Suppressed grain growth and enhanced irradiation resistance of nano-grained waste form under electronic energy loss

The effects of both nuclear energy loss and electronic energy loss need to be taken into consideration in the ceramic-based waste forms under repository environment. However, the irradiation responses of ceramic-based waste forms to each type of energy loss are somewhat different. In this study, the microstructure evolutions of ultrafine nano and micro Gd₂Zr₂O₇-based waste forms were systematically studied under predominant electronic energy loss simulated by multi-energy He⁺ irradiation, and compared to those under predominant nuclear energy loss. The results reveal that the fewer He bubble chains, ribbon-like He bubbles and smaller microcracks were observed in the irradiated nano-grained sample. Additionally, nano-grained sample displayed a lower degree of amorphization and higher atomic order compared to micro-grained samples when subjected to predominant electronic energy loss. Moreover, the irradiation dominated by nuclear energy loss can easily induce the grain growth of nano-grained Gd₂Zr₂O₇-based waste form, but in the present study this phenomenon was not observed under multi-energy He⁺ irradiation. Consequently, under predominant electronic energy loss, the thermodynamic instability and driving force for grain growth due to excess surface energy in the ultrafine nano sample can be suppressed. As a result, the sample demonstrated enhanced irradiation resistance due to the more efficient absorption and elimination of defects at grain boundaries induced by electronic excitation. We elucidated that enhanced irradiation resistance of the waste forms by tailoring the grain size requires the consideration of the effects of electronic energy loss and nuclear energy loss, which can provide guidance for the design and optimization of highly irradiation-resistant nuclear waste forms.     

Irradiation response of Al2O3-ZrO2 ceramic composite under He ion irradiation

The dense Al₂O₃-ZrO₂ ceramic composite prepared by spark plasma sintering was irradiated by 500 keV He ions with different fluences and temperatures. The microstructural evolution and mechanical properties were investigated. The results showed that peak broadening and shifts at RT revealed by GIXRD and Raman are associated with damage induced microstrain and formation of point defects. The recovery at 500 °C suggested the reduction of irradiation induced damage. Compared with α-Al₂O₃, t-ZrO₂ exhibited a reverse trend in lattice parameters change and lattice expansion. Many helium bubbles with oblate and ribbon-like shape were mainly formed in α-Al₂O₃ grains at He concentration peak at 1.0 × 10¹⁷ ions/cm². With increasing of fluence at RT, ribbon-like helium bubbles developed into microcracks at 4.0 × 10¹⁷ ions/cm². Though evident structural changes, no full amorphization was observed at 4.0 × 10¹⁷ ions/cm². Formation of ribbon-like bubbles and microcracks is the main mechanism for degradation of mechanical properties of irradiated Al₂O₃-ZrO₂ composite.     

Defect-fluorite Gd2Zr2O7 ceramics under helium irradiation: Amorphization,cell volume expansion,and multi-stage bubble formation

Here, we report a study on the radiation resistance enhancement of Gd₂Zr₂O₇ nanograin ceramics, in which amorphization, cell volume expansion, and multi-stage helium (He) bubble formation are investigated and discussed. Gd₂Zr₂O₇ ceramics with a series of grain sizes (55-221 nm) were synthesized and irradiated by 190 keV He ion beam up to a fluence of 5 × 10¹⁷ ions/cm². Both the degree of post irradiation cell volume expansion and the amorphization fraction appear to be size-dependent. As the average grain size evolves from 55 to 221 nm, the degree of post irradiation cell volume expansion increases from 0.56% to 1.02%, and the amorphization fraction increases from 6.8% to 11.1%. Additionally, the threshold He concentrations (at.%) of bubbles at different formation stages and locations, including (a) bubbles at grain boundary, (b) bubble-chains, and (c) ribbon-like bubbles within the grain, are all found to be much higher in the nanograin ceramic (55 nm) compared with that of the submicron sample (221 nm). We conclude that grain boundary plays a critical role in minimizing the structural defects, and inhibiting the multi-stage He bubble formation process.     

High density nano-grained Gd2Zr2O7 ceramic prepared by combined cold and microwave sintering

In this study, nano-grained Gd₂Zr₂O₇ (NGZO) ceramic with a high relative density was prepared by a novel cold sintering process (CSP) assisted by microwave sintering (CSP-MS). The CSP with water as the liquid phase at 280 °C yielded nano-grained ceramic with a relative density of 71.5%. NGZO with a high relative density of 97.1% and average grain size of 73 nm was obtained by subsequent microwave sintering of the cold-sintered sample at 1300 °C. Therefore, CSP-MS is feasible in preparing dense NGZO ceramics with small grain sizes at relatively low sintering temperatures.     

