Carlos Gómez-Yáñez

Mechanical activation of the synthesis reaction of BaTiO3 from a mixture of BaCO3 and TiO2 powders

Abstract The influence of long term milling in an attritor of a mixture of BaCO3 and TiO2 powders on the reaction synthesis of BaTiO3 was studied. Thermal analysis (TG and DTA) of the unmilled and milled powders and X-ray diffraction of powders calcined at different temperatures were undertaken. Milling does not change the reaction sequence between BaCO3 and TiO2. BaTiO3 and Ba2TiO4 are produced in both kinds of powders. However, milling reduces the formation temperature and accelerates the formation rate of these two phases. The production of final BaTiO3 is incomplete in the milled powder, probably because of the formation of big crystals of Ba2TiO4. As a consequence, the milled powder contains large amounts of Ba2TiO4 after calcination at 1200°C. Milling produces allotropic transformations in TiO2 from anatase phase, to α-PbO2-like phase and finally to rutile structure.

Analysis and perspectives concerning CO2 chemisorption on lithium ceramics using thermal analysis

CO2 removal from flue gas has been proposed as one of the most reliable solutions to mitigate global greenhouse emissions. Lithium ceramics are among several materials that have potential applications in CO2 removal. Lithium ceramics are able to chemisorb CO2 in a wide temperature range, presenting several interesting properties. All lithium ceramics present a similar CO2 chemisorption reaction mechanism that has been described at the micrometric scale. However, there are several issues that have not been fully elucidated. The aim of this study is to re-analyze different experiments related to the CO2 chemisorption on lithium ceramics and to propose how different factors control this process. This study focuses on diffusion controlled CO2 chemisorption, which has been shown to be the limiting step of the CO2 chemisorption process. Diffusion controlled CO2 chemisorption appears to be mainly influenced by the chemical composition of a product’s external shell.

Structural and Thermochemical Chemisorption of CO2 on Li4+x(Si1–xAlx)O4 and Li4–x(Si1–xVx)O4 Solid Solutions

Different Li(4)SiO(4) solid solutions containing aluminum (Li(4+x)(Si(1-x)Al(x))O(4)) or vanadium (Li(4-x)(Si(1-x)V(x))O(4)) were prepared by solid state reactions. Samples were characterized by X-ray diffraction and solid state nuclear magnetic resonance. Then, samples were tested as CO(2) captors. Characterization results show that both, aluminum and vanadium ions, occupy silicon sites into the Li(4)SiO(4) lattice. Thus, the dissolution of aluminum is compensated by Li(1+) interstitials, while the dissolution of vanadium leads to lithium vacancies formation. Finally, the CO(2) capture evaluation shows that the aluminum presence into the Li(4)SiO(4) structure highly improves the CO(2) chemisorption, and on the contrary, vanadium addition inhibits it. The differences observed between the CO(2) chemisorption processes are mainly correlated to the different lithium secondary phases produced in each case and their corresponding diffusion properties.

Colloidal processing of BaTiO3 using ammonium polyacrylate as dispersant

Abstract The variation of the ζ potential of BaTiO3 particles in aqueous suspension as a consequence of changing the pH and the concentration of ammonium polyacrylate (NH4PA) has been studied. An isoelectric point at pH ∼2.5 was found. The ζ potential decreases upon addition of NH4PA up to 0.1 g of dispersant/g of powder. Using sedimentation tests, it was found that the stabilization of suspensions is attained at pH greater than or equal to 8 and concentrations greater than or equal to 1.5×10−3 g of dispersant/g of powder. By rheological measurements, a pseudoplastic behavior and little thixotropy was observed in the suspensions. The maximum solid content attained during this work was 48 vol%. Infrared spectroscopy of the sedimented and dried powder was carried out. It was observed the presence of BaCO3 and the change of ionization of the NH4PA molecules when the pH changes. High green density values (up to ∼60% of the theoretical density) in BaTiO3 cakes were achieved at pH of 11.3, at 2.5×10−3 g of dispersant/g of powder and with a solid content between 40 and 48 vol%.

Synthesis of advanced ceramics by hydrothermal crystallization and modified related methods

The present article aims to give a brief overview about the advantages of the hydrothermal crystallization method for the synthesis of advanced ceramics. Emphasis is given, not only on the conventional hydrothermal crystallization, but also on some of its variants; such as ultrasound-assisted, electrochemical-assisted, microwave-assisted and surfactant-assisted hydrothermal methods which open up new opportunities for the synthesis of ceramic materials with novel properties demanded for advanced applications. In the current work the synthesis of barium titanate (BaTiO3), lithium metasilicate (Li2SiO3) and sodium-potassium niobate (Na, K)NbO3 powders are reported as cases of study.

Tunneling electroresistance effect in a Pt/Hf0.5Zr0.5O2/Pt structure

The present work reports the fabrication of a ferroelectric tunnel junction based on a CMOS compatible 2.8 nm-thick Hf0.5Zr0.5O2 tunnel barrier. It presents a comprehensive study of the electronic properties of the Pt/Hf0.5Zr0.5O2/Pt system by X-ray photoelectron and UV-Visible spectroscopies. Furthermore, two different scanning probe techniques (Piezoresponse Force Microscopy and conductive-AFM) were used to demonstrate the ferroelectric behavior of the ultrathin Hf0.5Zr0.5O2 layer as well as the typical current-voltage characteristic of a ferroelectric tunnel junction device. Finally, a direct tunneling model across symmetric barriers was used to correlate electronic and electric transport properties of the ferroelectric tunnel junction system, demonstrating a large tunnel electroresistance effect with a tunneling electroresistance effect ratio of 20.

A Complementary Metal Oxide Semiconductor Process-Compatible Ferroelectric Tunnel Junction

In recent years, experimental demonstration of ferroelectric tunnel junctions (FTJ) based on perovskite tunnel barriers has been reported. However, integrating these perovskite materials into conventional silicon memory technology remains challenging due to their lack of compatibility with the complementary metal oxide semiconductor process (CMOS). This communication reports the fabrication of an FTJ based on a CMOS-compatible tunnel barrier Hf0.5Zr0.5O2 (6 unit cells thick) on an equally CMOS-compatible TiN electrode. Analysis of the FTJ by grazing angle incidence X-ray diffraction confirmed the formation of the noncentrosymmetric orthorhombic phase (Pbc21, ferroelectric phase). The FTJ characterization is followed by the reconstruction of the electrostatic potential profile in the as-grown TiN/Hf0.5Zr0.5O2/Pt heterostructure. A direct tunneling current model across a trapezoidal barrier was used to correlate the electronic and electrical properties of our FTJ devices. The good agreement between the experimental and theoretical model attests to the tunneling electroresistance effect (TER) in our FTJ device. A TER ratio of ∼15 was calculated for the present FTJ device at low read voltage (+0.2 V). This study suggests that Hf0.5Zr0.5O2 is a promising candidate for integration into conventional Si memory technology.