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Fostering the properties of Zr_0.8Sn_0.2TiO₄ (ZST) ceramics via freeze granulation without sintering additives

S. M. Olhero,   Ajay Kaushal and   J. M. F. Ferreira

The present paper reports the overall benefits of freeze granulation for enhancing the properties of zirconium tin titanate Zr_0.8Sn_0.2TiO₄ (ZST) ceramics in the total absence of sintering additives. The ZST powder was synthesized by solid state reaction and attrition milled in ethanol for 10 h. This starting non-granulated powder (NG-ZST), without and with 1 wt% ZnO as sintering additive, was used to consolidate green bodies by dry pressing. The pure ZST powder was also dispersed in aqueous media to obtain stable suspensions with high solid loadings. Free flowing spherical homogeneous granules were prepared by freeze granulation and used to consolidate ZST green bodies by dry pressing (FG-ZST). The effects of processing variables and sintering temperature (1300–1450 °C) on densification and on the structural, mechanical and electrical properties of ZST ceramics were systematically investigated. Our study clearly reveals the superior properties of FG-ZST ceramics, which derive from an enhanced sintering behaviour associated with the absence of sintering additives.

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A comparison between spray drying and spray freeze drying for dry powder inhaler formulation of drug-loaded lipid–polymer hybrid nanoparticles

Spray-freeze-drying for protein powder preparation: particle characterization and a case study with trypsinogen stability

Sonner C, Maa YF, Lee G

This work investigates the use of spray freeze-drying (SFD) to produce protein loaded particles suitable for epidermal delivery. In the first part of the study, the effects of formulation and process conditions on particle properties are examined. Aqueous solutions of trehalose produce SFD particles in the size range 20-80 microm, with a smooth, textured surface, but having high internal porosity. The latter was visualized using SEM and a novel particle embedding and sectioning technique. Use of an annealing step during the freeze-drying cycle caused the particles to shrink, reducing hereby porosity and also the measured rate of moisture uptake into these amorphous particles. SFD pure mannitol was approximately 40% amorphous, but not hygroscopic. Incorporation of dextran 37,500 into a combined amorphous trehalose/mannitol formulation led to increased particle shrinkage and lower particle porosity on annealing. The model protein trypsinogen lost approximately 15% activity during SFD of solutions containing 50 mg/mL protein, but was only marginally aggregated (1.4%). It is suggested that trypsinogen forms an irreversible partially unfolded state or molten globule on SFD/rehydration. The pure protein was also partially inactivated without aggregation during atomization into air. Surprisingly, neither activity loss nor aggregation were detected on atomization of the protein solution into liquid nitrogen. Quench-freezing of small droplets may reverse the partial unfolding of trypsinogen occurring on atomization into air. The origin of the trypsinogen inactivation during SFD must therefore be the subsequent freeze-drying step of this multistep process. Isolated freeze drying of trypsinogen produces strong aggregation and equivalent inactivation. This result suggests that trypsinogen behaves differently during freeze drying from frozen droplets and from bulk solution in a vial. In the former case the protein forms an irreversible partially unfolded state, whereas in the latter case aggregates are formed. Trypsinogen inactivation during SFD could be completely prevented by the presence of trehalose in the formulation. Electron Spectroscopy for Chemical Analysis (ESCA) showed a high surface excess of the protein in the SFD particles, which was reduced on inclusion of Polysorbate 80, but not trehalose. Taken together, these results help to elucidate the complex destabilization behavior of trypsinogen during SFD.

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Impedance analysis of 0.5Ba(Zr_0.2Ti_0.8)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ ceramics consolidated from micro-granules

Abstract

This work aims to contribute new data on the dielectric relaxation behavior of the lead-free piezoelectric 0.5Ba(Zr_0.2Ti_0.8)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ (BZT–BCT) material processed in aqueous media. To obtain a BZT–BCT ceramic with high density, an aqueous suspension of BZT–BCT material was successfully transferred to spherical granules via freeze granulation, followed by sintering of a consolidated high green dens compact at 1350 °C for 4 h. The dielectric relaxation behavior in the temperature range 300–600 °C over the frequency range of 0.1 Hz–1 MHz has been carried out to display the characteristic of dielectric relaxation behavior of the BZT–BCT ceramic. Two distinct temperature dependent peaks at characteristic frequencies were observed, which shift towards the high frequency end with increasing temperature, depicting a thermally activated relaxation phenomena in the BZT–BCT ceramic. The dependences of dielectric parameters by fitting data with Cole–Cole equations (Nyquist plot) on temperature have been discussed in detail. Relaxation time was found to decrease with increasing temperature and to obey the Arrhenius relationship. The values of calculated resistances were found to be of same order for bulk (R_b) and grain boundary (R_gb) contributions, whereas capacitance values calculated for bulk (C_b) were found to be 2 orders more than that of capacitance values calculated for grain boundaries (C_gb).

Keywords

BZT–BCT ceramics, Impedance spectroscopy, AC conductivity, Electric modulus

New approach for distribution of carbon nanotubes in alumina matrix

Keywords

• Al₂O₃;
• Carbon nanotubes;
• Freeze granulation;
• Microstructure;
• Electrical conductivity