ZrO2 HfO2 Research Articles

Physicochemical Properties of Sm2O3–ZrO2–HfO2 Ceramics for the Development of Promising Thermal Barrier Coatings

Physicochemical Properties of Sm2O3–ZrO2–HfO2 Ceramics for the Development of Promising Thermal Barrier Coatings

Improvement of Ferroelectricity and Fatigue Property of Thicker HfxZr1−XO2/ZrO2 Bi-layer

HfxZr1−xO2 (HZO) has been an attractive material for future ferroelectric memory devices because of their high scalability ~10 nm, stable ferroelectricity over a wide Hf:Zr composition range, and compatibility with CMOS manufacturing process. [1] Considering the ferroelectric device manufacturing, the stable ferroelectricity in the thicker film region is required. However, it has been reported that the ferroelectricity of HZO films is generally degraded in the thick ferroelectric film region > 10 nm. [2] To obtain the superior ferroelectricity in the thicker film region, we pay attention to the ferroelectricity of the HZO based thick ferroelectric structures (> 20 nm) with paraelectric HfO2 and anti-ferroelectric ZrO2 layers. In this work, we studied the ferroelectricity and fatigue properties of the thick HZO/ZrO2 and HZO/HfO2 stack structures fabricated using atomic layer deposition (ALD).The TiN/HZO/ZrO2/TiN capacitors were fabricated as follows: An HZO layer was deposited on TiN bottom electrode by ALD at 300 °C using (Hf/Zr)[N(C2H5)CH3]4 (Hf:Zr = 1:1) cocktail precursor and H2O gas. The thickness of the HZO layer were varied from 0 to 10 nm. Next, a 10-nm-thick ZrO2 layer was deposited at 300 °C using (C5H5)Zr[N(CH3)2]3 and H2O gas. After fabrication of HZO/ZrO2 bi-layer, post deposition annealing (PDA) was performed at 600 °C for 1 min in a N2 atmosphere. Finally, TiN top electrode was fabricated by DC sputtering. The capacitors with HZO single layer and HZO/HfO2 bi-layer were also prepared as references.Figure 1 shows the remanent polarization (2P r) of the HZO single layer, HZO/HfO2, and HZO/ZrO2 bi-layers from polarization-electric field (P-E) hysteresis curves as a function of the HZO thickness while the HfO2 and ZrO2 thicknesses were kept at 10 nm. The maximum 2P r (12 µC/cm2) of the HZO single layer was obtained when the thickness of the HZO layer was 10 nm. However, the 2P r of the HZO single layer gradually decreased with the HZO thickness due to the formation of the stable monoclinic phase which shows paraelectricity. [2] The ferroelectric properties of ZrO2 and HfO2 single layers were hardly observed because both layers exhibited anti-ferroelectricity and paraelectricity, respectively. The HZO/HfO2 bi-layer maintained extremely small 2P r regardless of the HZO thickness. On the other hand, the switching properties of HZO/ZrO2 bi-layer were divided into three regions such as anti-ferroelectricity in less than 12.5 nm, mixing phases of anti-ferroelectricity and ferroelectricity in 15 nm, and ferroelectricity in over 17.5 nm. The maximum 2P r of 11 µC/cm2 could be obtained in the 20-nm-thick of HZO/ZrO2 bi-layer, and was approximately 2 times larger than that (6 µC/cm2) of the 20-nm-thick HZO single layer. This superior ferroelectricity of the HZO/ZrO2 bi-layer may be due to the improvement of quality of the HZO layer and the transformation from anti-ferroelectric to ferroelectric phase in the ZrO2 layer. [3, 4]Figure 2 shows endurance properties of HZO (10 nm) single layer, HZO/ZrO2 (20 nm), and HZO/HfO2 (20 nm) bi-layers using positive-up negative-down (PUND) method. For the HZO single layer and the HZO/ZrO2 bi-layer cases, the wake-up effect was observed until 104 cycles, while the HZO/HfO2 bi-layer had small 2P r in the entire switching cycle region. The large difference of 2P r properties between the HZO layer and the HZO/ZrO2 bi-layer was observed in over 104 cycles. Noted that the HZO/ZrO2 bi-layer was larger 2P r than that in pristine condition after 106 cycles, while the 2P r of the HZO single layer was dropped by around 30%. Therefore, it is thought that ZrO2 layer play an important role in preventing the HZO/ZrO2 bi-layer from the degradation of ferroelectricity.In conclusion, the thick HZO/ZrO2 bi-layer exhibited superior ferroelectricity and endurance properties compared to the HZO/HfO2 bi-layer and thin HZO single layer. Based on these results, the HZO/ZrO2 bi-layer using the combination of HZO and ZrO2 layers is one of the promising structures for future ferroelectric devices application.This study was partially supported by JSPS KAKENHI (JP18J22998 and K4H002). The authors thank all staff members of the Nanofabrication Group of NIMS for their support in fabricating the MFM capacitors.[1] J. Müller et al., Nano Lett. 12, 4318 (2012).[2] M. H. Park et al., Appl. Phys. Lett. 102, 242905 (2013).[3] T-.Y. Hsu et al., Smart Mater. Struct. 28, 084005 (2019).[4] S. Shibayama et al., Jpn. J. Appl. Phys. 59, SMMA04 (2020). Figure 1

