Transparent Zirconia Composition and Process for Making Same (1969)
U.S. Patent No. 3,432,314, granted on March 11, 1969, to Khodabakhsh S. Mazdiyasni, Charles T. Lynch, and Jonathan S. Smith II (assigned to the United States of America as represented by the Secretary of the Air Force), describes a breakthrough method for producing high-density, polycrystalline cubic zirconium oxide that is optically transparent and thermally stable above 2000°C.
This invention solved a major roadblock in materials science and aerospace engineering: refractory polycrystalline zirconia made by conventional ceramic methods was inherently opaque, porous, and prone to catastrophic cracking under thermal cycling. The inventors developed a chemical synthesis route that achieves complete cubic phase stabilization using significantly less rare-earth stabilizer while imparting the ability to transmit both visible and infrared radiation.
The Innovation: The Alkoxide Decomposition Route
Pure zirconium oxide undergoes destructive phase transformations upon heating and cooling (monoclinic to tetragonal to cubic), which causes severe volume changes that fracture solid ceramic bodies. Traditional approaches required adding large amounts of stabilizers—typically 8 to 15 mole percent of expensive rare-earth oxides—and firing at extreme temperatures, which left residual pores, coarse grains, and secondary phases that scattered light and rendered the material opaque.
The inventors bypassed conventional mechanical powder blending entirely by turning to alkoxide precursors dissolved in mutual organic solvents. By simultaneously hydrolyzing zirconium and rare-earth metal alkoxides, they produced molecularly uniform, submicron powders in a highly reactive state.
Why Alkoxide Synthesis?
True Solid Solution: The simultaneous precipitation ensures the stabilizing oxide is dispersed at the molecular scale, entering the crystal lattice directly without forming light-scattering secondary phases.
Lower Stabilizer Requirement: Full stabilization into the isotropic cubic phase is achieved with only 6 mole percent additive, compared to 8 to 15 mole percent in prior art methods.
Negligible Porosity: The extreme surface area and cleanliness of the submicron powder allow cold-pressed shapes to sinter to near theoretical density (5.5 to 6.0 g/cm³ versus theoretical 6.1 g/cm³), eliminating the microscopic voids that cause optical opacity.
Key Chemical Components
The transparent refractory body consists of a synergistic, ultra-high-purity oxide system:
| Component | Function |
| Zirconium Oxide (ZrO2) | Primary refractory base ceramic providing chemical inertness, high mechanical strength, and a melting point exceeding 2700°C. |
| Yttrium Oxide (Y2O3) | Primary stabilizing additive (6 mole percent) that locks the crystal structure into the cubic phase, preventing destructive phase changes. |
| Dysprosium / Ytterbium Oxide (Dy2O3 / Yb2O3) | Alternative plus-three valence rare-earth additives matching the ionic radius and electronegativity of yttrium for equivalent stabilization. |
| Lithium Fluoride (LiF) | Optional sintering aid (0.1% to 3%, typically 2%) that reduces required compaction pressures down to 25,000 psi and vaporizes out during sintering. |
Performance: Extreme Heat Stability and Optical Clarity
The patent presents rigorous testing data confirming structural survival under harsh aerospace environments and verified optical transmissivity.
Extreme Thermal Stability Test:
- 262-Hour Air Roast at 2050°C: A specimen tested in flowing air inside a gas-fired furnace showed no destabilization. Grain growth remained exceptionally small (growing from 2–5 microns to 35–50 microns with no irregular growth).
- Phase Retention: X-ray diffraction confirmed the body remained 100% cubic with zero reversion to the destructive monoclinic phase. Commercial CaO- or MgO-stabilized zirconia disintegrated under equivalent cycling.
- Chemical Integrity: Density remained virtually unchanged (within 0.01 g/cm³), and the 6 mole percent stabilizer level held constant throughout the test.
Optical and Infrared Transmission:
- Visible Light: Samples 1.2 mm thick transmit visible light cleanly without distortion, allowing printed lettering on paper to be read directly through the ceramic.
- Near and Mid-Infrared: 1.2 mm specimens transmit 10% of incident radiation across 0.8 to 1.5 microns (8,000 to 15,000 Angstroms), 4% out to 8.0 microns, and continue transmitting out to 40.0 microns. Thin sections (0.5 mm) exceed 10% transmission across wide bands.
The Manufacturing Process
The inventors established a precise multi-step synthesis and sintering protocol:
- Synthesize Solution: Dissolve high-purity zirconium isopropoxide and yttrium isopropoxide (or Dy/Yb equivalents) in a mutual solvent such as n-hexane or benzene.
- Hydrolyze: Add triply distilled or demineralized water dropwise with vigorous agitation, causing quantitative precipitation of intimately mixed submicron oxides.
- Dry and Comminute: Dry the solvent-wet powder in air at 110°C for at least 24 hours, grind to eliminate aggregates, and store over desiccant.
- Calcine: Heat the powder in air at 800°C for 1 to 4 hours to convert it to oxide form, followed by regrinding into a fine powder.
- Cold Press: Uniaxially or isostatically compress the powder into shapes at pressures between 50,000 and 200,000 psi.
- Sinter: Fire the green compacts in air at 1450°C for at least 16 hours to produce a fully cubic, low-porosity, transparent body.
Historical and Scientific Impact
Patented during the height of Cold War aerospace development, Mazdiyasni, Lynch, and Smith’s work at the Air Force Materials Laboratory pioneered modern sol-gel processing of advanced ceramics.
- Aerospace and Defense Windows: Transparent cubic zirconia provided a material capable of serving as infrared sensor domes, nose cones, heat shields, and antenna windows for hypersonic flight vehicles subjected to aerodynamic heating above 2000°C.
- High-Temperature Lighting: With a melting point above 2700°C, it surpassed aluminum oxide (melting point approximately 2050°C) as a gas-tight envelope material for ultra-high-temperature arc lamps.
- Energy and Electrochemical Applications: The fully stabilized cubic Y2O3-ZrO2 structure served as a precursor for high-temperature solid electrolyte membranes in advanced fuel cells.
About the Inventors: Khodabakhsh S. Mazdiyasni et al.
Dr. Khodabakhsh S. Mazdiyasni, along with Charles T. Lynch and Jonathan S. Smith II, were research scientists working at Wright-Patterson Air Force Base in Dayton, Ohio. Mazdiyasni was a world-renowned pioneer in chemical synthesis of ceramics; his development of metal alkoxide hydrolysis laid the foundation for modern sol-gel processing and nano-powder synthesis, revolutionizing structural ceramics, optical coatings, and refractory aerospace composites.
Summary of Claims
The patent explicitly claims:
- A high-density, polycrystalline fully stabilized cubic zirconia body having fine, uniform grain size, negligible porosity, stability in oxidizing environments above 2000°C, and transmissivity in visible and infrared radiation.
- A composition consisting of high-purity zirconia and about 6 mole percent of a stabilizer selected from yttrium oxide, dysprosium oxide, or ytterbium oxide.
- The method of manufacture comprising the mixing of high-purity oxides, calcining in air at 800°C for 1 to 4 hours, regrinding, cold pressing at 50,000 to 200,000 psi, and sintering at 1450°C for at least 16 hours.
