High Temperature Materials Laboratory third annual report

The High Temperature Materials Laboratory has completed its third year of operation as a designated DOE User Facility at the Oak Ridge National Laboratory. Growth of the user program is evidenced by the number of outside institutions who have executed user agreements since the facility began operation in 1987. A total of 88 nonproprietary agreements (40 university and 48 industry) and 20 proprietary agreements (1 university, 19 industry) are now in effect. Sixty-eight nonproprietary research proposals (39 from university, 28 from industry, and 1 other government facility) and 8 proprietary proposals were considered during this reporting period. Research projects active in FY 1990 are summarized

Cooperative international program on mechanical strength measurements of ceramics

This report consists of viewgraphs for the presentation on the cooperative international program on mechanical strength measurements of ceramics for advanced heat engines. (JDL

Effects of alternate fuels report No. 8: analysis of degradiation of magnesia-based refractory bricks from a residual oil-fired rotary cement kiln

Residual oil was used as an alternate fuel to natural gas to supply heat in a rotary cement kiln. Principal impurities in the residual oil were Ca, Fe, Mg, Na, Ni, P.S. and V. the kiln operators were concerned about the effects of these oil impurities on observed degradation of the magnesia-based bricks used as a liner in the burning zone of the kiln. Two degraded bricks, which had been in service for six to nine months, were analyzed to determine the role of fuel impurities on the observed degradation. The maximum hot-face temperature of the refractory during service was about 1500/sup 0/C. One brick had decreased in thickness about 45%, the about 15%. Various analytical measurements on these samples failed to reveal the presence of fuel impurities at or near the hot face of the bricks, and therefore it is concluded that the relatively short service life of these refractories was not due to use of residual oil as the fuel in the kiln. The observed degradation, therefore, was attributed to other reactions and to thermal mechanical conditions in the kiln, which inevitably resulted in extensive erosion of the bricks

Corrosion of a stainless steel waste heat recuperator

Waste heat recuperation has significant potential for saving energy in fossil-fuel-fired industrial furnaces. Preheating the air used to burn the fuel can significantly reduce fuel consumption. The US Department of Energy is contracting several high-temperature waste heat recuperation demonstrations with the objective of using successful efforts to stimulate the industrial utilization of these devices. One of the recuperator demonstration contracts has as an objective the successful operation of a concentric-shell radiation recuperator of a new design on aluminum-scrap-remelting furnaces. The design employs type 309 stainless steel reradiant inserts within the type 309 stainless steel inner shell to increase heat radiation to the recuperator partition, thereby increasing the heat exhanger's effectiveness. The first demonstration recuperator in this program was installed on a furnace fired with No. 2 oil and melting about 60 Mg (66 tons) of aluminum per 24-h day. The unit operated for about 30 d and provided air to the burner at 540/sup 0/C. during this period, a burner control misoperation provided very fuel-rich gases to the base of the recuperator. This fuel combined with safety dilution air at the recuperator base and burned within the recuperator. Also, during this period, air flow loss was detected at the burner. An inspection revealed that this was caused by failure of the partition wall separating the primary and secondary sides of the recuperator. Extensive corrosion of the partition wall and reradiant inserts was also observed. The recuperator was removed from the furnace for an analysis of the failure

Stress relaxation and creep of high-temperature gas-cooled reactor core support ceramic materials: a literature search

Creep and stress relaxation in structural ceramics are important properties to the high-temperature design and safety analysis of the core support structure of the HTGR. The ability of the support structure to function for the lifetime of the reactor is directly related to the allowable creep strain and the ability of the structure to withstand thermal transients. The thermal-mechanical response of the core support pads to steady-state stresses and potential thermal transients depends on variables, including the ability of the ceramics to undergo some stress relaxation in relatively short times. Creep and stress relaxation phenomena in structural ceramics of interest were examined. Of the materials considered (fused silica, alumina, silicon nitride, and silicon carbide), alumina has been more extensively investigated in creep. Activation energies reported varied between 482 and 837 kJ/mole, and consequently, variations in the assigned mechanisms were noted. Nabarro-Herring creep is considered as the primary creep mechanism and no definite grain size dependence has been identified. Results for silicon nitride are in better agreement with reported activation energies. No creep data were found for fused silica or silicon carbide and no stress relaxation data were found for any of the candidate materials. While creep and stress relaxation are similar and it is theoretically possible to derive the value of one property when the other is known, no explicit demonstrated relationship exists between the two. For a given structural ceramic material, both properties must be experimentally determined to obtain the information necessary for use in high-temperature design and safety analyses

Effects of alternate fuels report No. 7: analysis of failure of a mullite-based refractory brick in an industrial oil-fired burner

Industrial conversion from natural gas to alternate fuels, such as residual oils and coal, often results in accelerated degradation of refractory materials due to chemical reactions with the metal impurities in the alternate fuels. The cause of failure of a refractory brick used in an industrial burner firing an alternate fuel is described. The burner, which was used to calcine CaSO/sub 4/ in a lime-type kiln, was fired with No. 6 residual oil. The refractory lining in the burner was constructed of aluminosilicate brick, castable, and mortar in contact with one another. The lining deteriorated after about 1000 h, during which the maximum hot-face temperature was about 1750/sup 0/C. The degraded refractories were subjected to chemical analyses, ceramography, x-ray diffraction, scanning electron microscopy, and electron microprobe analysis. Liquid phases that formed in the castable and mortar during operation of the burner at temperatures above about 1600/sup 0/C reacted with the brick, resulting in decomposition of mullite. Contamination of the original refractory with CaO and V/sub 2/O/sub 5/ resulted in the formation, during cooling, of compounds which are less refractory than the original castable and mortar. It was concluded that failure was initiated by melting in the castable and mortar. Large concentrations of aggressive oxide liquid were in the burner lining at the service temperature. The liquid phase eventually advanced into the refractory from the hot face to the extent that the brick grossly deteriorated. Therefore, rapid degradation of the refractory system was due to a combination of excess temperature and fluxing by process carry-over and impurities from the fuel oil