Compositional dependence of crystallization temperatures and phase evolution in hafnia-zirconia (HfxZr1−x)O2 thin films

Polymorphic (HfxZr1−x)O2 (HZO) thin films exhibit ferroelectric, dielectric, and antiferroelectric properties across a wide compositional range due to the existence of orthorhombic, monoclinic, and tetragonal phases. To better understand the phase stability across the HfO2–ZrO2 compositional range, we investigate the structural evolution of HZO thin films in situ via high-temperature x-ray diffraction (HTXRD) for five different compositions [ZrO2, (Hf0.23Zr0.77)O2, (Hf0.43Zr0.57)O2, (Hf0.67Zr0.33)O2, and HfO2]. The real-time monitoring of HZO crystallization reveals a competing driving force between the tetragonal and monoclinic phase stabilities for HfO2-rich vs ZrO2-rich compositions. Additionally, we confirm an XRD peak shift toward lower 2θ with increasing temperature in ZrO2, (Hf0.23Zr0.77)O2, and (Hf0.43Zr0.57)O2 films, which we ascribe to the appearance of a metastable orthorhombic phase during heating. A monotonic trend for the onset crystallization temperature is reported for five compositions of HZO and reveals an increase in onset crystallization temperature for HfO2-rich compositions. Relative intensity fraction calculations suggest a higher fraction of monoclinic phase with increasing annealing temperature for (Hf0.67Zr0.33)O2. This study of phase stability and onset crystallization temperatures offers insight for managing the thermal budget for HZO thin films, especially for temperature-constrained processing.

Whether Ge-Rich ZrO2 and Ge-Rich HfO2 Materials Have Similar Reaction on Annealing Treatment?

Usually, HfO2 and ZrO2 are considered as twin-oxides that have to demonstrate similar reaction on their doping with group IV elements as well as stable tetragonal “high-k” phase. However, in most cases the films produced with different approaches were compared. In this work, both Ge doped HfO2 and Ge-doped ZrO2 films were prepared using the same equipment that allows the same Ge content in both materials to be achieved via monitoring of deposition conditions. Thus, the effect of Ge content and thermal treatment on optical and structural properties of these films was studied to obtain direct experimental confirmation whether the formation of corresponding solid solutions and their phase separation stimulated by thermal treatment is similar for both oxides.The films were grown on 6-inch Si wafers by confocal magnetron sputtering using the deposition unit with installed Ge, HfO2 and ZrO2 targets simultaneously. After the deposition, the wafers with the films were cut on smaller samples and rapid thermal annealing in nitrogen atmosphere was performed at 300-900ºC during 30 s. The samples were investigated by means of spectroscopic ellipsometry, Raman scattering, FTIR and TEM methods.As-deposited pure ZrO2 and Ge-ZrO2 films as well as those annealed at TA≤600°C were found to be amorphous and homogeneous. Annealing at higher TA stimulates a phase separation and a formation of Ge nanoclusters (Ge-ncs). They crystallize at TC=640-700°C showing the tendency to the TC decrease with Ge content rise. An appearance of tetragonal ZrO2 phase in Ge-rich films was observed at TC=680-700°C. Such behavior of ZrO2 host shows the possibility to form Ge-ncs in amorphous matrix at TA=640-680°C that offers success application of such films.Contrary to pure ZrO2 films, as-deposited pure HfO2 films were found to be crystalline. Their doping with [Ge]≥5at% reduces the degree of film crystallinity towards amorphous structure that was found to be stable for TA£600°C. At higher TA the formation of Ge-ncs and tetragonal HfO2 phase was detected. However, this latter was found to be formed at TC=600-670 °C, while the Ge-ncs crystallize at TC=700-800°C. Note that the formation of larger Ge-ncs in Ge-ZrO2 films than that in Ge-HfO2 ones at the same annealing conditions was observed.Obtained results support the theoretical consideration about stability of tetragonal “high-k” phases in annealed films. However, they show also the difference in the reaction of Ge-HfO2 and Ge-ZrO2 materials on thermal treatment followed by phase separation. Its mechanism will be discussed in details.

