Effect of CuO on CaTiO3 perovskite ceramics prepared using a direct sintering process

Effect of CuO on CaTiO₃ (CT) ceramics prepared using a direct sintering process (reaction-sintering process) was investigated. The mixture of raw materials was pressed and sintered into ceramics without any calcination stage involved. Pure CT could be obtained. The degree of densification in CT via reaction-sintering process is lower than traditional oxide route but the grains grew easier in CT via reaction-sintering process. A density 3.63 g/cm³ (90.3% of ρ_th) is obtained in CT pellets after 1500 °C/16 h sintering. With 3 wt.% CuO addition, density 3.92 g/cm³ (97.5% of ρ_th) is obtained after 8 h sintering at 1500 °C due to the liquid phase sintering. The liquid phase at grain boundaries appeared significantly at a lower sintering temperature for longer soak time.     

Microwave sintering of 8 mol% yttria-zirconia (8YZ): An inert matrix material for nuclear fuel applications

This study focused on reducing overall processing time and temperature for fully stabilized zirconia, an inert matrix material candidate, to minimize the loss of actinides (that will be incorporated into the matrix material), while maintaining at least 90% theoretical density (TD). The effects of different processing routes on bulk density and microstructure were evaluated. The results obtained by adopting microwave sintering for 8 mol% Y₂O₃-ZrO₂ were compared to conventional sintering. A 20 min soak time at 1300 °C resulted in pellets with 90% TD for microwave-processed samples, compared to 77% TD for pellets processed conventionally. A similar density was obtained at lower temperature (1200 °C) by increasing the soak time to 100 min in microwave processing. This time and temperature resulted in 60% TD conventionally processed pellets. Compressive strength values obtained for a 1300 °C (20 min soak time) microwave-processed sample were higher (1600 MPa) as compared to a conventionally processed sample (1300 MPa).     

Diffuse reflectance and X-ray photoelectron spectroscopy of uranium in ZrO2 and Y2Ti2O7

Diffuse reflectance measurements were made over the wavenumber range of 4000-20,000 cm⁻¹ at room temperature on monoclinic and stabilised ZrO₂, together with Y₂Ti₂O₇ having the pyrochlore structure, all of which were doped with U and sintered in various atmospheres. X-ray photoelectron spectroscopy measurements were also carried out on selected samples. In monoclinic and stabilised zirconia, U exhibited valence states of +4 and/or +5, depending on the sintering atmosphere and the presence of appropriate charge compensators. Using both diffuse reflectance and X-ray photoelectron spectroscopy, U was also observed as mainly U⁴⁺ and/or U⁵⁺ in U-doped Y₂Ti₂O₇ sintered at 1400 °C in air or Ar, although a small amount of U⁶⁺ also appeared to be present in some U-doped Y₂Ti₂O₇ samples heated in air.     

Study of silver diffusion in silicon carbide

Diffusion of silver in 6H-SiC and polycrystalline CVD-SiC was investigated using α-particle channeling spectroscopy and electron microscopy. Fluences of 2 × 10¹⁶ cm⁻² of ¹⁰⁹Ag⁺ were implanted with an energy of 360 keV at room temperature, at 350 °C and 600 °C, producing an atomic density of approximately 2% at the projected range of about 110 nm. The broadening of the implantation profile and the loss of silver through the front surface during vacuum annealing at temperatures up to 1600 °C was determined. Fairly strong silver diffusion was observed after an initial 10 h annealing period at 1300 °C in both polycrystalline and single crystalline SiC, which is mainly due to implant induced radiation damage. After further annealing at this temperature no additional diffusion took place in the 6H-SiC samples, while it was considerably reduced in the CVD-SiC. The latter was obviously due to grain boundary diffusion and could be described by the Fick diffusion equation. Isochronal annealing of CVD-SiC up to 1400 °C exhibited an Arrhenius type temperature dependence, from which a frequency factor D_o ∼ 4 × 10⁻¹² m² s⁻¹ and an activation energy E_a ∼ 4 × 10⁻¹⁹ J could be extracted. Annealing of 6H-SiC above 1400 °C shifted the silver profile without any broadening towards the surface, where most of the silver was released at 1600 °C. Electron microscopy revealed that this process was accompanied by significant re-structuring of the surface region. An upper limit of D < 10⁻²¹ m² s⁻¹ was estimated for 6H-SiC at 1300 °C.     

