Cengiz Kaya

Nanostructured ceramic powders by hydrothermal synthesis and their applications

Abstract Nanoparticles of 3:2 mullite (3Al2O3·2SiO2)+5 wt.% zirconia, boehmite (γ-AlOOH) and pure ZrO2 suitable for applications in the field of monolithic/fibre reinforced ceramic composites, nanoceramics and ceramic fibre coating, respectively, have been produced by hydrothermal synthesis (HS) at moderately low temperatures. The effect of the process parameters or starting precursor materials properties on the particle size, shape and structure have been examined by particle size measurements, TEM and scanning electron microscopy. The mullite powders synthesised tend to be highly irregular in morphology with a wide particle size distribution from 55 nm to 7.1 μm, whereas the boehmite (γ-AlOOH) particle morphology and size are determined by the pH value of the initial hydrothermal solution. As the pH changes from low (acid) to high (alkaline), the boehmite morphology changes from a “needle-like” one to a “platelet” one. At a pH value of 10, the boehmite particles exhibit platelet morphologies, with a maximum face dimension of about 40 nm and a thickness of approximately 5 nm. The synthesised boehmite particles were successfully used to produce monolithic alumina components at lower sintering temperatures with sub-micrometer grain sizes. Near-spherical shape monoclinic zirconia particles with dimensions in the range of 20–60 nm were also produced by HS. The successful application of the synthesised powders in the field of mesoporous fibre-reinforced ceramic composites, high-density monolithic ceramics and ceramic fibre coatings (formation of weak inter-phase between fibre and matrix) are also presented.

Use of electrophoretic deposition in the processing of fibre reinforced ceramic and glass matrix composites: a review

Abstract Electrophoretic deposition (EPD) is a simple and cost-effective method for fabricating high-quality ‘green’ composite bodies which, after a suitable high-temperature treatment, can be densified to a composite with improved properties. In this contribution, we describe the use of EPD technique in the fabrication of fibre reinforced composites, with an emphasis on composites with glass and ceramic matrices containing metallic or ceramic fibre fabric reinforcement. EPD has been used to infiltrate preforms with tight fibre weave architectures using different nanosized ceramic particles, including silica and boehmite sols, as well as dual-component sols of mullite composition. The principles of the EPD technique are briefly explained and the different factors affecting the EPD behaviour of ceramic sols and their optimisation to obtain high infiltration of the fibre preforms are considered. In particular, the EPD fabrication of a model alumina matrix composite reinforced by Ni-coated carbon fibres is presented. The pH of the solution and the applied voltage and deposition time are shown to have a strong influence on the quality of the infiltration. Good particle packing and a high solids-loading were achieved in most cases, producing a firm ceramic deposit which adhered to the fibres. Overall, the analysis of the published data and our own results demonstrate that EPD, being simple and inexpensive, provides an attractive alternative for ceramic infiltration and coating of fibre fabrics, even if they exhibit tight fibre weave architectures. The high-quality infiltrated fibre mats are suitable prepregs for the fabrication of advanced glass and ceramic matrix composites for use in heat-resistant, structural components.

Antileishmanial effect of silver nanoparticles and their enhanced antiparasitic activity under ultraviolet light

Leishmaniasis is a protozoan vector-borne disease and is one of the biggest health problems of the world. Antileishmanial drugs have disadvantages such as toxicity and the recent development of resistance. One of the best-known mechanisms of the antibacterial effects of silver nanoparticles (Ag-NPs) is the production of reactive oxygen species to which Leishmania parasites are very sensitive. So far no information about the effects of Ag-NPs on Leishmania tropica parasites, the causative agent of leishmaniasis, exists in the literature. The aim of this study was to investigate the effects of Ag-NPs on biological parameters of L. tropica such as morphology, metabolic activity, proliferation, infectivity, and survival in host cells, in vitro. Consequently, parasite morphology and infectivity were impaired in comparison with the control. Also, enhanced effects of Ag-NPs were demonstrated on the morphology and infectivity of parasites under ultraviolet (UV) light. Ag-NPs demonstrated significant antileishmanial effects by inhibiting the proliferation and metabolic activity of promastigotes by 1.5- to threefold, respectively, in the dark, and 2- to 6.5-fold, respectively, under UV light. Of note, Ag-NPs inhibited the survival of amastigotes in host cells, and this effect was more significant in the presence of UV light. Thus, for the first time the antileishmanial effects of Ag-NPs on L. tropica parasites were demonstrated along with the enhanced antimicrobial activity of Ag-NPs under UV light. Determination of the antileishmanial effects of Ag-NPs is very important for the further development of new compounds containing nanoparticles in leishmaniasis treatment.

