High temperature composites and their application

This application claims the benefit of U.S. Provisional Application No. 61/586,001, filed Jan. 12, 2012, which is hereby incorporated by reference in its entirety as if fully set forth herein.

The present invention relates to the field of high temperature coating, pavement concrete joints, sealant and materials for blast furnaces, and shielding materials for nuclear reactors.

There are two types of sealant materials currently available for high temperature application: silicon based materials and glass based sealant materials.

Glass-ceramics, derived from “invert” alkali earth silicate glasses, are used as sealing materials for solid oxide fuel cell applications. The glass based ceramics materials are borosilicate glass-ceramics, boron-free alkaline earth silicates, and phospho-silicate glass-ceramics, which can withstand up to 800° C.

The silicon based ceramics are manufactured by various companies such as Dow Corning, Insulflex, and Cytec. These materials are stable up to 600° F. Some of the materials react or decompose in organic solvent, ultraviolet light and other chemicals. Currently silicon based materials have been used to seal the concrete joints.

These existing sealant materials are not stable at high temperature and decompose in harsh chemical environments.

Needs exist for improved high temperature sealant materials.

The present invention provides a high temperature composite that includes a binder, cement or geopolymer and ceramic filler, negative thermal expansion materials such as zirconium tungstate. The material is compatible with concrete, any ceramics and metal. The material is heat shock resistant and stable in harsh chemical environments and is impermeable to most solvents. The new sealant materials can be used as sealants, heat shock resistant structural materials and coatings.

The new ceramics based sealant materials are stable up to 1,000° C., which is much higher than existing capabilities technologies. The new sealant materials are robust in most chemical environments. The new sealant materials are impermeable to water and oils. The new sealant materials do not degrade under UV radiation.

These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.

FIGS. 1A-1E depict the shape of the specimen in FIG. 1A , a synthesis protocol for manufacturing the sealant materials in FIG. 1B , fabrication of a cement based specimen in FIG. 1C , prefabricated specimens in FIG. 1D and cross sectional view of the specimen in FIG. 1E .

FIGS. 2A-C are graphs of the thermo-gravimetric analysis (TGA) data, showing normalized weight as a function of temperature.

FIGS. 3A and 3B show specimens in FIG. 3A and a graph of the change in volume as a function of thermal cycle in FIG. 3B .

FIG. 4 shows a chemical resistivity test of cement-zirconium tungstate.

FIG. 5 shows the thermal expansion of specimens 1 as a function of temperature.

FIGS. 6A and 6B show a schematic in FIG. 6A and lab setup in FIG. 6B for a simulated exhaust test.

FIG. 7A-C show lab testing at 500° F. in FIG. 7A , and a specimen before heating in FIG. 7B and the specimen after heating at 500° F. in FIG. 7C .

FIGS. 8A-C show results of lab testing at 1700° F. of cement-zirconium tungstate (ZT3).

The invention is based on compensating for the expansion of pavement concrete with a matching contraction of cement-zirconium tungstate (CZT) composites in the joints. To achieve this, the coefficient of thermal expansion (CTE) of the composite should be lower or a match to the positive CTE of concrete (7-12×10 −6 /° C.). The required value was achieved by adding negative CTE materials to a cement matrix. The expected coefficient of thermal expansion was estimated using Turner's model shown in Table 1. Based on these estimations, cement-quartz (Q1), cement-quartz-zirconium tungstate (Q-ZT-2) and cement-zirconium tungstate (ZT3) were considered as shown in Table 2.

As shown in FIG. 1A , specimens having 2″×0.25″ square cross sections and ends flat and parallel to within 0.001 inch were fabricated as per ASTM E228-06, “Standard Test Method for Linear Thermal Expansion of Solid Materials”.

The general synthesis scheme for making the sealant materials is summarized in FIG. 1B . Three different types of sealant material specimens 1 were prepared: cement quartz (Q1), cement quartz zirconium tungstate (Q-ZT-2) and cement zirconium tungstate (ZT3). The sample materials were mixed with water for 10 minutes to make pastes. The water content was varied to obtain consistencies suitable for molding.

The pastes were placed in a wood mold 3 and cured at 100% humidity for 4 days as shown in FIG. 1C . The specimens 1 were extracted from the mold 3 as shown in FIG. 1D and were tested without further processing. FIG. 1E is a cross-sectional view of specimen 1 .