Whether Ge-Rich ZrO2 and Ge-Rich HfO2 Materials Have Similar Reaction on Annealing Treatment?

The properties of Ge-HfO2 and Ge-ZrO2 films were studied versus Ge content and annealing temperature by means of Raman scattering, FTIR and X-ray diffraction methods. Ge-HfO2 films with [Ge]<5 at.% were polycrystalline, whereas higher Ge content stabilized amorphous structure that was stable for T<600°C. The T increase caused the formation of Ge clusters and tetragonal HfO2 phase. This latter appeared at T=600-670°C, while the Ge clusters crystallized at T=700-800°C. Unlike to Ge-rich HfO2 films, as-deposited Ge-ZrO2 films and those annealed at T≤600°C were amorphous. In these films, formation of Ge clusters and their crystallization occurred at T=640-700°C depending on Ge content, whereas ZrO2 phase crystalized at T=680-700°C that allowed to form Ge nanocrystals in amorphous matrix at T=640-680°C. For T>800°C, the films showed appearance of monoclinic HfO2 or ZrO2 phases that was explained by the Ge content decrease via volatile GeO formation. Obtained results supported theoretical prediction of the stability of tetragonal phases in these oxides via Ge doping. However, our results showed also some difference in the phase separation process of Ge-rich HfO2 and Ge-rich ZrO2 upon thermal treatment.

Crystal Phase Distribution and Ferroelectricity in Ultrathin HfO2–ZrO2 Bilayers

(Hf,Zr)O2 ultrathin films are used as ferroelectric layers in emerging digital logic and nonvolatile memory devices. The ferroelectric properties of (Hf,Zr)O2 can be improved by interface engineering, such as the formation of nanolaminates with distinct HfO2 and ZrO2 layers. Herein, the ferroelectric performance of HfO2–ZrO2 ultrathin bilayer devices is shown to depend on the stacking order of HfO2 and ZrO2, which affects the quantity of the noncentrosymmetric orthorhombic Pca21 crystal phase. By combining X‐ray diffraction with a novel extended X‐ray absorption fine structure (EXAFS) analysis procedure, the orthorhombic, tetragonal, and monoclinic phase fractions are quantified for bilayers composed of 3 nm HfO2 and 3 nm ZrO2. A significantly larger orthorhombic ZrO2 phase fraction is found when ZrO2 has an unconstrained surface during annealing, whereas the presence of a ZrO2 interface with the substrate results in a substantial tetragonal ZrO2 phase fraction and a 2.4× smaller remanent polarization. HfO2 is found to be less susceptible than ZrO2 to crystal phase templating. The methods presented herein enable mechanistic studies of ferroelectric wake‐up, fatigue, and processing effects in (Hf,Zr)O2 films, accelerating the development of electronic devices that rely on ferroelectric oxides.

