Standard Transition Aluminas. Electron …

1 Standard Transition aluminas . Electron microscopy StudiesP. Souza Santosa, H. Souza Santos*b, rio de Mat rias-Primas Particuladas e S lidos N o Met licos,Departamento de Engenharia Qu mica, Escola Polit cnica,Universidade de S o Paulo, S o Paulo, SP, BrazilbLaborat rio de Microscopia Eletr nica, Departamento de F sica Geral,Instituto de F sica, Universidade de S o Paulo, S o Paulo, SP, BrazilReceived: May 30, 2000; Revised: November 17, 2000 The aim of this paper is to present the results of characterization of the particle shapes of sixstandard Transition aluminas samples using transmission and scanning Electron microscopies;selected area Electron diffraction, in parallel with X-ray powder diffraction were used for confirma-tion of the different Transition aluminas types.

2 The Transition aluminas - chi; kappa; gamma; theta;delta; and eta were supplied by ALCOA Central Laboratory. The chi-; kappa-;gamma- anddelta-Al203microcrystals are pseudomorphs from their respective precursors gibbsite and , theta-Al203microcrystals are not pseudomorphs after the Standard , eta-Al203are not pseudomorphs after bayerite : Transition aluminas , alumina precursors, gibbsite, Electron microscopies1. CorundumA crystalline element or a solid inorganic chemicalcompound formed by a geological process is a mineral andtherefore receives a specific name. Natural crystalline alu-minum oxide or alumina (Al203) is named corundum; themonocrystals* are colorless and transparent.

3 If they con-tain, intimately mixed, microcrystals of magnetite (Fe304)or hematite (Fe203), they are called emery. If the corundummonocrystals contain some elements in isomorphic substi-tutiton for Al3+in their structure, gemstones and semi-pre-cious gemstones are formed, also receiving specific namesfor their colors: (a) sapphire - blue (Fe, Ti); (b) ruby - red(Cr); oriental topaz - yellow (Fe3+,Fe2+); amethyst - purple(Fe, Mn, Ti); oriental emerald - green (Fe2+).Since, by definition, a mineral is formed in Nature, theexpression synthetic mineral may be criticized. How-ever, it is used for synthetic rubies and sapphires that areindustrially produced today by the Verneuil (oxi - hydrogenflame) or by hydrothermal is a common practice in X-Ray Crystallography totake the crystalline structures of some minerals (and also ofsome chemical compounds) as standards , these structuresreceiving the name of the mineral.

4 The crystal structure ofcorundum is called corundum structure, but also is called -Al203or alpha-alumina structure (Krebs3). Alpha-aluminaIt was an old recommendation in gravimetric chemicalanalysis to heat the white gelatinous precipitate of alumi-num hydroxide at temperatures above 1200 C to stabi-lize it for weighing the alumina formed at lowertemperatures; the aluminas , if not heated above 1200 C,adsorbed water vapor very fast and no equilibrium wasreached for the weighing. The alumina heated above1200 C was named alpha-alumina in 1916 by Rankin andMerwin4; the use of Greek letters was an old commonpractice in Chemistry to distinguish between compoundswith exactly the same composition, but having differentphysico-chemical properties.

5 These authors also proposeda new beta-alumina, which will be discussed later. Thename -Al203was also given by Rankin + Merwin4to a Na- containing alumina. The -alumina prepared by heatingaluminum hydroxides is a finely divided powder consti-tuted by micrometer sized particles. After the Debye-Scher-rer X-ray powder method was developed in 1916, soon itwas shown that the -alumina particles are micro-crystalswith the same crystalline structure of the mineral corun-dum. Today, the expression more frequently used is -alu-mina crystalline structure. Presently, -Al203has many*Departamento de F sica Geral, Instituto de F sica da Universidade deS o Paulo.

6 66318, S o Paulo - SP. e-mail: (mono) crystal consists of a three dimensional periodicalarrangement of atoms, each of which will scatter an incident Research, Vol. 3, No. 4, 104-114, 2000. 2000uses in Traditional and Advanced Ceramics; particularly,is an important substrate for the growth of the superconduc-tors and semiconductors by molecular beam Transition AluminasThe aluminas , heated at temperatures under 1100 C,were considered in the early days of X-ray powder diffrac-tion method as amorphous or non-crystalline materials, inspite of the fact that they have important adsorptive andcatalytic properties; in other words, they were consideredvery ill defined and complex materials, that above 1100 Cchanged into crystalline -alumina.

7 Sometimes, they wereloosely called gamma-alumina by , it was recognized6the existence of five crystal-line aluminum hydroxides: gibbsite; bayerite; nord-strandite; diaspore; and boehmite; they have crystalsvarying from micro to milimetric size; their dehydroxila-tion occurs by heating between 300 C and 600 C andaluminas are formed, all as micrometer sized powders;these fine grained powders change their structure at1100 C, but remain as powders; increasing the tempera-tures from 1100 C up to near the melting point of -Al203(2050 C), the powder particles start to coalesce, sinter andrecrystallize as microplatelets of hexagonal profile or evenhexagonal prisms with -Al203structure; they are called tabular crystals of aluminas and must not be confusedwith the commercial product patented under the name tabular alumina.

8 The formation of the hexagonal crystalsof -Al203may be accelerated by the addition of (from ALCOA S Research Laboratories,now Technical Center, Pittsburgh, USA), showed that,between the temperatures of dehydroxilation of the alumi-num hydroxides and the alpha-alumina first crystallization,a number of well characterized and reproducible interme-diate crystalline alumina structures are formed; each onehas a stable different crystalline structure at a given tem-perature range, with just one exception which is amor-phous .The type or structure of each alumina and its tempera-ture range of existence are determined by the structure ofthe starting or precursor hydroxide; they are different forgibbsite, bayerite, nordstrandite, boehmite or diaspore*.

9 Extensive literature exists on the dehydroxilation of thecrystalline hydroxides, in special on gibbsite, because it isthe phase formed in the industrial Bayer Process. Theseseven aluminas are called Transition aluminas and re-ceived Greek letters to identify them: gamma; delta; theta;kappa; chi; eta and rho. Figure 1 is adapted from the mostrecent review7of the thermal transformation sequence:aluminum hydroxide Transition aluminas -Al203 From Fig. 1, may be concluded that all TransitionAluminas may be produced from gibbsite; that fact is onereason more for the industrial importance of the Bayergibbsite. All Transition aluminas are five crystalline hydroxides can be synthetic ornatural; from either origin, the Transition aluminas areformed under firing.

10 These same Transition aluminas mayalso be formed by pyrolysis of crystalline hydrates ofaluminum salts as sulphate; nitrate; ammonium alum; chlo-ride; formate; and hydroxiacetate. Other methods for pre-paring Transition aluminas are reviewed by Wefers andMisra7.* In order to be coherent, the crystalline Al-hydroxides should have Standard names for their structures; in fact they have, but nordstrandite has none7;so, it became common practice to use the mineral name to designate the hydroxides, natural or transformation sequence of the aluminum hydroxides (adapted from Ref. 7).Vol. 3, No. 4, 2000 Transition aluminas . Electron Microscopy105In spite of its Greek letter, -Al203is not a transitionalumina, but is the last crystalline material formed by theeffect of increasing temperatures on the Transition alumi-nas.