In situ ERDA measurements of hydrogen isotope concentrations in palladium at atmospheric pressure

In situ elastic recoil detection analysis (ERDA) measurements in gases at atmospheric pressure have been carried out using 15 MeV ⁴He ion beams. The beams are extracted through a molybdenum foil having a thickness of 5 μm. The maximum depth of analysis is about 4 μm for the palladium hydride and palladium deuteride (PdH_x and PdD_x, x = 0.7-0.8) samples. The temperature of the samples rises stepwise from room temperature to 180 °C. ERDA spectra are obtained every 2 min. Hydrogen and deuterium in the samples are discharged in the temperature range of 120-140 °C in a vacuum. Decrease in the hydrogen concentration in the PdH_x sample heated in a vacuum follows a first order in the value of x and an apparent activation energy of discharge of hydrogen is 1.05 eV. On the other hand, the hydrogen and deuterium concentrations decrease at about 80 °C in air. No isotope effects are observed in both a vacuum and air. The temperature at which the hydrogen concentration decreases in helium gas is almost the same as that in a vacuum. It indicates that hydrogen and deuterium atoms are discharged by chemical reactions with air and that there are no effects of cooling of the thermocouple by convection of air.     

The interdiffusion and solid-state reaction of low-energy copper ions implanted in silicon

At room temperature, single-crystal silicon was implanted with Cu⁺ ions at an energy of 80 keV using two doses of 5 × 10¹⁵ and 1 × 10¹⁷ Cu⁺ cm⁻². The samples were heat treated by conventional thermal annealing at different temperatures: 200 °C, 230 °C, 350 °C, 450 °C and 500 °C. The interdiffusion and solid-state reactions between the as-implanted samples and the as-annealed samples were investigated by means of Rutherford backscattering spectrometry (RBS) and X-ray diffraction (XRD). After annealing at 230 °C, the XRD results of the samples (subject to two different doses) showed formation of Cu₃Si. According to RBS, the interdiffusion between Cu and Si atoms after annealing was very insignificant. The reason may be that the formation of Cu₃Si after annealing at 230 °C suppressed further interdiffusion between Si and Cu atoms.     

Synthesis & fabrication of lithium-titanate pebbles for ITER breeding blanket by solid state reaction & spherodization

⁶Li produces tritium by (n, α) nuclear reaction, ⁶Li + ¹n → ⁴He + ³H. Lithium titanate (Li₂TiO₃) enriched with ⁶Li, is the most promising candidate for solid test blanket module (TBM) material for fusion reactors. Various processes are reported in literature for the fabrication of Li₂TiO₃ pebbles for its use as TBM material. A process has been developed based on the solid state reaction of lithium-carbonate and titanium-dioxide for the synthesis of lithium titanate and pebble fabrication by extrusion, spherodization and sintering. This paper discusses the sequence of steps followed in this process and the properties obtained. Analytical grade titanium-dioxide and lithium-carbonate were taken in stoichiometric ratio and were milled to ensure thorough intimate mixing and obtain fine particles less than 45 μm before its calcination at 900 °C. Following calcination, the agglomerated product was again milled to fine particles of size less than 45 μm. Aqueous solution of ploy-vinyl-alcohol was added as binder prior to its feeding in the extruder. The extruded strips were spherodized and spherical pebbles were dried and sintered at 900 °C for different duration. Pebbles of desired density and porosity were obtained by suitable combination of sintering temperature and duration of sintering. Properties of the prepared pebbles were also characterized for sphericity, pore size distribution, grain size, crushing load strength, etc. The values were found to be conforming to the desired properties for use as solid breeder. The attractive feature of this process is almost no waste generation.     