Production of Ceramics Based on the Y2O3–ZrO2–HfO2 System for Casting Molds

The potential of the Y2O3–ZrO2–HfO2 system for synthesizing the casting mold ceramic of the composition 15 mol % Y2O3 : 60 mol % ZrO2 : 25 mol % HfO2 was investigated. The effect of technological parameters on the morphology and main properties of ceramic molds produced by investment casting using the ceramic of the above composition was studied, and the basic recommendations for the further development of their production technology were considered. It was determined that a high-refractory casting mold stable to 2773 K can be obtained by conventional investment casting, in which the solid-phase sintering of the ceramic does not rule out the simultaneous combination with the synthesis of the chosen ternary solid solution of the composition 15 mol % Y2O3 : 60 mol % ZrO2 : 25 mol % HfO2, acting as a high-temperature binder.

First-principles study of impurity segregation in zirconia, hafnia, and yttria-stabilized-zirconia grain boundaries

The impurity segregation behavior in symmetric tilt Σ5 (310)/[001] grain boundaries (GBs) of cubic ZrO2, HfO2, and yttria-stabilized ZrO2 (YSZ) were studied using first-principles calculations. The substitutional impurities, including divalent (Mg, Ca), trivalent (Al, Y), and tetravalent (Si, Ti) elements, were considered. It is found that divalent and trivalent impurities tend to segregate to a narrow region within 4 Å of the GB plane, while the tetravalent impurities have a much more scattered segregation profile extending 8–15 Å from the GB plane. For ZrO2 GB, the calculated grain boundary impurity enrichment factor for yttrium dopant is about 1.7, which is in good agreement with the experimental value of 1.9. For YSZ GB, there exist a strong GB segregation tendency for Mg, Ca, and Si, and a very weak segregation tendency for Ti in the YSZ GB, which is consistent with experimental findings.

Thermodynamic control of ferroelectric-phase formation in HfxZr1−xO2 and ZrO2

Since ferroelectric HfxZr1−xO2 is different from other ferroelectric doped-HfO2 materials in terms of the dopant sensitivity of the formation of a ferroelectric phase, the mechanism of formation of a ferroelectric phase is investigated in comparison with that in ZrO2. It is found that ferroelectric HfxZr1−xO2 follows a unique phase-transition pathway from tetragonal to monoclinic phases similar to that in other doped HfO2 materials. The phase diagram of the HfO2–ZrO2 solid-solution system is reconsidered thermodynamically by taking into account the effects of both film thickness and nonequilibrium fabrication conditions on structural phase stability. The formation mechanism of a ferroelectric phase in the HfxZr1−xO2 system over a wide range of Hf/Zr concentration ratios is qualitatively (but clearly) understandable from these thermodynamic considerations. Finally, it is demonstrated that both ferroelectric and antiferroelectric ZrO2 films can be formed on the same substrate by controlling the nucleation of monoclinic and tetragonal phases, respectively.

Crystallization behavior and ferroelectric property of HfO2–ZrO2 films fabricated by chemical solution deposition

In this study, we investigate the crystallization behavior of the precursor HfO2–ZrO2 (HZO) thin film to increase the fraction of the ferroelectric phase with orthorhombic symmetry in the resulting crystallized films. The precursor film of Hf0.5Zr0.5O2 prepared by the chemical solution deposition (CSD) process was crystallized to the tetragonal phase (at 600 °C) or a mixture of the orthorhombic and monoclinic phases (at 700 °C and above), depending on the crystallization temperature. A saturated P–E hysteresis loop with a remanent polarization (Pr) of 8 µC/cm2 was observed for the HZO films crystallized at 700 °C, whereas a distorted P–E curve with a lower Pr value was confirmed for the film crystallized at 600 °C.

