Patrick S. Nicholson

Fracture of a brittle particulate composite

Previous models for crack-particle interactions in brittle composites are modified to account for penetrable obstacles, obstacle shape and secondary crack interactions. The modified model is applied to a glass-unbonded nickel sphere composite system, the experimental aspects of which were summarized in Part 1. Increases in fracture energy are explained in terms of local crack blunting. It is shown that these results fall, as expected, between those for an entirely sharp crack front and an entirely blunt one.

Functionally graded ceramic/ceramic and metal/ceramic composites by electrophoretic deposition

Constant current electrophoretic deposition (EPD) has been used to synthesise Al2O3/YSZ,Al2O3/MoSi2, A12O3/Ni and YSZ/Ni functionally graded materials (FGM). EPD is a cheap and simple technique to fabricate complicated ceramic shapes. By this technique it is possible to synthesize step FGMs as well as continuous-profile FGMs. The profile can be controlled precisely by controlling the deposition current density, second component flow rate, suspension concentration, etc. The microstructures of the FGMs produced were characterized by optical and electron microcopy and micro-indentation was used to track the Vickers hardness and fracture toughness variation across the composition profiles.

The relative stability of spray-frozen/freeze-dried β″-Al2O3 powders

The relative stability of Na-β″- Al2O3 powders made from soluble chemicals by spray-freeze/freeze-drying and 5 hr/1250°C calcination, has been investigated. The β″-Al2O3 phase-stabilizer (Li2O, MgO) and the level of excess Na2O were varied. Low f(β) (< 0.1), high density (98 ± 0.5% theoretical), β″-Al2O3-ceramics are produced after 5 mins/1600°C sintering β″-Al2O3-powder compositions stabilized with (2% MgO + 2% wt % extra Na2O). No post-sintering anneal procedure was employed.

Electrophoretic deposition of alumina suspension in a strong magnetic field

Textured monolithic alumina ceramics were synthesized by electrophoretic deposition (EPD) in strong magnetic field of 10 T. Single crystalline, granular α-alumina particles in aqueous suspensions were rotated due to their anisotropic diamagnetic susceptibility and then deposited on substrate. A multilayered alumina composite of oriented and randomly oriented layers was also synthesized by the alternate EPD of alumina suspensions which were placed in and out of a superconducting magnet. It was demonstrated that the EPD in a strong magnetic field is a promising ceramic processing technique for fabricating sophisticated ceramic composites.

Creep of hot-pressed silicon nitride

Creep tests were undertaken on hot-pressed silicon nitride in the temperature range 1200 to 1400° C. The activation energy for creep was determined to be 140 kcal mol−1 and the stress exponent of creep rate was 1.7. The creep behaviour is ascribed to grain-boundary sliding accommodated by void deformation at triple points and by limited local plastic deformation. Electron microscopic evidence supporting this mechanism is presented.

Influence of elastic and thermal mismatch on the local crack-driving force in brittle composites

An expression is derived for the change of localK1 value of a crackfront near circular and spherical inclusions with elastic moduli and thermal expansion coefficient different from those of the matrix. The derivation is based on the concept of an “image stress” which is imposed on the crack, to illustrate the interaction between elastic and thermal stress concentrations developed around inclusions in a composite material and the crack-tip stress field.

The mixed alkali effect in (Na+−K+) β″-alumina

The electrical conductivities of single crystals of mixed alkali (Na+−K+)β″-alumina were measured for different alkali compositions. Typical log σT versus 1T curves exhibit three regions indicative of three activation energies. Electrical conductivity isotherms show that the mixed alkali effect disappears >200°C and the eletrical conductivity does not change significantly from [K+][K+]+[Na+]=0.0 to 0.9, at low temperatures. For [K+][Na+]+[K+]>0.9, the conductivity increased sharply. This β″-alumina behaviour is entirely different from that of β-alumina. The observed results are discussed in terms of the degree of disorder and the site distribution of alkali ions on the conduction plane. A model is proposed which assumes the cooperative motion of Na+ and K+.

The influence of oxygen partial pressure and temperature on Bi‐Pb‐Sr‐Ca‐Cu‐O 110 K superconductor phase formation and its stability

A systematic study of the influence of oxygen partial pressure on 110 K phase formation in the Bi‐Pb‐Sr‐Ca‐Cu‐O system has been undertaken on the optimum nominal composition to promote the Bi2Sr2Ca2Cu3Ow (2223, 110 K) phase, i.e.; Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06Ow. The relationship between 110 K phase fraction, oxygen partial pressure, sintering temperature, and time has been mapped for this composition via experimental data. The optimum conditions for 110 K phase formation were identified as 0.01≤PO2≤0.35 atm, at 820≤T≤880 °C for ≤36 h with intermediate grinding every 6 h. The single 110 K phase stability range was determined as PO2≤0.5 atm. The variation of oxygen partial pressure during the 110 K phase formation process was monitored via a Y2O3‐ZrO2 oxygen sensor and a possible mechanism for its formation and the effect of PO2 thereon is discussed.

Polycrystalline H3O+−β″/β-aluminuma-abrication characterization and use for steam electrolysis

Abstract Polycrystalline H 3 O + -β″/β-alumina ceramics have been fabricated and used succesfully to electrolyse steam at ∼ 100°C. The ceramic precursor is a mixed alkali β/β″-alumina of sufficient mechanical strength to withstand the mecessary potassium melt and field assisted acid ion exchange steps required to produce the hydronium polycrystals.

The preparation and characterization of dense, highly conductive Na5GdSi4O12 nasicon (NGS)

Abstract Na 5 GdSi 4 O 12 has been prepared via conventional ball-milling technique and through spray-freezing/freeze-drying. The ball-milled materials were calcined at 700°C or 925°C and sintered at 1050°C/3.5h to 89% dense multiphase ceramics. Spray-frozen/freeze-dried powders were calcined at 530°C and sintered at 1050°C/25 min to 99 ± 1% theoretical density. The latter material was single-phase NGS nasicon with a 300°C resistivity of 3.8 Ω·cm, an activation energy for Na + -conduction of 4.4 kcal/mol, a flexural strength of 105 MPa, a duplex grain structure with average sizes 0.4 μm and 3 μm and a unique linear thermal expansion coefficient (25–540°C; α = 12.6 × 10 −6 /°C±4%).