Method to measure composition modifications in polyethylene terephthalate during ion beam irradiation

Matter losses of polyethylene terephthalate (PET, Mylar) films induced by 1600 keV deuteron beams have been investigated in situ simultaneously by nuclear reaction analysis (NRA), deuteron forward elastic scattering (DFES) and hydrogen elastic recoil detection (HERD) in the fluence range from 1 × 10¹⁴ to 9 × 10¹⁶ cm⁻². Volatile degradation products escape from the polymeric film, mostly as hydrogen-, oxygen- and carbon-containing molecules. Appropriate experimental conditions for observing the composition and thickness changes during irradiation are determined. ¹⁶O(d,p₀)¹⁷O, ¹⁶O(d,p₁)¹⁷O and ¹²C(d,p₀)¹³C nuclear reactions were used to monitor the oxygen and carbon content as a function of deuteron fluence. Hydrogen release was determined simultaneously by H(d,d)H DFES and H(d,H)d HERD. Comparisons between NRA, DFES and HERD measurements show that the polymer carbonizes at high fluences because most of the oxygen and hydrogen depletion has already occured below a fluence of 3 × 10¹⁶ cm⁻². Release curves for each element are determined. Experimental results are consistent with the bulk molecular recombination (BMR) model.     

Mössbauer study and structural characterization of UO2–Gd2O3 sintered compounds

Samples of UO₂and up to 10 wt% of Gd₂O₃ were prepared by solid-state reaction under a reducing atmosphere, in a thermal path comprising ramps and dwell times in the temperature range of 900–1750 °C. The sintered material was analyzed by X-ray diffraction and ¹⁵⁵Gd Mössbauer spectroscopy. The results showed that for samples annealed up to 900 °C, the gadolinium sesquioxide remained unreacted. However, when the temperature was increased to 1300 °C, a solid-state reaction took place forming mixed oxides. For the more severe sintering condition, at 1750 °C, gadolinia left urania partially unreacted producing a material consisting of two compositions, UO₂ (with no dissolved gadolinium) and (U, Gd)O₂. The proposed heating cycle provided pellets free from Gd₂O₃ phase and may be used by the nuclear fuel industry as a suitable sintering process.     

Synthesis and characterization of Li4SiO4 nano-powders by a water-based sol-gel process

The water-based sol-gel process for the synthesis of Li₄SiO₄ nano-powders was reported for the first time. LiOH·H₂O and aerosil SiO₂ were used as the starting materials with citric acid (C₆H₈O₇·H₂O) as the chelating agent. Li₄SiO₄ powders with particle size as small as 100 nm were successfully synthesized at the temperature as low as 675 °C. Phase analysis, morphology, sintering behavior of the powders and ionic conductivity of the sintered bodies were investigated systematically. The experimental results showed that the powders obtained by the water-based sol-gel process (SG) possessed excellent sinterability, exhibiting a linear shrinkage of 5.2% while sintered to 900 °C, more than 3 times that of the powders obtained by solid state reaction (SSR). The bulk conductivity of the SG sintered bodies was much higher than that of the SSR samples at the same testing temperature.     

Direct carbothermic reduction of actinide oxalates: Example of Nd(III) oxalate-carbon mixtures conversion

Mixed uranium-plutonium monocarbide was obtained by carbothermic reduction of mixture of UO₂, PuO₂ and C at around 1500 °C under vacuum or argon for 4 h followed by sintering at around 1650 °C under mixture of Ar + 8%H₂ for 10 h. Moreover, PuO₂ was synthesized by oxalic precipitation followed by calcination at around 700 °C (PUREX process). This work deals with a process simplification, i.e. fabrication of carbide by calcination of actinide oxalate, precipitated in presence of carbon, at 1600 °C for 13 h under argon. In this preliminary study, neodymium was used as a surrogate of actinides. No carbon influence was noticed during calcination of neodymium oxalate into oxide. Carbon content was the same before and after calcination making possible conversion to carbide directly from mixture of neodymium oxalate and C.     

Permeability of hydrogen and deuterium of Hastelloy XR

Permeation of hydrogen isotope through a high-temperature alloy used as heat exchanger and steam reformer pipes is an important problem in the hydrogen production system connected to be a high-temperature engineering test reactor (HTTR). An experiment of hydrogen (H₂) and deuterium (D₂) permeation was performed to obtain permeability of H₂ and D₂ of Hastelloy XR, which is adopted as heat transfer pipe of an intermediate heat exchanger of the HTTR. Permeability of H₂ and D₂ of Hastelloy XR were obtained as follows. The activation energy E₀ and pre-exponential factor F₀ of the permeability of H₂ were E₀=67.2±1.2 kJ mol⁻¹ and F₀=(1.0±0.2)×10⁻⁸ m³(STP) m⁻¹ s⁻¹ Pa^−0.5, respectively, in the pipe temperature ranging from 843 K (570 °C) to 1093 K (820 °C). E₀ and F₀ of the permeability of D₂ were respectively E₀=76.6±0.5 kJ mol⁻¹ and F₀=(2.5±0.3)×10⁻⁸ m³(STP) m⁻¹ s⁻¹ Pa^−0.5 in the pipe temperature ranging from 943 K (670 °C) to 1093 K (820 °C).     