Preparation and Characterization of Nanoceramics for Solid Oxide Fuel Cells

Precursor powders in the ZrO2–HfO2–Y2O3–CeO2, In2O3–ZrO2, and NiO–Nd2O3 systems for components of solid oxide fuel cells have been prepared by liquid-phase synthesis. We have determined formation conditions and the particle size of ZrO2- and In2O3-based solid solutions and neodymium nickelate (Nd2NiO4), demonstrated the feasibility of producing nanocrystalline powders (10–30 nm) of tailored chemical composition in the temperature range 500–900°C, and optimized powder consolidation conditions. Nanoceramics with a crystallite size from 60 to 90 nm have been obtained and their microstructure and phase composition have been investigated. We have studied the electrical properties of the ZrO2- and In2O3-based solid solutions and the Nd2NiO4 compound and established the range of their electrical conductivity at temperatures from 300 to 1000°C: 2.27 × 10–3 to 2.51 S/cm for the ZrO2-based solid solution, 8.91 × 101 to 6.59 × 103 S/cm for the In2O3-based solid solution, and 3.98 × 102 to 5.02 × 102 S/cm for Nd2NiO4.

Synthesis of the study of solid solutions based on the ZrO2–HfO2–Y2O3(CeO2) system

The results of the studies of the conditions of the liquid-phase synthesis of highly dispersed xerogels with a low degree of agglomeration and precursor nanopowders (~10–12 nm) based on zirconium dioxide in the ZrO2–HfO2–Y2O3(CeO2) system are presented. The thermal decomposition of xerogels and formation of crystalline solid solutions with the structure of fluorite are investigated. The optimal conditions for the solidification of nanodispersed powders for fabricating compact ceramics based on solid solutions of ZrO2 and the physical–chemical properties of these ceramics are studied.

Synthesis, vaporization and thermodynamics of ceramic powders based on the Y2O3–ZrO2–HfO2 system

Nano-particles of refractory oxides based on the Y2O3–ZrO2–HfO2 system (cubic phase) were synthesized using the glycol-citrate method. The dependence of the morphology and fineness of the powder obtained on the heat treatment conditions was considered. Vaporization processes and thermodynamic properties of ceramic powder in the Y2O3–ZrO2–HfO2 system were studied by the Knudsen effusion mass spectrometric method in the temperature range 2700–2925 K. It was found that at the temperature (2773 ± 40) K the ceramic powder containing Y2O3:ZrO2:HfO2 as 0.15:0.60:0.25 mol fractions underwent a congruent sublimation. The phenomenon observed was confirmed during ten hours vaporization under conditions intermediate to the equilibrium state and the evaporation from the exposed surface. This experimental observation may be successfully used for the development of advanced high temperature coatings based on the ceramics of the Y2O3–ZrO2–HfO2 system.

Phase Diagrams of Refractory Oxide Systems and Microstructural Design of Materials

It is shown that the phase diagrams of refractory oxide systems based on ZrO2, HfO2, Al2O3, and rare earth oxides underlie the microstructural design of various high-performance materials. Process steps to produce coarse-grained ceramics in the HfO2–ZrO2–Y2O3, ZrO2–Y2O3–Sc2O3, HfO2–ZrO2–Sc2O3, Y2O3–Er2O3, Y2O3–ZrO2, Y2O3–HfO2, Y2O3–Al2O3, Y2O3–SiO2, and Y2O3–La2O3 systems to perform at temperatures up to 2200°C are designed. Process steps to produce high-performance fine-grained composites in the HfO2–ZrO2–Y2O3 (Ln2O3) (Ln–Dy, Ho, Er, Tm, Yb), ZrO2–Y2O3–Sc2O3, ZrO2–Y2O3–Sc2O3, Al2O3–Zr(Hf)O2–Ln(Y)2O3 (Ln–La, Nd, Sm, Gd, Er, Yb), and ZrO2–Y2O3–CeO2–Al2O3 systems are designed as well.

Ferroelectricity in Simple Binary ZrO2 and HfO2

The transition metal oxides ZrO(2) and HfO(2) as well as their solid solution are widely researched and, like most binary oxides, are expected to exhibit centrosymmetric crystal structure and therewith linear dielectric characteristics. For this reason, those oxides, even though successfully introduced into microelectronics, were never considered to be more than simple dielectrics possessing limited functionality. Here we report the discovery of a field-driven ferroelectric phase transition in pure, sub 10 nm ZrO(2) thin films and a composition- and temperature-dependent transition to a stable ferroelectric phase in the HfO(2)-ZrO(2) mixed oxide. These unusual findings are attributed to a size-driven tetragonal to orthorhombic phase transition that in thin films, similar to the anticipated tetragonal to monoclinic transition, is lowered to room temperature. A structural investigation revealed the orthorhombic phase to be of space group Pbc2(1), whose noncentrosymmetric nature is deemed responsible for the spontaneous polarization in this novel, nanoscale ferroelectrics.

