# Utilizing Fly Ash from Coal-Fired Power Plants to Join ZrO2 and Crofer by Reactive Air Brazing

## TL;DR

This study explores using fly ash to improve the air brazing of ZrO2 and Crofer, finding that 5% fly ash helps maintain airtightness while higher amounts cause leaks.

## Contribution

The novel use of 5% fly ash in reactive air brazing improves joint airtightness without compromising structural integrity.

## Key findings

- Adding 5% fly ash maintains airtightness for 280 hours at room temperature and 24 hours at 600°C.
- 10% fly ash leads to visible voids and cracks, reducing airtightness due to brittle oxide formation.
- 5% fly ash with Ag-rich paste is feasible for airtight applications in ZrO2-Crofer joints.

## Abstract

This study attempts to use fly ash as the brazing filler additive to increase the sustainable use of coal-fired power plant by-product materials. The experimental results show that adding 5 wt% fly ash into the Ag paste filler contributes to the interfacial reactions in heterogeneous reactive air brazing (RAB) of the ZrO2 and Crofer alloy. The Ag-rich phase dominates the brazed zone. The interfacial reaction layers contain oxidation of the Cu-Ti coating layer, Crofer alloy, and the Si/Al-rich oxides from the fly ash particles. The 5% fly ash RAB joint maintained airtightness for 280 h under 2 psig helium at room temperature. When the test temperature was raised to 600 °C for 24 h, the pressure of the joint assembly still did not drop. When the fly ash addition was increased to 10 wt%, the joint assembly was no longer leak-free at room temperature. Many visible voids and cracks exist in the brazed zone and at the ZrO2/braze and braze/Crofer interfaces. A high volume fraction of the fly ash particles results in many brittle Si/Al-rich oxides in the joint after RAB, and the fracture of these oxides significantly deteriorates the airtightness of the joint. This study shows the feasibility and potential of introducing 5 wt% fly ash particles to the Ag-rich paste filler during the RAB of ZrO2 and Crofer for airtight applications.