Helium irradiation induced microstructural damages and mechanical response of Al2O3-ZrO2-SiC composites

Ceramic composites are promising candidates as structural materials for future fission and fusion reactors. In present work, Al₂O₃-ZrO₂-SiC ternary ceramic composites were irradiated with 2.0 MeV He-ions at 300 and 800 ℃. Grazing incidence X-ray diffraction results confirmed that there was irradiation induced shift and broadening of diffraction peaks, but no amorphization of Al₂O₃-ZrO₂-SiC composite were observed up to fluence of 1.72 × 10¹⁸ ions/cm². Transmission electron microscopy observations showed that throughout the entire irradiation region, nano-sized helium bubbles are mostly distributed in Al₂O₃ grains and partly in ZrO₂ grains, while no detectable bubbles are observed in SiC grains. No obvious agglomeration of bubbles was found at grain/phase boundaries. By using nanoindentation technique, slight hardening or softening was confirmed for the samples irradiated at 300 and 800 ℃ respectively. The absence of amorphization and surface exfoliation indicating the Al₂O₃-ZrO₂-SiC composite exhibits remarkable resistance to He-ions irradiation.     

Phase transitions and He bubble evolution under sequential He ions implantation and Fe ions irradiation: New insight into the irradiation resistant of Ti3AlC2

The phase transitions and He bubble evolution in Ti₃AlC₂ with the sequential He ions implantation and Fe ions irradiation at room temperature were investigated with grazing incidence X-ray diffraction and transmission electron microscopy. The pre-implanted He makes an obviously effect of suppressing the phase transitions caused by the following Fe ions irradiation. The following Fe ions irradiation was also found to create a competitive effect on the evolution of the pre-formed He bubbles. It can cause not only the growth of He bubbles, but the shrinkage (or re-solution of He bubbles). This will improve the resistance to He bubbles induced damage for Ti₃AlC₂. Both He ions implantation and Fe ions irradiation actually inhibit each other's irradiation damage to the material. This may play a positive role in reducing irradiation damage to the Ti₃AlC₂ material, and also provides new insight into irradiation resistance of this material.     

He irradiation-induced lattice distortion and surface blistering of Gd2Zr2O7 defect-fluorite ceramics

Gd₂Zr₂O₇ ceramics with different grain sizes ranging from nanoscale to submicron scale (91, 204, and 634 nm) were irradiated at room temperature using 190 keV He ions with doses ranging from 5 × 10¹⁶ to 5 × 10¹⁷ ions/cm². We fully characterized the pre- and post-irradiation samples using grazing-incidence X-ray diffraction (GIXRD), scanning electron microscope (SEM), and atomic force microscope (AFM) as the grain size and degree of irradiation vary. The results suggested that all three Gd₂Zr₂O₇ samples demonstrate outstanding radiation tolerance to displacement damage by retaining their crystallinity after irradiation at 5 × 10¹⁷ ions/cm². which is equal to 16 displacement per atom (dpa) at peak positions. Although lattice expansion was observed at a He irradiation at 5 × 10¹⁶ ions/cm² and beyond, the lattice remained stable for the nanograin ceramic, while the degree of distortion for the sample with the largest grain size (634 nm) continuously increased. Moreover, a delayed He bubble evolution process was seen for the nanograin ceramic, which did not appear for the submicron-grained sample. Interestingly, the grain size-dependent surface blistering was also found to be a function of ion fluence. After He irradiation at 5 × 10¹⁷ ions/cm² the AFM root-mean-square(RMS) roughness variation for Gd₂Zr₂O₇ ceramics of 91, 204, and 634 nm were 4.8, 7.0, and 11.1 nm, respectively.     

Formation of nanostructures in Ti2AlC induced by high-temperature helium irradiation

The effects of helium (He) irradiation on Ti₂AlC at different temperatures were studied in this work. He irradiation at room temperature (RT) induced severe lattice distortion and caused serious cracks in the samples. During He irradiation, Ti-Al bonds were easily broken and He atoms tended to accumulate at basal planes forming microcracks after irradiations at RT and 430 °C. Stacking faults and nanotwins were formed after He irradiation at all temperatures. Three kinds of nanostructures were formed after He irradiation: 1) face-centered cubic (fcc) nanotwins generated by outward diffusion of Al, accumulation of C interstitials and detwinning of Ti₂C slabs; 2) fcc nanograins induced by the formation of fcc structure and its subsequent amorphization by irradiation; 3) Ti₂AlC nanotwins induced by recrystallization from fcc nanotwins at 750 °C. All the results indicate that Ti₂AlC is highly resistant to He radiation and has excellent damage recovery at high temperatures.     