Sol–gel derived silica-based organic–inorganic hybrid materials as “composite precursors” for the synthesis of highly homogeneous nanostructured mixed oxides: an overview

The present contribution reports on our results concerning the synthesis of different binary and ternary oxide systems by using hybrid materials as “composite” precursors. In the last years, we have developed and explored a valuable strategy to yield a very homogeneous dispersion of nanoparticles of early metal transition oxide, MO2 (M = Zr, Hf) inside a silica matrix. This route is based on the use of the sol–gel process to obtain organic–inorganic hybrid silica-based materials embedding the oxide precursors (Zr and/or Hf oxoclusters), which are then calcined at high (T > 500 °C) temperatures to give the desired oxides. The “precursor” hybrid materials are prepared by a modified sol–gel process, involving the copolymerisation of the organically modified oxozirconium or oxohafnium clusters (M4O2(OMc)12 (M = Zr, Hf and OMc = methacrylate) with (methacryloxymethyl)triethoxysilane (MAMTES) or (methacryloxypropyl)trimethoxysilane (MAPTMS). Free radical copolymerisation of the 12 methacrylate groups of the oxoclusters with the methacrylate-functionalised siloxanes allows a stable anchoring of the oxoclusters to the silica network formed by the hydrolysis and condensation of the alkoxy groups. The sol–gel reactions of the two methacrylate-modified silanes methacryloxymethyltriethoxysilane and methacryloxypropyltrimethoxysilane were followed by using two independent time-resolved spectroscopic methods, viz., IR ATR and NMR with the aim to optimise their pre-hydrolysis times and consequently their use as precursors for hybrid materials. As mentioned, thermal treatment at high temperature of the hybrid yields a very homogeneous dispersion of ZrO2 and/or HfO2 nanoparticles in the silica matrix, since the molecular homogeneity of the starting hybrid is retained in the final mixed oxide. This route was successfully applied both to the synthesis of bulk materials and thin films characterised by different compositions (in term of M/Si molar ratios and M nature), heating route (conventional or microwave-assisted) and final temperature of annealing (from RT to 1,100 °C). The first example of the ZrO2–HfO2–SiO2 ternary oxide system was also prepared by this approach. The prepared systems, both in the form of hybrid materials as well as in the final form of binary or ternary oxides, were thoroughly characterised by a wide variety of analytical tools from a compositional, structural, morphological point of view. Moreover, in the case of the binary ZrO2–SiO2 bulk materials, also the evolution under heating was followed by different methods. In particular, the composition of the hybrid as well as of the final oxidic materials was determined by X-Ray Photoelectron Spectroscopy and elemental analysis, whereas FT-IR and multinuclear solid-state NMR spectroscopies shed light on the changes occurring in the composition upon thermal heating and the degree of condensation of the silica network. The morphology and the microstructure of the hybrids and of the oxides were studied by nitrogen sorption and Scanning Electron Microscopy. X-Ray Diffraction, Transmission Electron Microscopy and X-ray Absorption Fine Structure Spectroscopy X-ray Absorption Fine Structure Spectroscopy were used to follow the conversion of the amorphous oxides to the final materials consisting of crystalline zirconia or hafnia dispersed in amorphous silica. On selected systems, functional properties (surface reactivity, dielectric properties) were furthermore investigated. The obtained binary oxides were also used as substrates for functionalisation experiments with (1) dialkycarbamates and (2) long alkyl chains to produce functional materials for catalysis and HPLC applications, respectively.

