Effect of the Machining Method on the Catalycity and Emissivity of ZrB₂ and ZrB₂–HfB₂‐Based Ceramics

Abstract

The emissivity and the catalytic efficiency related to atomic oxygen recombination were investigated experimentally in the range 1000-2000 K for ZrB 2 and ZrB 2 -HfB 2 -based ceramics. In order to evaluate the effect of the machining method, two series of samples, one prepared by electrical discharge machining and the other machined by diamond-loaded tools, were tested. High emissivity (about 0.7 at 1700 K) and low recombination coefficients (on average 0.08 at 1800 K) were found for all the materials. The experimental data showed an effect of the surface machining on the catalytic behavior only on the ZrB 2 -based composite; conversely, small variations were found in the recombination coefficients of ZrB 2 -HfB 2 -based samples for the different machining processes. The surface finish affected the emissivity at lower temperatures in both compositions, with the effect becoming negligible at temperatures above 1500 K.

Dissociation of CO2 molecules in microwave plasma

“…Metals, on the other hand, have much higher recombination coefficients, usually between 0.1 and 1 [18,19]. Numerous studies on recombination of O atoms to parent molecules (O 2 ) have been performed [19,20], but to the best of our knowledge, no attempt has been done to measure the coefficient for heterogeneous surface recombination of CO and O atoms. Quite a few attempts have been done on quantification of the radical recombination on silicon dioxide materials.…”

Advances in Ultra High Temperature Ceramics for Hot Structures

“…An analysis of the surface of EDM machined samples has shown that the EDM induce a damage: the surface modifications seem more pronounced in material B due to its higher refractoriness. The SEM-EDS analyses upon the electrical discharged surfaces confirmed the formation of a partially oxidized thin layer, which in turn includes contaminants from the electrically active tools used for EDM 34) . Flexure strength measured upon samples machined with EDM showed a decreased mean value of about 26.4%, compared to that obtained from specimens machined with DLTs, the major reason for this being the micro-cracked surface layer induced by the action of the electrical discharges 32) .…”

“…Both the UHTC composition A and B, were tested, in samples machined by diamond loaded tools (DLT) or by electrical discharge machining (EDM) in order to evaluate the effect of the surface finish on those properties 33), 34) . Figure 3 reports the evolution of the total hemispherical emissivity versus temperature for the four selected materials: A-EDM (dotted line), A-DLT (full line), B-EDM (bold dotted line) and B-DLT (bold dotted line with space).…”

Material property requirements for analysis and design of UHTC components in hypersonic applications

“…Direct measurements of hemispherical emissivity at elevated temperatures are difficult, and few such data are available in the literature for UHTC materials. The measurements of Scatteia et al 58,59 for ZrB 2 -15SiC-2MoSi 2 , ZrB 2 -15SiC, and ZrB 2 -15SiC-10HfB 2 composites are a notable exception. Emissivity values ranged from 0.49 to 0.81, depending on material composition, temperature, machining method, and ambient oxygen environment.…”