Ian Nettleship

Tetragonal zirconia polycrystal (TZP)—A review

Abstract Tetragonal zirconia polycrystal (TZP) is a new type of zirconia-based engineering ceramic. TZP ceramics are attractive because of their excellent room-temperature mechanical properties, which include flexural strength values over 1 GPa and a fracture toughness greater than 15 MPam 1 2 . These, combined with superior wear properties and a thermal expansion coefficient close to that of iron and iron-based alloys, give rise to a class of desirable engineering ceramics. This review will attempt to summarise the considerable amount of work done on the microstructural development and mechanical properties of TZP, especially over the last few years. The paper is divided into four main sections: 1. (a) The ZrO 2 Y 2 O 3 phase diagram. 2. (b) Development and optimisation of Y-TZP. 3. (c) Low-temperature ageing of TZP. 4. (d) TZP developed by means of the ZrO 2 CeO 2 system

Chemical preparation and phase stability of Ca2SiO4 and Sr2SiO4 powders

A process has been developed to produce high surface area Ca2SiO4 and Sr2SiO4 powders using colloidal silica for the silicon precursor. This method was relatively straightforward and avoided the use of precipitation techniques and unstable alkoxides. Calcium carbonate and strontium carbonate were identified as intermediate compounds, and the silicates appeared to form by a low-temperature solid-state reaction. The effect of resin content and calcination conditions on the phase distribution and physical properties of the powders was examined. Finally, the sodium content of the colloidal silica was found to have a significant effect on the phase stability of β-Ca2SiO4 after calcination at 1400°C.

Antimycobacterial efficacy of silver nanoparticles as deposited on porous membrane filters.

Environmental mycobacteria pose a significant health burden. Non-tuberculous mycobacteria infections have been traced to water treatment networks, where mycobacterial biofilms are ubiquitous. Filters that remove potential pathogens have significant medical applications. The purpose of this study is to demonstrate that an antibacterial silver nanoparticle (AgNP) coating can prevent colonization and growth of a mycobacterial biofilm on a filter material. The antibacterial efficacy of commercially available AgNPs was measured against Mycobacterium avium, Mycobacterium smegmatis, and Mycobacterium marinum after 48 h in liquid culture. Nanoparticles were deposited on micro-porous track etched polycarbonate membranes. The growth of biofilms on the membranes was observed by microscopy and counting colony forming units. M. smegmatis was most susceptible to AgNPs, with a 98.7% reduction at 100 μM AgNP concentration. M. avium was reduced by 97.3% at 539 μM AgNP after 48 h. Deposited nanoparticles inhibited colonization and growth for both M. smegmatis and M. avium on the membrane surface. Similar to the liquid culture, M. avium (84.2% survival) was more resistant than M. smegmatis (0.03% survival).

The simulation and selection of shapes for the unfolding of grain size distributions

The determination of a grain size distribution from one planar section requires a representative shape that allows the distribution in particle size to be estimated from the size distribution of the particle sections. In this work a simulation has been developed to randomly generate convex polyhedra and calculate the distributions in section properties needed to unfold the particle size distribution into three dimensions. Furthermore a new shape selection process based on a common shape factor is described. The relatively high symmetry polyhedra commonly used to unfold grain size distributions underestimate the grain size compared with the randomly generated polyhedra of the simulation. The latter are thought to be more representative of real grain shapes in that they do not commonly possess pairs of parallel faces.

Mutation of environmental mycobacteria to resist silver nanoparticles also confers resistance to a common antibiotic

Non-tuberculous mycobacteria are a threat to human health, gaining entry to the body through contaminated water systems, where they form persistent biofilms despite extensive attempts at disinfection. Silver is a natural antibacterial agent and in nanoparticle form activity is increased by a high surface area. Silver nanoparticles (AgNPs) have been used as alternative disinfectants in circulating water systems, washing machines and even clothing. However, nanoparticles, like any other antibiotic that has a pervasive durable presence, carry the risk of creating a resistant population. In this study Mycobacterium smegmatis strain mc2155 was cultured in AgNP enriched agar such that only a small population survived. Surviving cultures were isolated and re-exposed to AgNPs and AgNO3 and resistance to silver was compared to a negative control. After only a single exposure, mutant M. smegmatis populations were resistant to AgNPs and AgNO3. Further, the silver resistant mutants were exposed to antibiotics to determine if general resistance had been conferred. The minimum inhibitory concentration of isoniazid was four times higher for silver resistant mutants than for strain mc2155. However, core resistance was not conferred to other toxic metal ions. The mutants had lower resistance to CuSO4 and ZnSO4 than the mc2155 strain.

