Characterization of high-fracture toughness K-fluorrichterite-fluorapatite glass ceramics

Stoichiometric K-fluorrichterite (Glass A) and the same composition with 2 mol% P2O5 added (Glass B) were prepared and then heat-treated isothermally from 550°1000°C with 50°C intervals. Samples were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The biaxial flexural strength and indentation fracture toughness of heat-treated glass specimens were also determined for both materials. XRD traces and TEM images showed similar phase evolution and fine microstructures for both systems at ≤950°C, with mica and diopside reacting with residual glass to form K-fluorrichterite as the temperature was increased from 650°C. However, in Glass B, fluorapatite was also present at >800°C. In contrast, coarser microstructures were observed at 1000°C, with larger K-fluorrichterite (20 μm) and enstatite (10 μm) crystals in Glasses A and B, respectively. The highest fracture toughness (2.69 ± 0.01 MPa·m(1/2)) and biaxial strength (242.6 ± 3.6 MPa) were recorded for Glass B heat-treated at 1000°C. This was attributed to the presence of enstatite coupled with an interlocked lath-like crystalline microstructure

Advances in cold sintering : Improving energy consumption and unlocking new potential in component manufacturing

Ceramics are traditionally sintered at high temperatures (~80% melting temperature (Tm)). There are numerous incentives to reduce processing temperature: the reduction in processing energy; integration of polymeric and non-noble metals; greater control of microstructure and final component geometries. ‘Cold sintering’ has been developed as a novel method of densification which uses a transient liquid phase, pressure and heat to achieve dense ceramics. This review explores the process of cold sintering and its potential to densify various ceramic materials and components at low temperatures (<300°C), primarily describing recent results at The University of Sheffield, UK

Design of a bilayer ceramic capacitor with low temperature coefficient of capacitance.

We show how a simple bilayer system that combines a layer of undoped BaTiO3 (BT) with a second layer of Ba0.975Na0.025Ti0.975Nb0.025O3 (2.5NNBT) can be used to improve the temperature coefficient of capacitance (TCC) of BaTiO3-based materials for capacitor applications. The bilayer system emulates the volume ratio between a conventional core and shell phase microstructure allowing a simple resource efficient approach to optimise the system for low TCC. Optimisation was achieved with a volume ratio of 0.67 2.5NNBT with 0.33 BT and results in a TCC of ±6% over the temperature range ∼25 to 125 °C whilst maintaining a permittivity of εr ∼ 3000 and low dielectric loss

Synthesis of magnetocaloric LaFe11.6Si1.4 alloy by spark plasma sintering

LaFe11.6Si1.4 alloys have been successfully fabricated by spark plasma sintering (SPS). An annealing study of the SPS LaFe11.6Si1.4 alloys at different temperatures ranging from 1373 to 1523 K for annealing times from 30 minutes to 72 hours was carried out. This annealing study showed that LaFe11.6Si1.4 samples annealed at 1473K (for annealing times between 30 minutes and 6 hours) have a significantly higher amount of the NaZn13-type phase compared to samples annealed at other temperatures. Thus the critical annealing temperature which enhances the formation of the NaZn13-type phase in SPS LaFe11.6Si1.4 compounds is 1473K. A second study investigated the effect of different particle sizes of the starting powders on the formation of the NaZn13-type phase. This study found that the samples synthesized using larger sized powder particles exhibited a significantly higher amount of the NaZn13-type phase compared to samples synthesized using smaller sized powder particles, for the same heat treatment

Maghemite-like regions at crossing of two antiphase boundaries in doped BiFeO3

We report the observation of a novel structure at the point where two antiphase boundaries cross in a doped bismuth ferrite of composition (Bi0.85Nd0.15)(Fe0.9Ti0.1)O0.3. The structure was investigated using a combination of high angle annular dark field imaging and electron energy loss spectroscopy spectrum imaging in the scanning transmission electron microscope. A three-dimensional model was constructed by combining the position and chemistry data with previous results and assuming octahedral coordination of all Fe and Ti atoms. The resulting structure shows some novel L shaped arrangements of iron columns, which are coordinated in a similar manner to FeO6 octahedra in maghemite. It is suggested that this may lead to local ferromagnetic orderings similar to those in maghemite

Optimization of magnetocaloric properties of arc-melted and spark plasma-sintered LaFe11.6Si1.4

