Towards a more environmentally benign synthesis of barium-based perovskites

and doped barium titanate

Danielle Harlan ’07

Jonathan Martin ’07

Samantha DeCarlo ’10

Heather Sheridan ’10

Anne Marteel-Parrish, Ph.D.

Outline

• Background on perovskites

• First goal: Benign by design synthesis of barium-based perovskites

• Achieving first goal

• Results

• Second goal: Environmentally benign doping of barium-based oxides

• Previous methods of doping

• Achieving second goal

• Results

• Conclusions

• Crystalline, lattice structure

• Molecular formula: ABO3 (A2+ and B4+)

• Ideal perovskite structure: cubic (insulator)

• Under specific conditions: tetragonal (capacitor and transducer)

• Both have applications in the electroceramic industry

• Multibillion-dollar global business

Background on perovskites

Background on perovskites

• BaTiO3: used in multi-layered ceramic capacitors in computers, aerospace, and communication technologies

• BaZrO3: used in superconducting applications; one of the most inert, stable, and corrosion-resistant perovskite

• BaSnO3: used in ceramic dielectric bodies to prepare thermally stable capacitors, and to fabricate ceramic boundary layer capacitors when combined with BaTiO3

• BaHfO3: used in optical coatings, and as a high-k dielectric in dynamic random access memory capacitors

First goal: Benign by design synthesis of barium-based perovskites

• Previous methods: solid-state method, microwave-assisted technique, low-temperature aqueous preparation, spray co-precipitation

• Poor stoichiometric control

• Uneconomical and environmentally unfriendly reaction conditions and starting materials

• Application of the catecholate method• Environmentally benign precursors

• Smaller quantities of solvents

• Stoichiometric control over Ba:Ti ratio

• Absence of byproducts

Achieving first goal

XO2H2SO4 / (NH4)2SO4

Δ

Acidic X solution H2O Dilute X solution

C6H4O2

(cat)+

NH4OH[NH4]n[X(cat)m]zH2O

Ba(OH)2·8H20Ba[X(cat)m]zH2O

BaXO3

Δ

Reference: Davies, J.A.; Dutremez, S.J. J. Am. Ceram, Soc. 1990, 73, 1429-1430.

Results

• Synthesis of barium zirconate and barium hafnate– When X = Zr or Hf, there were no difficulties in the

completion of the 5-step synthesis. However, low yields were obtained once the powders were calcinated in the tube furnace at 700 °C.

– IR spectroscopy and X-ray powder diffraction confirmed the presence of the expected products

• Synthesis of barium stannate– When X = Sn, the preparation of the Sn(IV) acidic

solution was unsuccessful.

Results

• Application of some of the 12 principles of green chemistry

• Principle 3: less hazardous chemical synthesis

• Principle 5: safer solvents

• Principle 6: design for energy efficiency

• Principle 8: reduce derivatives

• Principle 11: real-time analysis

Second goal: Environmentally benign doping of barium-based oxides

• Doping:– Replacing a small quantity of atoms in a structure with

atoms of a compatible element– Is a cation exchange

• Curie Temperature (Tc):

– The temperature at which structure, polarity, and electric properties of a perovskite transform

– In the case of BaTiO3:Tc 120 °C

• Doping a perovskite ultimately lowers the Curie temperature

Ideal structure: Cubic, non-polar, paraelectric, large dielectric constant

Tetragonal, polar, ferroelectric

Tc > 120 °C 5 °C < Tc < 120 °C

Second goal: Environmentally benign doping of barium-based oxides

Previous methods of doping

• “Sol-gel process and microwave sintering”

• “Wet chemical synthesis technique”

• “Polymeric citrate precursor method”

• “Microwave-hydrothermal route”

• “Simple direct precipitation method”

Previous methods of doping

• Major disadvantages:– Poor stoichiometric control

• BaxSryTiO3

• Require post-treatments

– Uneconomical and environmentally unfriendly• Low yields• Expensive and toxic precursors• High temperatures• High pressures• Handling, storage, and disposal of toxic precursors

Achieving second goal

• Develop a method in which the Ba : Sr ratio is under thermodynamic control in the presence of excess reagent

• Exposure of a solution of BaX to an excess of solid strontium salt, SrX– Cation exchange whose extent is governed by the ratio of

solubility products of BaX and SrX at the temperature of the solution

– Control of the doping level independent of the initial BaX concentration

– To prepare ceramics with different Ba : Sr ratios: select the strontium source where the ratio of the solubility products of BaX : SrX has the appropriate value

Achieving second goal

X Ksp(BaX) Ksp(SrX) Ksp(BaX)/Ksp(SrX) Product

CO32- 8.1×10-9 9.4×10-10 8.6 88%Ba +

12%Sr

C2O42- 2.3×10-8 5.0×10-8 0.46 51%Ba +

49%Sr

Achieving second goalExample with SrX=SrCO3

Ba[Ti(cat)3] + SrCO3 in excess

Dissolution in deionized water

Heated in microwave

60 min. + 10% power

Microwave Centrifuge

Ba[Ti(cat)3] + SrCO3 in excess

Dissolution in deionized water

Mixed in centrifuge for 20 minutes at 1000 RPM, then for 20 minutes at

3800 RPM

Filtration

Precipitate containing BaCO3 +

SrCO3

Solution containing Ba2+, Sr2+,

and Ti(cat)32-

Removal of water and calcination for 4 hours at 800 °C

BaxSryTiO3

with x+y=1

Results

• X-ray powder crystallography

– Confirmed the appropriate stoichiometry for BaxSryTiO3 using the centrifuge and microwave strategies for both sources of strontium

• Elemental analysis using ICP-MS in progress

Conclusions

• The catecholate method was successfully applied to the synthesis of barium zirconate and barium hafnate.

• Two environmentally benign strategies were applied to doping BaTiO3 using both SrC2O4 and SrCO3 as strontium sources. The microwave (requiring water as the only solvent) and the centrifuge (at room temperature) strategies offered better stoichiometric control.

QUESTIONS??????????