Structural integrity and damage of glass-ceramics after He ion irradiation: Insights from ZrO2-SiO2 nanocrystalline glass-ceramics

Developing new radiation-resistant materials and understanding the structural damages caused by radiation are persistent goals of material scientists. Here, we report on the structural integrity and damage to ZrO₂-SiO₂ nanocrystalline glass-ceramics after radiation with 1.4 MeV He ions at three different fluences: 1.0 × 10¹⁶ ions/cm² (low), 5.0 × 10¹⁶ ions/cm² (moderate), and 1.0 × 10¹⁷ ions/cm² (high) at 500 °C. Grazing incident X-ray diffraction shows the tetragonal-ZrO₂ to monoclinic-ZrO₂ phase transformation induced by microstrain from the irradiation. The addition of yttrium indicated tetragonal-ZrO₂ stabilization effect during irradiation. The irradiated glass-ceramics show a Raman signal-enhancement effect probably related to the electronic structure changes of the amorphous SiO₂ component in the glass-ceramics. The formation of microcracks and lattice defects within ZrO₂ nanocrystallites is the main structural damage caused by irradiation. There was no observable amorphization of ZrO₂ nanocrystallites due to irradiation. No obvious He bubbles were detected, either. The formation of microcracks results in a decrease of in the nanohardness of the glass-ceramics. The results provide fundamental experimental data to understand the structural integrity and damage caused by radiation, which could be useful to design radiation‐resistant nanocrystalline glass-ceramics for extremely radioactive environments.     

Fabrication and optical characterizations of hot-pressed transparent SrF2/Nd:SrF2/SrF2 composite ceramic

A novel transparent SrF₂/Nd:SrF₂/SrF₂ composite ceramic with sandwich-like laminar configuration was designed and successfully fabricated by the hot-pressed sintering method. The composite ceramic possesses an apparent transition interfacial (about 200 μm in thickness) between SrF₂ and Nd:SrF₂ layers, formed by non-uniform distribution of raw powders and the diffusion of Nd ions during the high temperature sintering. The average grain sizes of SrF₂ and Nd:SrF₂ layers are about 262.1 μm and 28.6 μm, respectively. For a 2-mm thick transparent SrF₂/Nd:SrF₂/SrF₂ composite ceramic hot-pressed at 900 °C for 2 h, the transmittance at 500 nm and 1200 nm are about 49.6 % and 62.3 %, respectively. The microstructure, emission spectra and thermal conductivities of ceramics are also detected and studied.     

Low-temperature sintering of Al2O3 ceramics doped with 4CuO-TiO2-2Nb2O5 composite oxide sintering aid

Dense alumina ceramics doped with 5 wt% 4CuO-TiO₂-2Nb₂O₅ composite sintering aids were obtained at low sintering temperatures of 950∼975 °C. The ceramic sintered at optimal condition shows good microwave dielectric properties (ε_r = 12.7, Q × f = 7400 GHz), high thermal conductivity (18.4 W/m K) and high bending strength (320 MPa). TEM and EDS analysis revealed that amorphous Cu-Ti-Nb-O interfacial films with nanometer thickness formed at the grain boundaries, which could provide paths of mass transportation for densification. Al³⁺ ions may be involved in mass transportation through substitution by Ti³⁺ and Ti⁴⁺ ions near the grain boundary during the sintering process. The accumulation of copper ions at the trigeminal grain boundary was observed. The migration and reaction of copper ions in grain boundaries may also play an important role in promoting mass transportation and low-temperature densification of alumina ceramics.     

Effects of Nd Concentration on Microstructure and Optical Properties of Nd: CaF2 Transparent Ceramics

Highly transparent Nd‐doped calcium fluoride (Nd: CaF₂) ceramics with different Nd‐doped concentrations were fabricated by hot‐pressed method using Nd: CaF₂ nanopowders synthesized by coprecipitation method. SEM observations indicated that the average grain size of nanopowders was about 16–30 nm and the average grain size of the ceramics was between 200 nm and 1 μm. The grain boundaries of the ceramics were clean and no pores or impurities were detected. For 2‐mm‐thickness sample, the transmittance of the as‐fabricated 5 at.% Nd: CaF₂ ceramic at 1200 nm was about 85%. The absorption coefficient and emission intensity of the Nd: CaF₂ ceramics were measured and discussed. From the Nd: CaF₂ ceramics fluorescent spectra and the decay curves, it was found that the fluorescent quenching effect became more evident with the increase in the Nd³⁺ ions‐doped concentration.     

