Transparent polycrystalline Gd₂Hf₂O₇ ceramics

Abstract

We have successfully developed transparent polycrystalline Gd2Hf2O7 ceramics with high in‐line transparency. A sol–gel process was used to synthesize the Gd2Hf2O7 powder. Simultaneous thermal gravimetric analysis and differential thermal analysis (TGA/DTA) was used to identify the decomposition sequence as a function of temperature for the as‐synthesized sol–gel powders. The calcined powder is single phase and was formed with an estimated average particle size of 120 nm. Crystallization was confirmed by x‐ray diffraction (XRD) and a single phase was achieved by calcining at 1000°C. The calcined powders were hot‐pressed at 1500°C to achieve >95% theoretical density with closed pore structure followed by a hot isostatic pressing at 1500°C at 207 MPa to achieve a fully dense structure. Microstructural characterization shows a uniform grain size distribution with an average grain size of about 11 μm. In‐line transmission measurements revealed high transparency in the red and infrared. Dielectric properties remain stable with relative permittivity values around 180 and loss tangents less than 0.005 up to 350°C. Thermal conductivity was measured to be ~1.8 W/m°K at room temperature, decreasing to ~1.5 W/m°K by 500°C.

Thermally Induced Disorder–Order Phase Transition of Gd2Hf2O7:Eu3+ Nanoparticles and Its Implication on Photo- and Radioluminescence

“…ref49,ref50 In addition, we have found that electronic transition involving ionized oxygen vacancies are responsible for such luminescence properties from our earlier work on Nd 2 Zr 2 O 7 and Gd 2 Zr 2 O 7 pyrochlore. 5,27 The used excitation energy (∼3.37 eV, 332 nm) is less than the band gap (3.42 eV) of GHO, 19,44 so direct transition from the valence band (VB) to the conduction band (CB) does not happen, and there exist certain localized defect states within the band gap of the GHO NPs. Such defects could arise during thermal treatment of the GHO NPs or may be present intrinsically in them.…”

“…18 GHO has a cubic structure as well as high Z eff , making it a potential host lattice for scintillators. 19,20 Moreover, given its high melting point and high structural and thermal stabilities, it can be a potential host for lanthanide ion-doped phosphor. The IRR of GHO is approximately around 1.48 (close to 1.46), 21 so it is expected that the DF and OP phases can coexist in GHO depending on the synthesis conditions.…”

A Review of Grain Boundary and Heterointerface Characterization in Polycrystalline Oxides by (Scanning) Transmission Electron Microscopy

“…Transparent polycrystalline ceramics have become of interest since Neodymium-doped yttrium aluminum garnet (Nd:YAG) was introduced as a lasing medium for solid state lasers in 1995 [201] and have applications in armors, opto-electrical devices such as solar cells, displays, and circuitries. Compared to single crystals, polycrystalline ceramics are costeffective, easier to produce, exhibit good mechanical, thermal and chemical stability, and shape-control availability [292,293]. However, certain defects such as impurities and porosity inside their microstructure scatter and refract light leading to optical losses, thus limiting wide employment of transparent polycrystalline ceramics [203,294].…”

An overview on transparent ceramics with pyrochlore and fluorite structures

“…Gd 2 Hf 2 O 7 transparent ceramics were prepared from powders synthesized by using a sol-gel process. 74 The powder calcined at 1000 ○ C had an average particle size of 120 nm. The calcined powders were hot-pressed at 1500 ○ C to relative densities of > 95%, whereas fully dense ceramics were obtained after HIP treatment at 1500 ○ C at a pressure of 207 MPa.…”