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INTRODUCTION

OBVIOUSLY, most synthetic ceramics and glass-ceramic composites were evaluated by means of microstructure and crystallinity. Whisker-like nanostructured ceramics based on raw materials have many fields of application because of their outstanding physical properties, such as high strength and heat resistance. However, the microstructure is the most important characteristic feature in synthetic materials (i.e., ceramic, glass-ceramic, and composites), and the microstructure can be used to evaluate the materials. The particle sizes in the microstructure of the investigated materials may be in the macro- (10 2 mm), micro- (10 6), nano(10 9 nm), or femto-scale (10 15 fm). Additionally, the crystallinity of the particle microstructure plays an important role in the properties of the materials. The crystallinity may be rounded, spherulitic, acicular, etc.; however, the whisker microstructure, such as interlocked fibers, needles, or rods, results in high-strength materials.

SAYED H. KENAWY is with the Refractories, Ceramics and Building Materials Department, National Research Center, 12622, Cairo, Egypt. ESMAT M.A. HAMZAWY is with the Glass Research Department, National Research Center, 12622, Cairo, Egypt. Contact e-mail: [email protected] Manuscript submitted September 7, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

Another characteristic feature of some synthetic minerals or compounds is the low or negative coefficient of thermal expansion (CTE). If we look to the synthetic materials, we find that some well-known minerals, such as quartz glass (SiO2, 5.0 9 10 7, 25 C to 1000 C), glass-ceramic of spodumene (LiAlSi2O6, 9.0 9 10 7/C, 25 C to 1000 C), b-eucryptite (LiAlSiO4, 62.0 9 10 7/C, 25 C to 1000 C), cordierite (Mg2Al4Si5O18, 14.0 9 10 7/C, 25 C to 1000 C), mono-celsian (BaAl2 Si2O8, 23.0 9 10 7/C, 25 C to 1000 C), and anorthite ceramic (CaAl2Si2O8, 52.0 9 10 7/C, 25 C to 1000 C), have low or ultra-low CTEs.[1–6] Some binary alkaline-earth borate glasses, such as CaO-B2O3 (56.4 9 10 7/C), SrO-B2O3 (57.9 9 10 7/C), and BaO-B2O3 (65.9 9 10 7/C), have a low CTE.[7] However, some ceramic bodies that were prepared from raw materials, such as cordierite (Mg2Al4Si5O18, 4.0 9 10 7/ C, 25 C to 1000 C) and mullite (Al6Si2O13, 50 to 54 9 10 7/C), have very low CTEs.[8–10] In the ternary Al2O3-SiO2-B2O3 system, Werding and Schreyer [11] predicted the synthesis of the boron mullite phase with vertices formed by 3:2, while 2:1 mullite Al5BO9 with mullite-type structure[12] was utilized to produce AlBO3 with a calcite-type structure.[13] Throughout the aluminoborate phases, a whisker-like microstructure was prepared through a ceramic route for bulk or sintered glass-ceramic and composites. Through an evaporation deposition route using BI3/H3BO3 vapor with an Ar gas precursor on an Al film substrate in the range from 750 C to 1000 C, Al18B4O33 and Al4B2O9, nanowires, and nanotubes were synthesized on sapphire substrates

covered by NiB powder, Pt powder, and Al films.[14] The other aluminum