Fabrication and characterisation of Ni-coated carbon fibre-reinforced alumina ceramic matrix composites using electrophoretic deposition

The present study explores the feasibility of fabricating Ni-coated carbon fibre-reinforced alumina ceramic matrix composites via a single-infiltration electrophoretic deposition (EPD) process performed in vacuum. The nano-size boehmite sol was seeded using nano-size δ-alumina powder in order to control the final sintered microstructure and then characterised using transmission electron microscopy, differential thermal and thermogravimetric analysis (DTA/TG) and X-ray disc centrifuge system (BI-XDC) in order to determine the sol microstructure, phase transformation temperatures and particle size (also degree of agglomeration), respectively. An EPD manufacturing cell for fabrication of Ni-coated carbon fibre reinforced alumina matrix composites was designed and experiments were conducted under vacuum (first time to date), resulting in full deposition of the sol material throughout the voids within/between the fibre tows. Composites with high green density (67% theoretical density) were produced using an applied voltage of 15 V d.c. and deposition time of 400 s. The sintered density after pressureless sintering at 1250°C for 2 h was 91% theoretical density. Crack path propagation test showed that the metallic Ni coating was able to provide a weak interface, as an indenter induced crack within the alumina matrix was deflected and arrested at the Ni interface.

Investigation of antileishmanial activities of Tio2@Ag nanoparticles on biological properties of L. tropica and L. infantum parasites, in vitro.

Leishmaniasis is a public health problem which is caused by protozoon parasites belonging to Leishmania species. The disease threatens approximately 350 million people in 98 countries all over the world. Cutaneous Leishmaniasis (CL) and Visceral Leishmaniasis (VL) are the mostly commonly seen forms of the disease. Treatment of the disease has remained insufficient since current antileishmanial drugs have several disadvantages such as toxicity, costliness and drug-resistance. Therefore, there is an immediate need to search for new antileishmanial compounds. TiO2@Ag nanoparticles (TiAg-Nps) have been demonstrated as promising antimicrobial agents since they provide inhibition of several types of bacteria. The basic antimicrobial mechanism of TiAg-Nps is the generation of reactive oxygen species (ROS). Even though Leishmania parasites are sensitive to ROS, there is no study in literature indicating antileishmanial activities of TiAg-Nps. Herein, in this study, TiAg-Nps are shown to possess antileishmanial effects on Leishmania tropica and Leishmania infantum parasites by inhibiting their biological properties such as viability, metabolic activity, and survival within host cells both in the dark and under visible light. The results indicate that TiAg-Nps decreased viability values of L. tropica, and L. infantum promastigotes 3- and 10-fold, respectively, in the dark, while these rates diminished approximately 20-fold for each species in the presence of visible light, in contrast to control. On the other hand, non-visible light-exposed TiAg-Nps inhibited survival of amastigotes nearly 2- and 2.5-fold; while visible light-exposed TiAg-Nps inhibited 4- and 4.5-fold for L. tropica and L. infantum parasites, respectively. Consequently, it was determined that non-visible light-exposed TiAg-Nps were more effective against L. infantum parasites while visible light-exposed TiAg-Nps exhibited nearly the same antileishmanial effect against both species. Therefore, we think that a combination of TiAg-Nps and visible light can be further used for treatment of CL, while application of TiAg-Nps alone can be a promising alternative in VL treatment.

Al2O3–Y-TZP/Al2O3 functionally graded composites of tubular shape from nano-sols using double-step electrophoretic deposition

Abstract Al2O3–Y-TZP/Al2O3 functionally graded composites of tubular shape incorporating a very tough central layer with graded composition (Al2O3–Y-TZP) and a hard outer surface layer of pure alumina were produced from nano-size sols using electrophoretic deposition (EPD) in an attempt to generate a continuously inhomogeneous property variation across the final component and to control the microstructure at a nanometer scale. It is shown that hardness, fracture toughness and alumina grain size within the graded layer are controlled by the volume fraction of TZP grains and the highest volume fraction (71%) of TZP phase provides a fracture toughness value of 7.1 MPa m1/2 and Vickers hardness of 10.4 GPa whilst the lowest volume fraction (13%) results in obtaining a fracture toughness value of 3.8 MPa m1/2 and hardness value of 15.7 GPa. The pure alumina surface layer (100 μm in thickness) with a high hardness value of 19.4 GPa is considered to be beneficial for tribological applications where high wear resistance is required.