Transparent Hafnia‐Containing β‐Quartz Glass Ceramics: Nucleation and Crystallization Behavior

Heterogeneous nucleation and crystallization of lithium alumosilicate glasses with hafnia‐containing nucleation agents was explored. Kinetic, thermodynamic, and structural data were considered to assess nucleation efficiency and to characterize the crystallization process. It is shown how lattice parameters and, particularly, anisotropy of the nuclei phase depend on the amount of ZrO2–HfO2 substitution in a specific base glass. For a given crystallization treatment, the size of the derived crystallites of β‐quartz structure is used as a measure of nucleation rates. A nonlinear dependence of nucleation efficiency on HfO2 content was established, with the supposedly most efficient nucleation corresponding to the lowest degree of anisotropy of the nuclei crystallites, i.e., when about 20%–30% of ZrO2 was substituted by HfO2. Apparent activation energies and estimates of the Avrami coefficient were determined from nonisothermal crystallization experiments for selected compositions to highlight the differences between hafnia and zirconia. Hafnia can be used alone or in combination with other agents to nucleate nanocrystalline glass ceramics with a low coefficient of thermal expansion.

Effect of Al2O3 concentration on zirconolite (Ca(Zr,Hf)Ti2O7) crystallization in (TiO2,ZrO2,HfO2)-rich SiO2–Al2O3–CaO–Na2O glasses

Glass-ceramic matrices containing zirconolite (nominally Ca(Zr,Hf)Ti2O7) crystals in their bulk that would incorporate high proportions of minor actinides (Np, Am, Cm) or plutonium could be envisaged for their immobilization. Zirconolite-based glass-ceramics can be prepared by controlled crystallization of zirconolite in glasses belonging to SiO2–Al2O3–CaO–Na2O–TiO2–ZrO2–HfO2 system. In this study, neodymium was used as trivalent actinides surrogate. Increasing Al2O3 concentration in glass composition had a strong effect on the nucleation rate I z of zirconolite crystals in the bulk, on the amount of neodymium incorporated in zirconolite phase and on the crystal growth rate of silicate phases (titanite + anorthite) from glass surface. These results could be explained by the existence of competition—in favor of aluminum—between Al3+ and (Ti4+, Zr4+, Hf4+) ions for their association with charge compensators cations to facilitate their incorporation in the glassy network. Differential thermal analysis (DTA) was used to study exothermal effects associated with bulk and surface crystallization. 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra showed that aluminum enters glasses network predominantly in 4-fold coordination. Neodymium optical absorption and fluorescence spectroscopies showed that the Al2O3 concentration changes performed in this study had not significant effect on Nd3+ ions environment in glasses.

The Zirconia–Hafnia System: DTA Measurements and Thermodynamic Calculations

The equilibrium temperature (T0) at which the tetragonal and monoclinic phases of either ZrO2 or HfO2 coexist is generally defined by the middle temperature of As (the onset transformation temperature on heating) and Ms (the onset transformation temperature on cooling). It cannot be directly determined due to the athermal nature of the martensitic transformation. Practically, the determination of T0 is important for the prediction of As and Ms in ZrO2 or HfO2‐based materials. In this work, the ZrO2–HfO2 system was studied experimentally by differential thermal analysis (DTA) to obtain the martensitic tetragonal ⇔ monoclinic transformation temperatures in the temperature range of 1273–1973 K. The T0 temperatures obtained for ZrO2 and HfO2 are 1367±5 and 2052±5 K, respectively. They are adopted for the assessments of the Gibbs energy parameters of these two oxides. A reasonably calculated ZrO2−HfO2 phase diagram is presented.

Raman spectra and structure of multicomponent oxide planar waveguides prepared by sol-gel

This work was aimed at understanding the structure of SiO2–MO2 (M = Ti, Zr, Hf) and SiO2–HfO2–MO2 (M = Ti, Zr) materials, used as mixed oxide glass hosts for Er3+ ions in the fabrication of optical planar waveguides by sol-gel processing. This structural study was performed by Waveguide Raman Spectroscopy (WRS), complemented with X-ray diffraction (XRD). The admixture of TiO2 to HfO2, SiO2–HfO2 and HfO2–ZrO2 compositions was found to cause precipitation of nanocrystals of tetragonal HfO2 or ZrO2, or the formation of hafnia-titania mixed crystals, depending on the HfO2/TiO2 molar ratio.