Fabrication of Large-Sized Invar Alloy Film with Low Thermal Expansion by Pulse Reverse Electrodeposition

Abstract

Invar alloy film with low thermal expansion is a key substrate for high-resolution organic light-emitting diodes (OLEDs). The preparation of Invar alloy film by electrodeposition has attracted increasing attention due to its low cost and simple process. In this work, a large-sized (8 × 10 cm) uniform Fe-Ni alloy film was fabricated by pulse reverse electrodeposition. The effects of the electrodeposition parameters on the chemical composition and microstructure of Fe-Ni alloy films were investigated. Results showed that greater Ni2+/Fe2+ in the electrolyte, lower pulse frequency (f), smaller reverse pulse coefficient (x), and lower electrodeposition temperature (T) favored the iron content. The Invar alloy film (64 wt% Fe) was electrodeposited at T = 50 °C, x = 0.2, f = 10 Hz, and Ni2+/Fe2+ = 1.9 when the average pulse current density was 30 mA cm−2. The structure of the film was composed of a mixture of fcc and bcc phases, where (110) and (111) preferential orientations were predominant. After heat treatment in H2 at 800 °C for 120 min, the bcc phase transformed into the fcc phase, and the grain size increased (>2 μm). As a result, the coefficient of thermal expansion of the film decreased from 11 × 10−6 to 1.5 × 10−6/°C, which is close to the standard for commercial application.