Grain boundary detection in microstructure images using computational intelligence

Two computational intelligence approaches, a fuzzy logic algorithm and a neural network (NN) algorithm, for grain boundary detection in images of superalloy steel microstructure during sintering are presented in this paper. The images are obtained from an optical microscope and are quite noisy, which adversely affects the performance of common image processing tools. The only known way to accurately determine the grain boundaries is digitizing by hand. This is a very time-consuming process, causes operator fatigue, and it is prone to human errors and inconsistency. An automated system is therefore needed to complete as much work as possible and we consider a fuzzy approach and a neural approach. Both methods performed better than the widely available standard image processing tools with the neural approach superior on images similar to those trained while the fuzzy approach showed more tolerance of disparate images.

Long-term three-dimensional perfusion culture of human adult bone marrow mononuclear cells in bioreactors

The construction and long‐term maintenance of three‐dimensional in vitro bone marrow models is of great interest but still quite challenging. Here we describe the use of a multi‐compartment hollow‐fiber membrane based three‐dimensional perfusion bioreactor for long‐term culture of whole human bone marrow mononuclear cells. We also investigated bioreactors with incorporated open‐porous foamed hydroxyapatite scaffolds, mimicking the in vivo bone matrix. Cells in bioreactors with and without scaffolds were cultured to 6 weeks and compared to Petri dish controls. Cells were analyzed for gene expression, surface markers by flow cytometry, metabolic activity, hematopoietic potential, viability, and attachment by immunocytochemistry. Cells in bioreactors were metabolic active during long‐term culture. The percentages of hematopoietic stem cell and mature endothelial cell fractions were maintained in bioreactors. The expression of most of the analyzed genes stabilized and increased after long‐term culture of 6 weeks. Compared to Petri dish culture controls, bioreactor perfusion culture improved in both the short and long‐term, the colony formation unit capacity of hematopoietic progenitors. Cells attached to the ample surface area provided by hydroxyapatite scaffolds. The implementation of a hydroxyapatite scaffold did not influence colony formation capacity, percentages of cell type specific fractions, gene expression, cell viability or metabolic turnover when compared to control cells cultured in bioreactors without scaffolds. In conclusion, three‐dimensional perfusion bioreactor culture enables long‐term maintenance of primary human bone marrow cells, with hydroxyapatite scaffolds providing an in vivo‐like scaffold for three‐dimensional culture. Biotechnol. Bioeng. 2015;112: 801–810.

Multiscale porous ceramic scaffolds for in vitro culturing of primary human cells

Abstract An emulsion casting method was chosen for the processing of highly porous hydroxyapatite (HAp) foams to be used for the in vitro culturing of primary human liver cells. The volume fraction of heptane added to the aqueous dispersion of HAp particles could be varied to achieve a wide range of porosities (10–90 vol.-%) and pore morphologies. The emulsion derived open foam has a distinctive morphology, including a continuous large pore phase and solid struts containing smaller isolated pores. The effect of the heptane content of the slip on this sintered structure was quantified using linear intercept measurement parameters. Additionally, macropores necessary for perfusion were created using polymer tubes placed in the casting mould. When the tube spacing approached the pore size in the foam, the pore morphology was strongly affected.

Two-stage data mining for flaw identification in ceramics manufacture

Advanced ceramics are commonly manufactured by sintering high-purity powders. The design of ceramic elements is governed by its fracture strength, which is greatly influenced by microstructural flaws. Three ceramic powder preparation methods for ceramics manufacturing are considered in this paper—uniaxial pressing followed by isostatic pressing, flocculated slip casting, and dispersed slip casting. Their effects on the growth and characteristics of microstructure flaws and damage on the ceramic surface are investigated using a two-stage data-mining approach. In the first stage, digital microstructural images are mined to characterize the flaws and surface damage. In the second stage, an extreme value probability distribution is fitted using the information from stage 1. The extreme value distribution estimates large flaws which are highly correlated with subsequent fractures. Results of the two-stage data mining show that ceramic production method significantly affects flaw characteristics that, in turn, determine the ceramics’ fracture strength.

A quantitative analysis of the effect of geometric assumptions in sintering models

Abstract Simple geometric models of the three stages of sintering are used to predict the path of microstructural change—the evolution of the microstructure as a function of relative density—in terms of stereological parameters. The geometric assumptions in the models are based on those frequently used in other sintering models. Two nondimensional parameters, the ratio of the mean grain and mean void intercepts and the ratio of the solid/solid and solid/void surface areas, are introduced so that the model predictions can be compared quantitatively with experimental results, independent of the characteristic length scale. The comparison is made with two alumina powders. The results suggest that an important contributor to the inaccuracy of sintering models based on simple geometric assumptions is the failure to account for nonuniformity of particle packing.