Microstructural development of woven mullite fibre-reinforced mullite ceramic matrix composites by infiltration processing

Abstract Mullite fibre (Nextel 720™)-reinforced mullite ceramic matrix composites (CMCs) with zirconia weak interface were fabricated from heterocoagulated nano-size boehmite/amorphous silica powder particles dispersed in water, using electrophoretic deposition (EPD) and pressure filtration (PF). The nano-size mullite precursor was first prepared and characterised in terms of short-range particle–particle interactions and particle size distribution. Woven Nextel 720 mullite fibres were first desized and then coated with hydrothermally derived zirconia using dip-coating. EPD was performed under constant voltage conditions with varying deposition times, to infiltrate the dispersed powder suspensions into mullite fibre preforms, enabling the parameters necessary for good deposition of stoichiometric mullite to be established. EPD formed bodies were further consolitated using PF. The EPD/PF prepared green body specimens were dried under controlled atmosphere conditions before being sintered at 1200°C for 2 h in air. Mullite fibre mats were fully infiltrated using EPD parameters of 12 V DC applied voltage with 4 min deposition time, then eight EPD infiltrated fibre mats were further consolidated together using PF. The resulting CMC produced contained 35 vol% fibre loading and showed 81% theoretical density aftersintering at 1200°C for 2 h.

Co-extrusion of Al2O3/ZrO2 bi-phase high temperature ceramics with fine scale aligned microstructures

Abstract Structural ceramic composites comprising continuous fibrillar microstructure are produced using sol-based technology which involves the extrusion, at room temperature, of a two-phase material (Al 2 O 3 /ZrO 2 ) resulting in an aligned bi-phase structure which is then multiple co-extruded to reduce the lateral dimensions of the phases. Two sol-derived pastes of differing chemistry (γ-AlOOH as alumina source and zirconia) are co-extruded in parallel, and layed-up in closed-packed linear array to form a heterogeneous macro-plug for subsequent extrusion with or without zirconia coating. The second and third extrusion steps produce a filament with markedly reduced lateral paste dimensions provided that the flow properties of the chemically different pastes are similar. The resulting extrudates in the form of continuous green monofilaments, are subsequently laid up in a mould where the structure is pressed and consolidated in desired shape, then pressureless sintered in air to form the multi-phase component. The developed process allows the microstructure to be controlled at a nanometer scale within each extruded filament and after the 3rd stage co-extrusion, each filament size within the final extrudate is reduced to ≈ 65 μm.

Electrophoretic deposition infiltration of 2-D metal fibre-reinforced cordierite matrix composites of tubular shape

Stainless steel (316L) fibre mats shaped into tubular geometry were used to reinforce cordierite. The ductile phase-reinforced cordierite matrix composites were manufactured by using electrophoretic deposition (EPD) and pressureless sintering. An EPD cell suitable for the fabrication of tubular composites was designed. The relevant process parameters required to infiltrate the fibre mats with nanosized cordierite powders and to obtain homogeneous electrophoretic cordierite deposits on the inner and outer surfaces of the fibrous substrate were optimised. EPD experiments were conducted under constant voltage conditions (5 V dc) with varying deposition times. The sintered composites having internal and external deposit thickness of about 1 mm were free of surface cracks when a deposition time of 2.5 min was used. The developed metal fibre reinforced cordierite composites may constitute a promising alternative for manufacturing damage-tolerant tubular components for applications at intermediate-temperatures (up to ∼900°C).

Zirconia-toughened alumina ceramics of helical spring shape with improved properties from extruded sol-derived pastes

High strength (1270 MPa) and tough (>9 MPa m1/2) zirconia-toughened alumina (ZTA) ceramics with a helical spring shape were produced from sol-derived pastes comprising a mixture of boehmite (γ-AlOOH) and nano-size zirconia powders using extrusion. It is shown that the mechanical response of the extruded ZTA ceramics can be controlled/altered by manipulating the microstructure.