatures in the range of 1500–2100 °C. These exceptional strengths are due to a core-shell microstructure, which leads to in-situ toughening and sub-grain refinement at elevated temperatures. Our findings promise to open a new avenue to designing materials that are super-strong at ultra-high temperatures

TEM analysis on TaSi2-containing ultra-high temperature ceramics

Ultra-high temperature ceramics are suitable structural ceramics for applications under high heat fluxes at temperature that can exceed 1600?C. Future hypersonic vehicles have a potential use temperature above 2000?C and require oxidation resistant materials. The ceramics object of this study, namely ZrB2, HfB2, HfC and TaC, possess a unique combination of properties including high melting point temperature (ZrB2: 3245?C, HfB2: 3250?C, HfC: 3890?C, TaC:3985?C), high hardness and strength, good oxidation resistance and high thermal conductivity. Ceramics based on borides and carbides of Zr, Hf and Ta were hot pressed at 1750?C-1900?C to full density thanks to the addition of 15 vol% of TaSi2. TaSi2 was selected to promote the densification, due to its high melting point (2200?C), its ductility at the sintering temperature and its capability to provide increased oxidation resistance. The microstructure of the composites was analyzed by X-ray diffraction, scanning and transmission electron microscopy in order to investigate the densification mechanisms occurring during sintering. In the boride-based composites the formation of (Ta,Me)B2 solid solution growing epitaxially on the matrix with low-angle grain boundary was observed. The chemistry of the triple points suggest that cation transfer is an active process and the passage through a liquid phase is also strongly hypothesized. The secondary phases identified were SiC, Ta5Si3 and Ta-oxides. Concerning the carbide-based materials, a higher solubility between Ta and Hf was observed both in the carbide grains and in the silicide. Also in these systems, Ta-rich solid solutions were observed surrounding the matrix. The microstructure evolution is discussed with respect to the chemistry of the elements involved, the phase diagrams and the thermodynamic

Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3. I. Structure and room temperature properties

Lead-free piezoelectric ceramics, (1-x-y)Bi0.5Na0.5TiO3-xBaTiO(3)-yK(0.5)Na(0.5)NbO(3) (0.05 <= x <= 0.07 and 0.01 <= y <= 0.03), have been synthesized by a conventional solid state sintering method. The room temperature ferroelectric and piezoelectric properties of these ceramics were studied. Based on the measured properties, the ceramics were categorized into two groups: group I compositions having dominant ferroelectric order and group II compositions displaying mixed ferroelectric and antiferroelectric properties at room temperature. A composition from group II near the boundary between these two groups exhibited a strain as large as similar to 0.45% at an electric field of 8 kV/mm. Polarization in this composition was not stable in that the piezoelectric coefficient d(33) at zero electric field was only about 30 pm/V. The converse piezoelectric response becomes weaker when the composition deviated from the boundary between the groups toward either the ferroelectric or antiferroelectric compositions. These results were rationalized based on a field induced antiferroelectric-ferroelectric phase transition.open12510

Impact of Specifically Adsorbing Anions on the Electroless Growth of Gold Nanotubes

Electroless metal deposition on nanochannel-containing templates is a versatile route towards metal nanotubes and nanowires if the plating reaction can be sufficiently controlled. In this study, disulfitoaurate-formaldehyde-based gold plating baths were modified by the addition of halides, pseudohalides, and EDTA. The introduction of specifically adsorbing anions strongly affected the heterogeneously autocatalyzed plating reaction and allowed the regulation of the reaction rate and the product morphology. The new plating baths showed enhanced stability and allowed the synthesis of homogeneous nanotubes of high aspect ratios (>150) in 30 μm thick ion track-etched polymer templates. Depending on the reaction conditions, solid and porous structures consisting of gold nanoparticles of differing size and shape were accessible. The presented strategy offers adapted gold thin films, nanotubes, and nanowires for applications in catalysis or sensing

Domain morphology of newly designed lead‐free antiferroelectric NaNbO₃‐SrSnO₃ ceramics

Reversible antiferroelectric‐ferroelectric phase transitions were recently observed in a series of SrSnO₃‐modified NaNbO₃ lead‐free antiferroelectric materials, exhibiting well‐defined double polarization hysteresis loops at ambient conditions. Here, transmission electron microscopy was employed to investigate the crystallography and domain configuration of this newly designed system via electron diffraction and centered dark‐field imaging. It was confirmed that antiferroelectricity is maintained in all compositions, manifested by the characteristic ¼ superlattice reflections in the electron‐diffraction patterns. By investigating the antiferroelectric domains and domain boundaries in NaNbO₃, we demonstrate that antiphase boundaries are present and their irregular periodicity is responsible for the streaking features along the ¼ superlattice reflections in the electron‐diffraction patterns. The signature domain blocks observed in pure NaNbO₃ are maintained in the SrSnO₃‐modified ceramics, but disappear when the amount of SrSnO₃ reaches 7 mol.%. In particular, a well‐defined and distinct domain configuration is observed in the NaNbO₃ sample modified with 5 mol.% SrSnO₃, which presents a parallelogram domain morphology

Coherent Precipitates with Strong Domain Wall Pinning in Alkaline Niobate Ferroelectrics

High‐power piezoelectric applications are predicted to share approximately one‐third of the lead‐free piezoelectric ceramic market in 2024 with alkaline niobates as the primary competitor. To suppress self‐heating in high‐power devices due to mechanical loss when driven by large electric fields, piezoelectric hardening to restrict domain wall motion is required. In the present work, highly effective piezoelectric hardening via coherent plate‐like precipitates in a model system of the (Li,Na)NbO₃ (LNN) solid solution delivers a reduction in losses, quantified as an electromechanical quality factor, by a factor of ten. Various thermal aging schemes are demonstrated to control the average size, number density, and location of the precipitates. The established properties are correlated with a detailed determination of short‐ and long‐range atomic structure by X‐ray diffraction and pair distribution function analysis, respectively, as well as microstructure determined by transmission electron microscopy. The impact of microstructure with precipitates on both small‐ and large‐field properties is also established. These results pave the way to implement precipitate hardening in piezoelectric materials, analogous to precipitate hardening in metals, broadening their use cases in applications