LaFe11.6Si1.4 alloy has been synthesized in polycrystalline form using both arc melting and spark plasma sintering (SPS). The phase formation, hysteresis loss and magnetocaloric properties of the LaFe11.6Si1.4 alloys synthesized using the two different techniques are compared. The annealing time required to obtain the 1:13 phase is significantly reduced from 14 days (using the arc melting technique) to 30 min (using the SPS technique). The magnetic entropy change (ΔSM) for the arc-melted LaFe11.6Si1.4 compound, obtained for a field change of 5 − 0T (decreasing field), was estimated to be 19.6 J kg−1 K−1. The effective RCP at 5T of the arc-melted LaFe11.6Si1.4 compound was determined to be 360 J kg−1 which corresponds to about 88 % of that observed in Gd. A significant reduction in the hysteretic losses in the SPS LaFe11.6Si1.4 compound was observed. The ΔSM, obtained for a field change of 5 − 0T (decreasing field), for the SPS LaFe11.6Si1.4 compound decreases to 7.4 J kg−1 K−1. The TC also shifts from 186 (arc-melted) to 230 K (SPS) and shifts the order of phase transition from first to second order, respectively. The MCE of the SPS LaFe11.6Si1.4 compound spreads over a larger temperature range with the RCP value at 5T reaching 288 J kg−1 corresponding to about 70 % of that observed in Gd. At low fields, the effective RCP values of the arc-melted and spark plasma-sintered LaFe11.6Si1.4 compounds are comparable, thereby clearly demonstrating the potential of SPS LaFe11.6Si1.4 compounds in low-field magnetic refrigeration applications

Controlling mixed conductivity in Na 1/2 Bi 1/2 TiO 3 using A-site non-stoichiometry and Nb-donor doping

Precise control of electronic and/or ionic conductivity in electroceramics is crucial to achieve the desired functional properties as well as to improve manufacturing practices. We recently reported the conventional piezoelectric material Na1/2Bi1/2TiO3 (NBT) can be tuned into a novel oxide-ion conductor with an oxide-ion transport number (tion) > 0.9 by creating bismuth and oxygen vacancies. A small Bi-excess in the nominal starting composition (Na0.50Bi0.50+xTiO3+3x/2, x = 0.01) or Nb-donor doping (Na0.50Bi0.50Ti1−yNbyO3+y/2, 0.005 ≤ y ≤ 0.030) can reduce significantly the electrical conductivity to create dielectric behaviour by filling oxygen vacancies and suppressing oxide ion conduction (tion ≤ 0.10). Here we show a further increase in the starting Bi-excess content (0.02 ≤ x ≤ 0.10) reintroduces significant levels of oxide-ion conductivity and increases tion ∼ 0.4–0.6 to create mixed ionic/electronic behaviour. The switch from insulating to mixed conducting behaviour for x > 0.01 is linked to the presence of Bi-rich secondary phases and we discuss possible explanations for this effect. Mixed conducting behaviour with tion ∼ 0.5–0.6 can also be achieved with lower levels of Nb-doping (y ∼ 0.003) due to incomplete filling of oxygen vacancies without the presence of secondary phases. NBT can now be compositionally tailored to exhibit three types of electrical behaviour; Type I (oxide-ion conductor); Type II (mixed ionic-electronic conductor); Type III (insulator) and these results reveal an approach to fine-tune tion in NBT from near unity to zero. In addition to developing new oxide-ion and now mixed ionic/electronic NBT-based conductors, this flexibility in control of oxygen vacancies allows fine-tuning of both the dielectric/piezoelectric properties and design manufacturing practices for NBT-based multilayer piezoelectric devices

Origin of improved tunability and loss in N2 annealed barium strontium titanate films

Barium strontium titanate (BSTO) thin films were deposited on Pt(111) by high throughput evaporative physical vapor deposition and then annealed at 650 °C for 30 min under N2 atmosphere. Using advanced transmission electron microscopy, energy-dispersive x-ray spectroscopy and electron energy-loss spectroscopy, we directly show that not only does N substitute for O in the BSTO lattice but that it also compensates for Ti3+ ions, suppressing conductivity, thereby reducing dielectric loss and enhancing dielectric tunability. However, this effect is negated near the film edge where we speculate that exposed Pt acts as a reservoir of adsorbed/absorbed O and alters the local N2 concentration during annealing

Slipcasting of MAX phase tubes for nuclear fuel cladding applications

As a proof of concept, tubes of Ti3SiC2 MAX phase were slipcast in order to investigate its potential for the fabrication of fuel cladding for nuclear reactors. A slip consisting of 46% dry weight basis (dwb) water, 4% dwb polyethyleneimine (PEI), 0.5% dwb methylcellulose was used to cast the tubes, which were then sintered for 2 h under vacuum at 1450 °C. Silicon loss was observed at surface which resulted in the formation of TiC. The hoop stress to destruction of the tubes was measured and achieved a maximum of 9.1 ± 2.2 MPa/mm of tube thickness