Dynamic annealing study of SiC epilayers implanted with Ni ions at different temperatures

SiC epilayers grown on 4H-SiC single crystals were implanted with 850 keV Ni⁺ ions with fluences in the 0.5-9 × 10¹⁶ Ni⁺/cm² range. Most of the samples were implanted at 450 °C, but for comparison some implantations were performed at room temperature (RT). In addition, a post-implantation annealing was performed in N₂ at 1100 °C in order to recover the implantation-induced structural damage. The disorder produced by the implantation at 450 °C and the effect of the post-implantation annealing on the recrystallization of the substrates have been studied as a function of the fluence by Backscattering Spectrometry in channeling geometry (BS/C) with a 3.45 MeV He²⁺ beam. RT as-implanted samples showed a completely amorphous region which extends until the surface when irradiated with the highest dose, whereas in the case of 450 °C implantation amorphization does not occur. In general, partial recovery of the crystal lattice quality was found for the less damaged samples, and the dynamic recovery of the crystalline structure increases with the irradiation temperature.     

Swift heavy ion irradiation induced texturing in NiO thin films

NiO thin films grown on Si(1 0 0) substrate by electron beam evaporation and sintered at 500 and 700 °C were irradiated with 120 MeV Au⁹⁺ ions. The FCC structure of the sintered films was retained up to the highest fluence (3 × 10¹³ ions cm⁻²) of irradiation. In the low fluence (?1 × 10¹³ ions cm⁻²) regime however, the evolution of the XRD pattern with fluence showed a wide variation, critically depending upon their initial microstructure. Though irradiation is known to induce disorder in the structure, we observe improvement in crystallization and texturing at intermediate fluences of irradiation.     

Effects of composition and structure on hydrogen incorporation in tungsten oxide films deposited by sputtering

The effects of composition and structure on hydrogen incorporation in tungsten oxide films were investigated. Films were deposited on carbon and SiO₂ substrates using a reactive sputtering by varying the substrate temperature from 30 to 600 °C in argon and oxygen mixture. The films were characterized using X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), elastic recoil detection analysis (ERDA) and Raman scattering. XRD patterns showed amorphous structure in the films deposited below 400 °C and (0 1 0) oriented monoclinic WO₃ in the films deposited beyond 400 °C. The results of RBS and ERDA indicated that hydrogen concentration in the amorphous films increased from 0.1 to 0.7 H/W with changing the composition from WO_0.25 to WO₃. The hydrogen concentration in WO₃ films decreased to 0.4 H/W with increasing the substrate temperature during deposition. The Raman spectra of the WO₃ films revealed that decreasing of W⁶⁺O terminals was related to decreasing of the hydrogen concentration. It was considered that the incorporated hydrogen in tungsten oxide films was bonded at the end of W⁶⁺O terminals.     

Preparation and characterization of lithium-titanate pebbles by solid-state reaction extrusion and spherodization techniques for fusion reactor

For the development of TBM for fusion reactors, lithium containing ceramics as against the metal are preferred as tritium breeding material. Lithium titanate (Li₂TiO₃) is one such chosen ceramic tritium breeder. Li₂TiO₃ pebbles are conventionally prepared by sol-gel process and wet process. Solid state reaction of lithium carbonate with titanium dioxide is preferred route for the bulk production of Li₂TiO_3. Thermo-gravimetric and differential thermal analysis (TG-DTA) techniques have been used in the present study to understand the solid state reaction of intimate mixture of lithium carbonate and titanium dioxide. It was found out that single phase lithium titanate (Li₂TiO₃) is produced at 750 °C and the reaction is completed in 6 h. Fine powders of lithium titanate obtained after milling and classification were mixed with aqueous solution of PVA to prepare green pebbles of desired size and shape. The pebbles were subsequently sintered at 900 °C and the effect of sintering time on the properties of sintered pebbles was studied. The reaction mechanisms and the product qualities obtained by the solid state reaction, extrusion and spherodization techniques are discussed in this paper.     

