Crystallization of Electrostatically Seeded Lanthanum Hexaluminate Films on Polycrystalline Oxide Fibers

Abstract

Lanthanum hexaluminate (LaAl 11 O 18 ), which features easily cleavable atomic planes in the magnetoplumbite (MP) structure, is a good candidate material for the fiber/matrix interface in oxide/oxide-fiber-reinforced composites. Difficulties that are encountered when using sol-gel synthesis to produce this material include the high crystallization temperature and the occurrence of an intermediate phase. This paper presents a method to overcome these difficulties by introducing crystalline seeds into the LaAl 11 O 18 sol during the fiber coating. Seed particles are deposited initially on the fibers via electrostatic forces. Then, the sol is dip-coated on the preseeded fibers, and the final LaAl 11 O 18 coating is achieved via the use of heat treatments. The crystallization temperature can be reduced by at least 50°C, in comparison with that for the unseeded case, according to differential scanning calorimetry measurements, and no intermediate LaAlO 3 phase is formed. The estimated seed numbers are on the order of 10 9 seeds/cm 3 in the preseeded sol-coated fiber surfaces and 10 11 seeds/cm 3 in the 4-wt%-seeded gels. The crystallization and structure of the La 1؊x Al 12؊y O 19؊z MP phase in the 4-wt%-seeded gels that have been heated at 400°and 1200°C have been studied using powder neutron diffraction.

La-Doped Alumina, Lanthanum Aluminate, Lanthanum Hexaaluminate, and Related Compounds: A Review Covering Synthesis, Structure, and Practical Importance

“…LHA features prominently among the HAs used for the reinforcement of composite ceramics. ,, , − It appears that the relatively weak mirror planes in the layered HA structure play an important role in crack deflection, while LHA grains also contribute to the limitation of grain growth of the host material (e.g., of alumina in alumina ceramics) . Similar arguments apply to the use of HAs as fiber-matrix interphases in fiber-reinforced ceramics , , , and oxide composite fibers . An alumina-based insulator for spark plugs is a good practical example for a HA-reinforced ceramic.…”

“…However, quite expensive starting materials and adamant reaction conditions (pH, temperature, absence of oxygen and moisture) limit the suitability of most sol–gel processes for large-scale applications. , It should also be kept in mind that the amount of water used in the alkoxide hydrolysis step can have an influence on the morphology and the surface properties of the final product. ,− Jana et al (Table S1, entry 52) developed an advanced sol–gel process for the synthesis of high-purity LHA; cheaper starting materials like boehmite and La-nitrate were used, and seeding of the precursor solution with LHA crystals decreased the formation temperature by ∼100 to ∼1200 °C. An almost single-phase product was observed at 1450 °C, while after calcination at 1600 °C (2 h) a clean LHA phase was formed. , Similarly, Saruhan et al (entry 50) determined by DSC that seeding reduces the crystallization temperature of single-phase LHA by ∼50 to ∼1190 °C.…”

“…After precipitation, washing, drying and calcination (1200 °C, 2–10 h, air), surface areas of 18–49 m 2 /g are reported (entries 28, 29, 33, 36, 38, 39, 41), amounting to an average surface area of 32 m 2 /g. The surface areas obtained by sol–gel synthesis (entries 47–53) , , − , , are in the upper range of those reported for coprecipitation. Higher surface areas are clearly seen for samples prepared in microemulsions (entries 54–58) ,,− (e.g., 67 m 2 /g in entry 58).…”

Al–O–N and Al–O–B–N thin films applied on Si–O–C fibers

“…and to preserve the mechanical properties of the fibers. Different materials may be considered regarding these expected properties: refractory oxides such as ZrO 2 or Al 2 O 3 [5][6]; silicon based ceramics such as Si-B-C-N, Si-C-N [7][8] or Si-Al-C-N [9][10][11], monazite [12][13][14][15], BN [16][17][18][19][20][21] and LaAlO x [22][23][24][25][26].…”

Microwave Hybrid Post‐Heat Treatment of Reaction Sintered Alumina/Lanthanum Hexaaluminate Composite Ceramics

“…[1,2] Fields of application are, e.g., thermal barrier coatings and combustion catalysts. [3,4] LHA is one of the main compounds which is normally formed in addition to the lanthanum aluminate, LaAlO 3 , in the La 2 O 3 -Al 2 O 3 system studied by Bondar and Vinogradova.…”