Influence of grain size on the corrosion behavior of a Ni-based superalloy nanocrystalline coating in NaCl acidic solution

The effects of grain size on the electrochemical corrosion behavior of a Ni-based superalloy nanocrystalline (NC) coating fabricated by a magnetron sputtering technique, has been investigated in 0.5 M NaCl + 0.05 M H₂SO₄ solution. Coatings with grain sizes 10 nm, 50 nm and 100 nm were fabricated on glass and the superalloy substrates. The results indicate that a passive film with porous property, n-type semiconductive property and incorporation of chloride ions formed on the NC coating with 100 nm grain size, which increased the susceptibility to pitting corrosion. The NC coatings with 10 nm and 50 nm grain size formed compact, non-porous and p-type passive films without chloride ions, which improved resistance to pitting corrosion. The smaller grain size of the material decrease the amount of chloride ions adsorbed on the surface and promoted the formation of compact passive film, which significantly increased the material's resistance to pitting corrosion in acidic solution.     

Continuous aluminum oxide-mullite-hafnium oxide composite ceramic fibers with high strength and thermal stability by melt-spinning from polymer precursor

Continuous aluminum oxide-mullite-hafnium oxide (AMH) composite ceramic fibers were obtained by melt-spinning and calcination from polymer precursor that synthesized by hydrolysis of the aluminum isopropoxide, dimethoxydimethylsilane and hafnium alkoxide. Due to the fine diameter of 8–9 µm, small grain size of less than 50 nm and the composite crystal texture, the highest tensile strength of AMH ceramic fibers was 2.01 GPa. And the AMH ceramic fibers presented good thermal stability. The tensile strength retention was 75.48% and 71.49% after heat treatment at 1100 °C and 1200 °C for 0.5 h respectively, and was 61.57% after heat treatment at 1100 °C for 5 h. And the grain size of AMH ceramic fibers after heat treatment was much smaller than that of commercial alumina fibers even when the heat treatment temperature was elevated to 1500 °C, benefited by the grain size inhibition of monoclinic-HfO₂ (m-HfO₂) grains distributed on the boundary of alumina and mullite grains.     

Grain size engineered high-performance nanograined BaTiO3-based ceramics: Experimental and numerical prediction

The ultra-thin multilayer ceramic capacitors (MLCCs) with layer thickness less than 1 μm or even 0.5 μm are in urgent demand due to the rapid development of modern electronic industries. Notably, the dielectric and ferroelectric properties of nanograined BaTiO₃-based ceramics, which are widely used as dielectric materials in MLCCs, are highly related to grain size. In this work, nanograined BaTiO₃-based ceramics with various grain sizes (50-100 nm) were prepared via the chemical coating method. The grain size effect on the dielectric and energy storage properties were systematically investigated. TEM and EDS images demonstrate that the typical core-shell structure is obtained inside ceramic grains even if the grain size is reduced to 50 nm. The fine-grain ceramic displays a lower maximal polarization but a higher breakdown strength, which ascribes to its weaker ferroelectric contribution and higher grain boundary ratio, respectively. As a result, it is confirmed that there exists an optimal grain size around 70 nm where maximum discharge energy density is achieved under the synergy effect of breakdown strength and polarization, which is also verified by a finite element analysis based on a modified hyperbolic tangent model. All these features provide important guidance towards the design of ultra-thin layer MLCCs by optimizing the dielectric properties and energy storage performance while pursuing miniaturization.     

High-entropy transparent fluoride laser ceramics

For the first time, a transparent high-entropy fluoride laser ceramic has been prepared and characterized. X-ray diffraction (XRD) analysis of a CeNdCaSrBaF₁₂ (CNCSBF) transparent ceramic consolidated by vacuum hot pressing (VHP) reveals that Ce³⁺, Nd³⁺, Ca²⁺, Sr²⁺, and Ba²⁺ have formed a single-phased fluorite solid solution, with a lattice constant of 5.826 Å. Bulk density measurements produced a value of 6.15 g/cm³. Scanning electron microscopy (SEM) analysis of the ceramic revealed a uniform distribution of grain sizes in the material, with the average grain size being approximately 20 μm. The material exhibits a maximum in-line transmittance of approximately 60% at 1000 nm. A near-infrared range photoluminescence (PL) emission band was observed at 1057 nm, with a visible-range PL emission band being located at 440 nm.     