The composites were thoroughly characterized. Thermo-gravimetric analysis (TGA) was used to study the thermal stability of materials, in all three specimens on heating as shown in FIGS. 2A-C . The overall mass losses for cement quartz, cement quartz zirconium tungstate and cement zirconium tungstate were 12.3%, 19.5% and 16.2%, respectively, during the first cycle with no significant change observed in subsequent cycles. The cement-quartz specimen shows two steps, 3% from room temperature to 200° C. and 9.3% from 600 to 1000° C. Cement-quartz-zirconium tungstate and cement-zirconium tungstate show ˜10%, ˜4%, ˜6% mass loss in the 100-200° C., 450-500° C. and 600-1000° C. ranges. The changes in the low-temperature and high-temperature ranges can be attributed to trapped moisture and decomposition of a structural hydroxyl group on the specimen surface respectively.

The shrinkage and surface changes for Q1, Q-ZT-2 and ZT3 were examined by placing a 40×10×10 mm in an open air box furnace, heating the specimens to 1,000° C. and then allowing the specimens to cool to room temperature. Volume reductions of 24%, 12% and 14% were observed after 25 cycles in Q1, Q-ZT-2 and ZT3, respectively, as shown in FIGS. 3A-C . The first cycle resulted in maximum shrinkage in agreement with mass-loss data. The reduction was due to evaporation of water and consequent shrinkage of the matrix. Both are known characteristics of cement materials. Specimens of Q1 and Q-ZT-2 exhibited numerous stress cracks. These cracks are considered normal and can be avoided by modifying material composition.

Chemical stability of specimens 1 was determined by exposure to Anderol ROYCO 782 Hydraulic Fluid (MIL-H-83282), AeroShell Turbine Oil 500 (MIL-L-23699), and JP-5 jet fuel ( FIG. 4 ). All the samples showed negligible change in volume and no visible physical or chemical change. The increase in weight of 6.5-8% was observed in the specimens. The weight increases can be attributed to absorption of fluid into the pores of the specimens 1 as shown in Table 3.

Thermal expansion of the samples was measured as a function of temperature as shown in FIG. 5 . Q-ZT-2 and ZT-3 expanded from 20° C. to 100° C. and contracted from 100° C. to 1,000° C. The average coefficient of thermal expansion (CTE) was estimated based on a linear best-fit analysis of the data and listed in Table 4. The CTE of ZT3 specimen is found to be 8.9×10 −6 ° C. −1 for 20-100° C. and −18×10 −6 ° C. −1 in 100-1,000° C. temperature range. Those results showed a 40% less positive and 44% less negative CTE compared to the equivalent ranges in pure cement. The CTE of ZT3 is thus found to be in the desired range for concrete expansion joints.

As shown in FIG. 6A , in order to examine the stability of sealant materials 5 , tests were performed involving exposure of concrete panels 7 to gas flames 9 at 550±50° F. and 1,700° F. The test was used to study the thermal stability of specimens. FIG. 6B shows the laboratory set up with the concrete panels 7 , the sealant 5 , thermocouple 11 and the blow torch 13 .

Unreinforced 6×6×1 inch Portland cement concrete (PCC) panels were fabricated and used as shown in FIGS. 7A-C . A slot 0.25 inch wide and 0.50 inch deep was cut and filled with sealant. The samples were cured at 100% humidity for 4 days. The panels were clamped facing a gas flame 9 with thermocouples 11 installed at various locations on the sealant 5 , within the sealant and on the concrete 7 two inches away from the hottest spot 15 , as shown in FIGS. 7A-C .

In a typical test, the sealant surface was maintained at either ˜500° F. for 15 minutes or 1,700° F. for 20 seconds. After each exposure, the specimen 1 was visually inspected to document change in material appearance or loss of adhesion as shown in FIG. 7C . Measurements were repeated twenty times to evaluate long-term durability of the material.

No loss of material or adhesion was observed in the heated zone 17 as shown in FIG. 8 . No flame, charring or physical change was observed on the sealant 5 , suggesting that the sealant material retains its original physical properties. Preliminary studies show that the CZT composite outperforms existing polymer-based sealant materials.

While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention, which is defined in the following claims.