Abstract
Sintering under pressure by means of the spark plasma sintering (SPS) technique is a common route to reduce the sintering temperature and to achieve ceramics with a fine-grained microstructure. In this work, high-density bulk TiO2 was sintered by high pressure SPS. It is shown that by applying high pressure during the SPS process (76 to 400 MPa), densification and phase transition start at lower temperature and are accelerated. Thus, it is possible to dissociate the two densification steps (anatase then rutile) and the transition phase during the sintering cycle. Regardless of the applied pressure, grain growth occurs during the final stage of the sintering process. However, twinning of the grains induced by the phase transition is enhanced under high pressure resulting in a reduction in the crystallite size.
Highlights
Nb-doped Titanium dioxide (Nb-TiO2 ) is a promising cheap, chemically stable and nontoxic transition metal oxide for high temperature thermoelectric (TE) devices working in oxidizing environments
To study the influence of pressure on the densification and the phase transition phenomena, four TiO2 :2%Nb pellets were sintered by spark plasma sintering (SPS) at 76, 200, 300 and 400 MPa at a constant temperature of 700 ◦ C according to the thermal cycle described in the experimental part
It can be observed from X-ray diffraction (XRD) profiles (Figure 2) that the phase transition shifted to lower temperatures as the pressure increased
Summary
Nb-doped Titanium dioxide (Nb-TiO2 ) is a promising cheap, chemically stable and nontoxic transition metal oxide for high temperature thermoelectric (TE) devices working in oxidizing environments. A good TE efficiency requires materials with low thermal conductivity and high electrical conductivity. While a small amount of Nb (2–4 at.%) increases the electrical conductivity of TiO2 by several orders of magnitude, it has been recently shown that the thermal conductivity of bulk nano-structured Nb-TiO2 obtained by spark plasma sintering (SPS) is significantly lowered by decreasing the average grain size [1]. SPS has proven its efficiency in fast densification with limiting grain growth [2,3]. This work provides a detailed investigation of the sintering mechanism of Nb-TiO2 during the SPS process and of the effect of the high pressure with the aim of identifying the parameters that control the average grain size in the nano-structured materials
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