Grain boundary sliding mechanism in plastic deformation of nano-grained YAG transparent ceramics: Generalized self-consistent model and nanoindentation experimental validation

The elastic-plastic deformation behaviors of nano-grained and coarse-grained yttrium aluminum garnet (YAG) transparent ceramics are investigated using nanoindentation. An inverse Hall-Petch relation is observed for the nano-grained YAG ceramic and a forward Hall-Petch relation is observed for the coarse-grained YAG ceramic. In addition, the plastic work ratio as a function of applied load for the nano-grained YAG ceramic shows a different trend than that for the coarse-grained YAG ceramic. These observations suggest that the plastic deformation of the nano-grained YAG ceramic cannot be attributed to the normal dislocation mechanism and is controlled by grain boundary sliding. A generalized self-consistent model for studying the mechanical behavior of the nano-grained YAG ceramic is developed and validated by experimental results. The stress-strain relationship predicted by this model is embedded in finite element simulations which confirmed that the plastic deformation of the nano-grained YAG ceramic indeed can be attributed to grain boundary sliding.     

Microstructure,dielectric, piezoelectric,and ferroelectric properties of fine-grained 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics

For preparing fine-grained 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ lead-free ferroelectric ceramics, the precursor powders were synthesized via sol-gel method and calcined at various temperatures. The precursor powders calcined at 520 °C, 550 °C, and 600 °C exhibit mean grain sizes of 30 ± 4 nm, 54 ± 3 nm, and 78 ± 5 nm, respectively. By optimizing the synthesis parameters, the fine-grained ceramics with high relative densities (>97%) and mean grain size around 100 nm were prepared. The ferroelectric, dielectric, and piezoelectric behavior were investigated. The ceramics prepared using the different precursor powders show different piezoelectric, ferroelectric, and dielectric behavior. The ceramic calcined at 550 °C and sintered at 900 °C exhibits the breakdown strength higher than 85 kV/cm, which exhibits the maximum polarization of 38.4 ± 0.3 μC/cm², remanent polarization of 20.6 ± 0.4 μC/cm².     

Fabrication of [1 0 0]-oriented bismuth sodium titanate ceramics with small grain size and high density for piezoelectric materials

A layered titanate H_1.07Ti_1.73O₄·nH₂O (HTO) with a plate-like particle morphology was used as a template for the fabrication of [1 0 0]-oriented bismuth sodium titanate (Na_0.5Bi_0.5TiO₃, or BNT) ceramics by a reactive-templated grain growth (RTGG) method. The oriented BNT ceramic with a high degree of orientation (95%), high density (98%), and small grain size (2 μm) was fabricated for the first time by the RTGG method using a HTO-TiO₂-Bi₂O₃-Na₂CO₃ reaction system. The oriented BNT ceramic is formed by a topotactic transformation reaction of plate-like HTO template particles to plate-like BNT mesocrystal particles, and then epitaxial crystal growth of BNT on the BNT mesocrystal particles. The epitaxial crystal growth reaction is affected by TiO₂/HTO mole ratio, chemical component of the starting material, and calcination temperature program. The fabricated oriented BNT ceramic shows a higher d₃₃* value than the non-oriented BNT ceramic, suggesting the promising application to high performance Pb-free piezoelectric materials.     

Effects of He Irradiation on Yttria‐Stabilized Zirconia Ceramics

Effects of He irradiation on polycrystalline yttria‐stabilized zirconia (YSZ) are studied with the focus on irradiation‐induced damage buildup, He behavior, and volume swelling. The evolution of irradiation‐induced structural damage in polycrystalline YSZ, which is independent of grain orientation, is described by a multistep damage accumulation model. A three‐step damage evolution process was found, and different types of defects were observed in the different damage steps. Compared with single‐crystal YSZ, the second damage step occurs at a lower dose in polycrystalline YSZ due to the initial defects and strain. The implanted He ions are readily trapped along the grain boundaries and the mobility of He ions is greatly increased. The enhanced He mobility along the grain boundary leads to a lower threshold irradiation dose and a larger penetration depth for bubble formation. Similar morphologies are observed for the He bubbles in the polycrystalline YSZ and in single‐crystal YSZ, and the formation of He bubbles in polycrystalline YSZ is not influenced by grain orientation. As both the extended defects and He bubbles can induce volume swelling, the variation in volume swelling as a function of dose can be divided into a two stage process.