HPMA modified aluminium nitride powder for aqueous tape casting of AlN ceramic substrates

Aluminum nitride(AlN)is considered one of the most desirable materials for integrated circuits and electronic packaging substrates.However,raw AlN powder reacts easily with water,forming Al(OH)3 or AlOOH on the surface and hindering the development of an aqueous tape-casting process for preparing AlN ceramic substrates.In this study,hydrolyzed polymaleic anhydride(HPMA)was used to modify AlN powder,which has good water solubility and dispersibility.The AlN powder was modified with 5 wt%HPMA remained stable in water for at least 90 h under magnetic stirring condition and 24 h under ball milling condition,indicating that HPMA-modified AlN powder has good resistance to hydrolysis.The action mechanism of HPMA is revealed.Firstly,–COOH of the HPMA polymer and the oxide layer on the surface of the AlN powder underwent a dehydration condensation reaction to form a compound.Secondly,long chains of the polymer further coated the surface of the AlN powder,forming an anti-hydration layer with a thickness of about 7.0 nm on the surface of the AlN particles.In addition,AlN green sheets were successfully prepared by aqueous tape casting using the HPMA-modified AlN powder without additional dispersants.Subsequently,AlN ceramic substrates were obtained by sintering at 1750℃for 4 h under an N2 atmosphere with a pressure of 0.2 MPa.The relative density and thermal conductivity were tested to be 97.3%and 122 W/(m·K),respectively.

Recent development of A_2B_2O_7 system transparent ceramics

A_2B_2O_7 system compounds, which usually present three phase structures mainly based on the ionic radius ratios of r_A and r_B(r_A/r_B), have been studied for potential applications in many fields, such as thermal barrier coatings, luminescence powders, fast-ion conductors, photocatalysts, and matrices for immobilization of highly active radionuclides. Since 2005, La_2Hf_2O_7 was fabricated into transparent ceramics and much more attentions were paid on A_2B_2O_7 transparent ceramics for new applications. In this review, the development of A_2B_2O_7 system transparent ceramics was described. The structure characteristics, powder synthesis method, and sintering techniques of the final A_2B_2O_7 transparent ceramics were summarized. After that, the mostly reported A_2Hf_2O_7, A_2Zr_2O_7, and A_2Ti_2O_7 system transparent ceramics were systematically introduced. The potential application fields and future development trends were also discussed, focusing on scintillators, optical elements, and other luminescent materials.

Gel-tape-casting of aluminum nitride ceramics

Aluminum nitride ceramic sheets were formed for the first time by a novel and simple method,namely gel-tape-casting process,where only two additives were used to prepare the slurries.PIBM (a water-soluble copolymer of isobutylene and maleic anhydride) acted as both dispersant and gelling agent,and PEG (polyethylene glycol) acted as plasticizer.Combining the advantages of gel-casting and tape-casting processes,flexible and uniform green tapes were obtained.The rheological properties of AlN slurries were studied to optimize the PIBM content and solids loading.After drying and debinding,the green sheets were sintered at 1840 ℃ in N2 atmosphere.Homogeneous microstructure with average grain size of about 7 μm was observed in the final AlN ceramics.Thermal conductivity of the AlN ceramics was 161 W/(m·K).

Correlated evolution of dual-phase microstructures,mutual solubilities and oxygen vacancies in transparent La2-xLuxZr2O7 ceramics

The A2B2O7 series of ternary oxides are derivatives of fluorite structure over a wide range of rA/rB.Competing by two rare-earths the A-site,La2-xLuxZr2O7 ceramics were found transparent only in pore-free microstructures with similar grain sizes of pyrochlore(PY)and defective fluorite(DF)phases.Mutual solubilities of Lu and La in both phases were found by imaging and energy-dispersive spectroscopy analysis in scanning electron microscope.The dual-phase microstructures were developed with liquid-phase resulted from the exothermal reactions,creating a miscibility gap between two structures to moderate their competing grain growth.Change in grain growth behaviors in liquid-phase is described by a nucleation line in La2Zr2O7-Lu2Zr2O7 phase diagram.Variations of solution levels in DF grains and co-existing of dual-phase grain clusters in common orientation were revealed in transparent ceramics by electron backscattered diffraction,resulted by epitaxial relation of two phases promoted by the liquid-phase.Oxygen vacancies and various hole states common in both phases were revealed by characteristic cathodoluminescence peaks.The collective effects of pores,phase and grain boundaries,oxygen vacancies on scattering or absorption of visible light enables to establish a hierarchical microstructure-transparency relationship in such complex oxide ceramics,which could be tailored or further optimized by controllable sintering.