Robert C. Pullar

Crystallisation of hexagonal M ferrites from a stoichiometric sol–gel precursor, without formation of the α-BaFe2O4 intermediate phase

Abstract Single phase BaM (BaFe 12 O 19 ) and SrM (SrFe 12 O 19 ) ferrites were made from stoichiometric aqueous sol–gel precursors, and two different types of precursor sol were used, containing either halide or nitrate anions. It was found that halides were retained in the ferrite precursors to over 900 °C and delayed the formation of the M ferrite phase. This resulted in the formation of M ferrite at a temperature up to 200 °C higher for the halide-derived samples, compared to the nitrate-derived samples. Single phase halide-derived BaM and SrM were formed at 1000 and 850 °C, respectively, while the nitrate-derived precursors formed single phase BaM and SrM at 750 and 700 °C, respectively. No α-BaFe 2 O 4 , γ-Fe 2 O 3 or any other ferrite intermediate was observed in the BaM precursor powder, the M ferrite forming directly from α-Fe 2 O 3 and barium salts. This indicated that the formation of α-BaFe 2 O 4 or γ-Fe 2 O 3 is not necessary for the formation of BaM ferrite.

The manufacture and characterisation of aligned fibres of the ferroxplana ferrites Co2Z, 0.67% CaO-doped Co2Z, Co2Y and Co2W

Abstract Gel fibres of Co 2 Z, 0.67% CaO-doped Co 2 Z, Co 2 Y and Co 2 W ferrite were blow spun from an aqueous inorganic sol and collected as an aligned tow blanket, with an alignment comparable to that found in commercial fibres. The fibres were then heated to produce the desired ceramic phases, characterised by various techniques and their ferromagnetic resonance spectra measured. Single phase Co 2 Z was found by X-ray diffraction to form at a relatively low temperature of between 1200°C and 1250°C, and the material exhibited the expected microwave properties. Furthermore, an addition of 0.67% CaO was found to promote the formation of Co 2 Z at an even lower temperature of below 1200°C and delay the exaggerated platy grain growth, which is normally encountered at the onset of formation of the Co 2 Z phase and which results in a mechanically weakened fibre.

Extraction and characterisation of apatite- and tricalcium phosphate-based materials from cod fish bones

Apatite- and tricalcium phosphate-based materials were produced from codfish bones, thus converting a waste by-product from the food industry into high added-valued compounds. The bones were annealed at temperatures between 900 and 1200 °C, giving a biphasic material of hydroxyapatite and tricalcium phosphate (Ca10(PO4)6(OH)2 and β-Ca(PO4)3) with a molar proportion of 75:25, a material widely used in biomedical implants. The treatment of the bones in solution prior to their annealing changed the composition of the material. Single phase hydroxyapatite, chlorapatite (Ca10(PO4)6Cl2) and fluorapatite (Ca10(PO4)6F2) were obtained using CaCl2 and NaF solutions, respectively. The samples were analysed by several techniques (X-ray diffraction, infrared spectroscopy, scanning electron microscopy and differential thermal/thermogravimetric analysis) and by elemental analyses, to have a more complete understanding of the conversion process. Such compositional modifications have never been performed before for these materials of natural origin to tailor the relative concentrations of elements. This paper shows the great potential for the conversion of this by-product into highly valuable compounds for biomedical applications, using a simple and effective valorisation process.

Temperature compensated niobate microwave ceramics with the columbite structure, M2+Nb2O6

Abstract Of the niobate ceramics with the formula M 2+ Nb 2 O 6 , several compounds (M 2+ =Zn, Mg, Ca and Co) have Qf values (at f =1–10 GHz) between 40 000 and 90 000 GHz, offering potential in dielectric resonator applications. However, their temperature coefficient of resonant frequency ( τ f ) values are too high for commercial development, at between −50 and −90 ppm. This paper details the doping of these materials with dielectric ceramics having a large positive τ f , in an attempt to reduce the overall τ f to zero, whilst maintaining a high quality factor ( Q ). It was also found that doping increased the relative permitivity ( e r ) of the niobates. Several materials have been made with near-zero τ f , such as 90% CoNb 2 O 6 /10% CaTiO 3 ( τ f =+2.0 ppm, e r =25.2 and Qf =21 700 GHz), and 94% CoNb 2 O 6 /6% TiO 2 ( τ f =+4.4 ppm, e r =29.6 and Qf =20 300 GHz)

The manufacture of yttrium aluminium garnet (YAG) fibres by blow spinning from a sol-gel precursor

Abstract Three different systems consisting of a yttria sol and alumina sols were investigated, and gel fibres were successfully spun which on subsequent heating produced ceramic fibres of yttrium aluminium garnet (YAG). The fibres were characterised by various techniques. The fibres were found to begin forming YAG between 700–750 °C and it was present as a single phase between 750–800 °C, the lowest reported temperature for the crystallisation of this material. The fibres had no discernible grain structure and the average crystallite size was calculated to be only 20 nm at this temperature; however, the fibres were estimated to remain 20% porous up to 1200 °C. The fibres were handleable after YAG crystallisation, but more investigation is required into sintering and the development of high temperature properties.