Defect characterization in boron implanted silicon after flash annealing

Flash-assisted rapid thermal processing (fRTP) has gained considerable interests for fabrication of ultra-shallow junction in silicon. fRTP can significantly reduce boron diffusion, while attaining boron activation at levels beyond the limits of traditional rapid thermal annealing. The efficiency of fRTP for defect annealing, however, needs to be systematically explored. In this study, a (1 0 0) silicon wafer was implanted with 500 eV boron ions to a fluence of 1 × 10¹⁵ cm⁻². fRTP was performed with peak temperatures ranging from 1100 °C to 1300 °C for approximately one milli-second. High resolution transmission electron microscopy and secondary ion mass spectrometry were performed to characterize as-implanted and annealed samples. The study shows that fRTP at 1250 °C can effectively anneal defects without causing boron tail diffusion.     

Fabrication and improvement of the density of Li2TiO3 pebbles by the optimization of a sol-gel method

Li₂TiO₃ is regarded as a promising candidate breeder in solid blanket concepts. Pebble configuration has been a preferred option due to its potential advantages in blanket design. Li₂TiO₃ pebbles were successfully fabricated by a water-based sol-gel method previously. However, the sintered density of the pebbles was very low (less than 70% theoretical density). The process parameters were optimized and the sintered density of the pebbles was improved significantly in this work. Li₂TiO₃ pebbles with density as high as 85% were obtained at a relatively lower sintering temperature (1100 °C) and shorter sintering time (4 h). The experimental results showed that the viscosity of the sol was influential to the sphericity of the gel-spheres and thus the sintered pebbles. The variety of lithium source, the pH value of the solution and the sintering conditions demonstrated significant influences on the microstructure and density of the sintered Li₂TiO₃ pebbles.     

Damage recovery and optical activity in europium implanted wide gap oxides

In this study we compare and discuss the defects and optical behaviour of sapphire and magnesium oxide single crystals implanted at room temperature with different fluences (1 × 10¹⁵-1 × 10¹⁶ cm⁻²) of europium ions.Rutherford backscattering channelling shows that for fluences above 5 × 10¹⁵ cm⁻² the surface disorder level in the Al-sublattice reaches the random level. Implantation damage recovers fast for annealing in oxidizing atmosphere but even for the highest fluence we recover almost completely all the damage after annealing at 1300 °C, independently of the annealing environment (reducing or oxidizing). Annealing above 1000 °C promotes the formation of Eu₂O₃ in the samples with higher concentration of Eu. The optical activation of the rare earth ions at room temperature was observed after annealing at 800 °C by photoluminescence and ionoluminescence. In Al₂O₃ lattice the highest intensity line of the Eu³⁺ ions corresponds to the forced electric dipole ⁵D₀ → ⁷F₂ transition that occurs ∼616 nm. For the MgO samples the Eu³⁺ optical activation was also achieved after implantation with different fluences. Here, the lanthanide recombination is dominated by the magnetic dipole ⁵D₀ → ⁷F₁ transition near by 590 nm commonly observed for samples were Eu³⁺ is placed in a high symmetry local site. The results clearly demonstrate the possibility to get Eu incorporated in optical active regular lattice sites in wide gap oxides.     

Proton irradiation study of GFR candidate ceramics

This work investigated the microstructural response of SiC, ZrC and ZrN irradiated with 2.6 MeV protons at 800 °C to a fluence of 2.75 × 10¹⁹ protons/cm², corresponding to 0.71-1.8 displacement per atom (dpa), depending on the material. The change of lattice constant evaluated using HOLZ patterns is not observed. In comparison to Kr ion irradiation at 800 °C to 10 dpa from the previous studies, the proton irradiated ZrC and ZrN at 1.8 dpa show less irradiation damage to the lattice structure. The proton irradiated ZrC exhibits faulted loops which are not observed in the Kr ion irradiated sample. ZrN shows the least microstructural change from proton irradiation. The microstructure of 6H-SiC irradiated to 0.71 dpa consists of black dot defects at high density.