טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentOstrovsky Avshalom Ze'ev
SubjectA New Method of Growing Pyroelectric Mg3B7O13Cl
Nano-Crystals with Preferred Crystallographic
Orientations
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Shlomo Berger
Full Thesis textFull thesis text - English Version


Abstract

Pyroelectric (hereafter PE) materials have a spontaneous electrical polarization and are used for a large variety of applications. The PE effect relates between the temperature and the polarization of the PE crystal, such that temperature modulation results in electrical current. PE devices are characterized by their respective figure of merit (FM) - the quotient of the pyroelectric coefficient to the dielectric permittivity. The PE device is better as its FM is higher.

Boracite (Mg3B7O13Cl) is a natural and rare water-insoluble mineral. Below 265ᵒC it has an orthorhombic ferroelectric (and thus PE) phase with a spontaneous polarization along [001]. At room temperature the bulk Boracite has a moderate pyroelectric coefficient (2.5 [μC•m-2•K-1]) and a low average dielectric permittivity (7.5 at 100 kHz), which results in a FM value of 35 [kV•m-1•K-1]. The dielectric and PE properties of the Boracite at the nanoscale range have not been reported yet.

This research reports, for the first time, on the growth of Boracite nano-crystals from aqueous solutions at low temperatures with preferred crystallographic orientations. The crystals are grown inside alumina pores having a diameter of about 52 nanometers, length of about 50 microns and surface density of 1011 [pores/cm2]. The porous alumina layer is prepared by the electrochemical anodization of aluminum in an oxalic acid liquid solution. The Boracite crystals are grown inside the porous alumina from aqueous solution of magnesium chloride hexahydrate and boric acid, in various molar ratios, below the bulk saturation limits of these materials. Two Boracite preparation conditions are reported in this work, PAA-B1 (molar ratio Mg/B = 0.43) and PAA-B2 (Mg/B=3.11).

The formation mechanism of the Boracite crystals inside the alumina nanopores with preferred crystallographic orientations along the longitudinal axis of the pores is explained thermodynamically by a three-step mechanism: (i) precipitation of solute ions from supersaturated liquid solution near the bottom of the pores, (ii) nucleation of the Boracite phase at these sites and (iii) its preferred growth along the longitudinal axis of the pores directed by the negatively charged alumina walls.

The dielectric and pyroelectric characteristics of the aforesaid nano-composites are reported, in which the pyroelectric coefficient increases by 2 orders of magnitude (250.6 ± 18.1 [μC•m-2•K-1] in PAA-B1 and 229.1 ± 11.9 [μC•m-2•K-1] in PAA-B2), and a FM values of up to 3774 [kV•m-1•K-1] and 3450 [kV•m-1•K-1] are received in PAA-B1 and PAA-B2, respectively.