Novel aqueous sol–gel preparation and characterization of barium M ferrite, BaFe12O19 fibres

Gel fibres of barium M ferrite, BaFe12O19, were blow spun from an aqueous inorganic sol and calcined at temperatures up to 1200°C. The ceramic fibres were shown by X-ray diffraction to be single phase crystalline M ferrite at 1000°C, and surface area and porosity measurements indicated an unusually high degree of sintering at this temperature. The fibres also demonstrated a favourable grain structure of less than 0.1 μm at this temperature and maintained a small grain size of less than 4 μm even up to 1200°C, an important factor in the magnetic properties of this material.

Structure and microwave dielectric properties of La(Mg0.5Ti0.5)O3-CaTiO3 system

(1-x)La(Mg 0.5 Ti 0.5 )O 3 (LMT)-xCaTiO 3 (CT) [0<x<1] ceramics were prepared from powder obtained by a nonconventional chemical route based on the Pechini method. The crystal structure of the microwave dielectric ceramics has been refined by Rietveld method using X-ray powder diffraction data. LMT and CT were found to form a solid solution over the whole compositional range. The 0.9LMT-0.1CT composition was refined using P2 1 /n space group, which allows taking into account B-site ordering. The compounds having x≥0.3 were found to be disordered and were refined using Pbnm space group. Microstructure evolution was also analysed. Dielectric characterization at microwave frequencies was performed on the LMT CT ceramics. The permittivity and the temperature coefficient of resonant frequency of the solid solutions showed a non-linear variation with composition. The quality factor demonstrates a considerable decrease with the increase of CT content.

Molecular modeling of the piezoelectric effect in the ferroelectric polymer poly(vinylidene fluoride) (PVDF)

In this work, computational molecular modeling and exploration was applied to study the nature of the negative piezoelectric effect in the ferroelectric polymer polyvinylidene fluoride (PVDF), and the results confirmed by actual nanoscale measurements. First principle calculations were employed, using various quantum-chemical methods (QM), including semi-empirical (PM3) and various density functional theory (DFT) approaches, and in addition combined with molecular mechanics (MM) methods in complex joint approaches (QM/MM). Both PVDF molecular chains and a unit cell of crystalline β-phase PVDF were modeled. This computational molecular exploration clearly shows that the nature of the so-called negative piezo-electric effect in the ferroelectric PVDF polymer has a self-consistent quantum nature, and is related to the redistribution of the electron molecular orbitals (wave functions), leading to the shifting of atomic nuclei and reorganization of all total charges to the new, energetically optimal positions, under an applied electrical field. Molecular modeling and first principles calculations show that the piezoelectric coefficient d33 has a negative sign, and its average values lies in the range of d33 ~ −16.6 to −19.2 pC/N (or pm/V) (for dielectric permittivity ε = 5) and in the range of d33 ~ −33.5 to −38.5 pC/N (or pm/V) (for ε = 10), corresponding to known data, and allowing us to explain the reasons for the negative sign of the piezo-response. We found that when a field is applied perpendicular to the PVDF chain length, as polarization increases the chain also stretches, increasing its length and reducing its height. For computed value of ε ~ 5 we obtained a value of d31 ~ +15.5 pC/N with a positive sign. This computational study is corroborated by measured nanoscale data obtained by atomic force and piezo-response force microscopy (AFM/PFM). This study could be useful as a basis for further insights into other organic and molecular ferroelectrics.

Calcium phosphate-based materials of natural origin showing photocatalytic activity

Calcium phosphate based materials of natural origin with photocatalytic properties were produced. Bones of Atlantic cod fish were treated in appropriate solutions (either Ca- or Ti-containing salts) and successively annealed. Results showed multiphasic materials (hydroxyapatite, β-tricalcium phosphate and anatase titania) with excellent photocatalytic performance under both UV and visible light, with an anatase concentration of only about 2 mol%. Results with such a low amount of anatase have never been reported before for a calcium phosphate-based material; this is attributed to the presence of anatase being mainly on the surface. Single-phase hydroxyapatite (Ca10(PO4)6(OH)2 or HAp) also showed some photocatalytic properties and antibacterial activity.

Local probing of magnetoelectric coupling in multiferroic composites of BaFe12O19–BaTiO3

The BaFe12O19–BaTiO3 composite ferroelectric/ferromagnetic ceramics were prepared by conventional solid-state sintering technique. The magnetic properties are consistent with the ratio of the magnetic phase present but ferroelectric properties are degraded due to a sufficiently high degree of conductivity in the ceramics. Magnetoelectric coupling was observed at a local level by means of the scanning probe microscopy (SPM). Piezoresponse and magnetic force modes of SPM were both utilized to assess strain-mediated magnetoelectric coupling between neighboring grains. The observed variation in the magnetic signal after the electrical poling with SPM was attested to the changes in the magnetic interactions and magnetic anisotropy leading to broadening of the magnetic domain wall.