1-1.3 Spectral Emissivity, e (X) - Aluminum Oxide

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1 1-1.3 Spectral Emissivity, e (X) - Aluminum Oxide Representative curves for the spectral emissivity of bulk alumina, bulk sapphire, and powdered sapphirc are presented in this section. Supplementary to this in Section are data for liquid alumina droplet emissivity and surface roughness effects onalumina emissivity. Additional data on emissivity are contained in Section , where the reflectance data are summarized. Since Kirchhoff's law I-R applies only to the S= proper angular complements, a conversion of all reflectance to emissivity has not been made. I Bulk Al 0 3 The bulk forms of Al 2 0 3, sapphire (e -AAl0 3 ) and alumina have different emissive properties, as shown in Figures I-I. 3a, b, and c. All bulk AlzO 3 shows a distinct emissivity maximum from approximately 4g/ to 111j, the oauset of this peak shifting to shorter wavelengths as the sample temperature is increased. Representative data for sapphire are from Stierwalt (Ref. 1SE-16) over a temperature range 4.2 K to K and a spectral range lp to 125 p ; the data of Blau (Ref. 1SE-5) for 99 percent pure alumina over a temperature range of 800 K to 1300 K is taken to be repres mtative of the pressed and sintered forms of A10 A. A precision of 4 ± percent as claimed by Blau (Ref. ISE-5) is taken to be representative of apl data shown in Figure and I-1. 3.Z, although no explicit statement of precision is given by the other references shown. The effect of differences in sample surface preparation on A)ll 3 emissivity has been studied by Richmond (Ref. 1SE-13) and has been found to be negligible below 14 p. The relatively minor effect of temperature can be seen in the data of Section Data from other researchers or many bulk forms of A10 3 are included in SectionII1-1.3 in graphical and tabular form, where are also more detailed descriptions of the representative data. 1-10

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6 Powde Ad Aluminum Oxide The representative spectral emissivity of powdered A over a particle size range of and a temperature of 3000K is shown in Figure The values of e(x) reported by Aronson (Ref. 1SE-2) and S reed (Ref. ISE-17) are much higher for X > 12p than the bulk materials, but do show the decrease at 1l1 from a plateau starting at approximately 4p. The sharp peak occuring at 3 pk is due to water contamination uf tlwc sample and is not prefent in the high temperature data. The complete set of particle emissivity data is contained in Section The representative curve is for 9 WCA alumina particles at 300 K as measured by Aronson (Ref. 1SE-2). 1-15

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10 1-1.4 Total Normal Emissivity, e (T) - Aluminum Oxide The total emissivities of bulk alumina and sapphire have been measured in the literature for temperatures ranging from 63 K to K. The representative curve for alumina data, constructed by fitting a third order polynomial to data from References ITE-4 and ITE-5 is shown in Table and Figure e(t) = 0.77 at low temperatures, then goes through an apparent minimum of about/ at 1660 K. No experimental error is quoted for these data. f (T) for sapphire is given in Section IH-1.4 in tabulated form in Ref. ITE-2 and ITE-6, and appears to be less than the alumina emissivity over the range of temperatues covered, K to K. One reference (1TE-3) gives c(t) for rocket exhaust particles from T = K to K. This is presented in Section 1-19

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