It has been over eight years since the subject of rapid solidification processing (RSP) has been given a general review. The early reviews of RSP were undertaken before the very rapid expansion of the late 1970s and early 1980s and the most recent general review was published in 1984, at approximately the height of RSP activity in the USA. More recent specialized reviews have been prepared for magnesium, titanium and aluminium alloys. The expansion of research and application testing resulted from the first major attempts to employ RSP on a large scale under both industrial and government sponsorship. The objective here is to develop the historical background of RSP, and then to update the present theoretical base for this technology. Examples of recent work on the rapid solidification of metals and ceramics will be given, along with examples of applications. Finally, an assessment of the future directions and prospects for RSP will be presented.

Rapid solidification processing

Biomaterial development is currently the most active research area in the field of biomedical engineering. The bioglasses possess immense potential for being the ideal biomaterials due to their high adaptiveness to the biological environment as well as tunable properties. Bioglasses like 45S5 has shown great clinical success over the past 10 years. The bioglasses like 45S5 were prepared using melt-quenching techniques but recently porous bioactive glasses have been derived through sol-gel process. The synthesis route exhibits marked effect on the specific surface area, as well as degradability of the material. This article is an attempt to provide state of the art of the sol-gel and melt quenched bioactive bioglasses for tissue regeneration. Fabrication routes for bioglasses suitable for bone tissue engineering are highlighted and the effect of these fabrication techniques on the porosity, pore-volume, mechanical properties, cytocompatibilty and especially apatite layer formation on the surface of bioglasses is analyzed in detail. Drug delivery capability of bioglasses is addressed shortly along with the bioactivity of mesoporous glasses. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1248-1275, 2016.

Review and the state of the art: Sol-gel and melt quenched bioactive glasses for tissue engineering.

Calcium-aluminum-rich inclusions in the Allende meteorite: evidence for a liquid origin

Thermodynamic data on activities, activity coefficients, and free energies of mixing in SiO/sub 2/--Al/sub 2/O/sub 3/ solutions were calculated from the phase diagram. Positive deviations from ideal mixing in the thermodynamic data suggest a tendency for liquid immiscibility in both SiO/sub 2/- and Al/sub 2/O/sub 3/-rich compositions. The calculated data were used to estimate regions of liquid-liquid immiscibility. A calculated metastable liquid miscibility gap with a consolute temperature of approx. = 1540/sup 0/C at a critical composition of approx. = 36 mol % Al/sub 2/O/sub 3/ was considered to be thermodynamically most probable; the gap extended from approx. = 11 to approx. = 49 mol % Al/sub 2/O/sub 3/ at 1100/sup 0/C. SiO/sub 2/-rich glass compositions showed evidence of glass-in-glass phase separation when examined by direct transmission electron microscopy.

Calculated Thermodynamic Data and Metastable Immiscibility in the System SiO2-Al2O3

Electrohydrodynamic atomization’has been adapted to produce Al2O3 powders ranging in size from 10 nm to 300 μm. Microstructural characterization using x-ray diffraction, scanning, and transmission electron microscopy reveals changes in phase selection as a function of particle size, hence supercooling. Amorphous powders are common below 100 nm in diameter. Cubic spinel γ is found in single phase form between 100 nm and 2μm, and partially transformed to δ between 2 and 20 μm. There is also evidence of σ and θ or a precursor of θ forming directly from the liquid above 5 μm. The stable corundum structure is consistently found above 20 μm but exhibits three different morphologies: faceted, dendritic, and cellular. Phase selection is examined on the basis of fundamental thermodynamic and kinetic considerations and results from computer models predicting the thermal history of the powders. It is concluded that metastable phases require the elimination of catalytic sites for the nucleation of a and are thus more likely to form in the smaller powders. Furthermore, submicron powders achieve sufficiently high cooling rates to preserve the metastable phases formed (γ), but those higher than ∼ 1 μm experience a thermal excursion long enough to transform γ to more stable forms of Al2O3.

Phase selection in electrohydrodynamic atomization of alumina

Glass spherules have been made by laser spin melting and free fall cooling techniques from ceramic rods. Thus, the materials were not in contact with a container at any time. Glasses of 100 to 800 μm diameter were formed from at least 80 wt% of the oxides Al2O3, Ga2O3, In2O3, La2O3, ZrO2, HfO2, Nb2O5 and Ta2O5 with 20 or less weight percent of CaO + SiO2. The best glass formers were in the Nb2O5- and Ta2O5-based systems. The indices of refraction of most of the glasses and Abbe numbers of many were measured.

Formation of new oxide glasses by laser spin melting and free fall cooling

Glass spherules were made from molten droplets spun off the ends of laser-heated ceramic rods and solidified by free-falling cooling. Glasses of several hundred μm diameter were formed from the pure lanthanide oxides Sc2O3, Y2O3, Sm2O3, Gd2O3, Yb2O3 and Lu2O3; glass formation decreased with addition of CaO. Stable glasses were also produced from binary La2O3 compositions with 25, 50 and 75 wt % of each of the oxides Y2O3, Yb2O3, Nb2O5 and Ta2O5, and from the ternary compositions 40 wt % La2O3 + 20 wt % CaO + 40 wt % Al2O3 or Ga2O3. The indices of refraction of all of the glasses and the Abbe numbers of several were measured.

L. E. Topol

Papers

Formation of new oxide glasses by laser spin melting and free fall cooling

Formation of new lanthanide oxide glasses by laser spin melting and free-fall cooling☆