Assessment of fibrous insulation materials for the selenide isotope generator system

Fibrous insulations for use in the converter and the heat source of the radioisotope-powered, selenide element, thermoelectric generator (selenide isotope generator) are assessed. The most recent system design and material selection basis is presented. Several fibrous insulation materials which have the potential for use as load-bearing or nonload-bearing thermal insulations are reviewed, and thermophysical properties supplied by manufacturers or published in the literature are presented. Potential problems with the application of fibrous insulations in the selenide isotope generator are as follows: compatibility with graphite, the thermoelectric elements, and the isolation hot frame; devitrification, grain growth, and sintering with an accompanying degradation of insulation quality; impurity diffusion from the insulation to adjoining structures; outgassing and storage of fibrous materials. Areas in which thermophysical data or quantitative information on the insulation and structural stability is lacking are identified

Materials analyses of ceramics for glass furnace recuperators

The use of waste heat recuperation systems offers significant promise for meaningful energy conservation in the process heat industries. This report details the analysis of candidate ceramic recuperator materials exposed to simulated industrial glass furnace hot flue gas environments. Several candidate structural ceramic materials including various types of silicon carbide, several grades of alumina, mullite, cordierite, and silicon nitride were exposed to high-temperature flue gas atmospheres from specially constructed day tank furnaces. Furnace charging, operation, and batch composition were selected to closely simulate industrial practice. Material samples were exposed in flues both with and without glass batch in the furnace for times up to 116 d at temperatures from 1150 to 1550/sup 0/C (2100 to 2800/sup 0/F). Exposed materials were examined by optical microscopy, scanning electron microscopy, energy dispersive x-ray analysis, x-ray diffraction, and x-ray fluorescence to identify material degradation mechanisms. The materials observations were summarized as: Silicon carbide exhibited enhanced corrosion at lower temperatures (1150/sup 0/C) when alkalies were deposited on the carbide from the flue gas and less corrosion at higher temperatures (1550/sup 0/C) when alkalies were not deposited on the carbide; alumina corrosion depended strongly upon purity and density and alumina contents less than 99.8% were unsatisfactory above 1400/sup 0/C; and mullite and cordierite are generally unacceptable for application in soda-lime glass melting environments at temperatures above 1100/sup 0/C

Effects of alternate fuels. Report No. 6. Analysis of low-alumina castable refractory degraded by residual oil combustion products

This is the sixth of a series of reports on analyses of several types of refractories used in industrial furnaces with fuels considered alternate to natural gas. Analyses were performed on a low-alumina castable used for only two months in the roof of a residual-oil-fired boiler. The maximum hot-face temperature during operation was about 1530/sup 0/K. The original microstructure of the castable, which consisted of mullite aggregate bonded with iron-containing gehlenite (2 CaO . Al/sub 2/O/sub 3/ . SiO/sub 2/), quartz and cristobalite, was totally altered during service in regions close to the hot face. At room temperature the altered microstructure consisted of corundum and gehlenite in a new oxide glass phase containing the elements Na, K, Ca, Fe, Ti, Al, Ni, and Si. The reactions of the fuel oil impurities Na, Fe, and Ni with mullite, quartz, and cristobalite in the original castable refractory caused the rapid degradation at the hot face during service in the boiler. Increasing the Al/sub 2/O/sub 3/ content of the castable by replacing mullite aggregate with alumina aggregate and using gehlenite with less iron impurity as the bonding material should improve the performance of this castable refractory or retard reactions of the castable with fuel oil combustion products including Na, Fe, and Ni

Study and analysis of the stress state in a ceramic, button-head, tensile specimen

The final results are reported for a study to identify and correct the causes of nongage-section failures (notably button-head failures) in ceramic tensile specimens observed in several laboratories. Numerical modeling of several candidate specimen gripping systems has shown inherent stress concentrations near the specimen button head at which the maximum stress may approach 75 to 100% of the gage-section stress for certain grip conditions. Empirical comparisons of both tapered- and straight-collet gripping systems revealed compromises in both systems. The straight-collet system, with deformable collets, is simpler to use but produces statistically significant greater average percent bending for all tests than those produced for the tapered-collet system, which is slightly more difficult to use. Empirical tensile tests of {approximately}50 aluminium oxide and {approximately}50 silicon nitride specimens were conducted to evaluate the loading capability of both gripping systems, the percent bending in each system, and the potential of consistently producing successful test results. These tests revealed that, due to variations in individuals specimens or the individual specimen/grip interfaces, neither of the gripping systems can consistently produce bending of less than 3 to 4% at failure although occasional values of {approximately}0.5% bending were attained. Refinements of grinding procedures and dimensional measurement techniques have shown critical details in both the practices and consistency of machining necessary for achieving the dimensional tolerances while minimizing subsurface damage. Numerical integration techniques indicate that up to a consistent 5.0% bending during fast- fracture tests can be tolerated before large influences are detected in the determination of the Weibull modulus and the Weibull characteristic strength