An Hardy

Study of the decomposition of an aqueous metal–chelate gel precursor for (Bi,La)4Ti3O12 by means of TGA–FTIR, TGA–MS and HT-DRIFT

Abstract A (Bi,La)4Ti3O12 acetate–citrate gel, prepared from an aqueous solution, was subjected to thermal decomposition in dry air and in inert atmosphere. In order to acquire the insight into its decomposition pathway, several hyphenated thermal analysis techniques were used. The evolved gases were characterized by TGA–MS and TGA–FTIR, while any changes in the solid phase upon heating were detected by means of HT-DRIFT. In dry air, the decomposition consists of four steps, centered at 80, 190, 340 and 455xa0°C. After drying of the sample in the first step, the second step is ascribed to the decomposition of ammonium acetate and citrate. Any metal acetates present in the gel would also decompose here. The third step comprises the decomposition/combustion of the citrate ligands. The last step consists of the combustion of all residual organic matter. In the third and fourth step, O2 plays an important role. XRD has proven that the desired Aurivillius oxide phase forms from the synthesized gel after an appropriate heat treatment in dry air. In inert atmosphere six steps can be distinguished, situated at 70, 200, 330, 420, 590 and 910xa0°C. The steps at 70, 200 and 330xa0°C correspond to the steps at 80, 190 and 340xa0°C in dry air. It is proposed that the steps above 400xa0°C correspond to the gradual decomposition of the residual organic matter. It has turned out to be impossible to remove all organic matter from the gel below 700xa0°C in inert atmosphere.

Synthesis of (Bi, La)4Ti3O12 by a new aqueous solution-gel route

Abstract A new aqueous solution-gel route for the synthesis of ferroelectric lanthanum substituted bismuth titanate was developed, based on aqueous Bi 3+ - and La 3+ -citrate solutions and an aqueous peroxocitrato-Ti(IV) solution. A homogeneous, amorphous precursor gel was prepared by evaporation of the BLT precursor solution. The homogeneity was maintained throughout the entire thermo-oxidative decomposition of the gel, as shown by TEM measurements on freestanding thin films. This molecular scale mixing and high degree of homogeneity led to the direct crystallization of the Aurivillius phase from the amorphous phase. As a consequence the crystallization temperature was as low as 525xa0°C, which was shown by a high temperature X-ray diffraction study of the stoichiometric precursor. The addition of an excess of Bi 3+ decreased the crystallization temperature, but also introduced a Bi 2 O 3 secondary phase.

Ti surface doping of LiNi0.5Mn1.5O4−δ positive electrodes for lithium ion batteries

The particle surface of LiNi0.5Mn1.5O4−δ (LNMO), a Li-ion battery cathode material, has been modified by Ti cation doping through a hydrolysis–condensation reaction followed by annealing in oxygen. The effect of different annealing temperatures (500–850 °C) on the Ti distribution and electrochemical performance of the surface modified LNMO was investigated. Ti cations diffuse from the preformed amorphous ‘TiOx’ layer into the LNMO surface during annealing at 500 °C. This results in a 2–4 nm thick Ti-rich spinel surface having lower Mn and Ni content compared to the core of the LNMO particles, which was observed with scanning transmission electron microscopy coupled with compositional EDX mapping. An increase in the annealing temperature promotes the formation of a Ti bulk doped LiNi(0.5−w)Mn(1.5+w)−tTitO4 phase and Ti-rich LiNi0.5Mn1.5−yTiyO4 segregates above 750 °C. Fourier-transform infrared spectrometry indicates increasing Ni–Mn ordering with annealing temperature, for both bare and surface modified LNMO. Ti surface modified LNMO annealed at 500 °C shows a superior cyclic stability, coulombic efficiency and rate performance compared to bare LNMO annealed at 500 °C when cycled at 3.4–4.9 V vs. Li/Li+. The improvements are probably due to suppressed Ni and Mn dissolution with Ti surface doping.

Properties and thermal stability of solution processed ultrathin, high-**k** bismuth titanate ( Bi_{2}Ti_{2}O_{7} ) films

Abstract Ultrathin bismuth titanate films (Bi 2 Ti 2 O 7 , 5–25xa0nm) are deposited onto SiO 2 /Si substrates by aqueous chemical solution deposition and their evolution during annealing is studied. The films crystallize into a preferentially oriented, pure pyrochlore phase between 500 and 700xa0°C, depending on the film thickness and the total thermal budget. Crystallization causes a strong increase of surface roughness compared to amorphous films. An increase of the interfacial layer thickness is observed after anneal at 600xa0°C, together with intermixing of bismuth with the substrate as shown by TEM-EDX. The band gap was determined to be ∼3xa0eV from photoconductivity measurements and high dielectric constants between 30 and 130 were determined from capacitance voltage measurements, depending on the processing conditions.