Atomic layer deposition of titanium phosphate onto reinforcing fibers using titanium tetrachloride, water, and tris(trimethylsilyl) phosphate as precursors

Abstract

A thermal atomic layer deposition process with precursors tris(trimethylsilyl) phosphate (TTMSP), titanium tetrachloride (TiCl4), and water was used with various pulse sequences in order to deposit titanium phosphate onto bundles of carbon fibers (diameter of one filament = 7 μm, 6000 filaments per bundle) and flat silicon substrates. Pulse sequence 1, TTMSP/N2/TiCl4/N2, which comprises no water, yields no significant deposition. Pulse sequence 2, TTMSP/N2/H2O/N2/TiCl4/N2, which comprises a water pulse, yields a mixed phosphate/oxide coating and shows a self-limiting character at 200 °C with a growth per cycle of 0.22 nm cycle−1. Wet chemical analysis of the coating revealed a ratio of Ti:P between 3:1 and 2:1 in reasonable agreement with the composition Ti2.4P1O7 obtained from X-ray photoelectron spectroscopy. Thus, the deposited material can approximately be described as a mixture of Ti¾PO4 and TiO2 in a molar ratio of 1:1.5. The coating shifts the temperature of the onset of oxidation—3% weight loss in thermogravimetry—of the carbon fibers from 630 °C (uncoated C-fiber) to 750 °C (with the titanium phosphate coating).

Amorphous Alumina Thin Films Deposited on Carbon Microfibers As Interface Layer for Thermal Oxidation Barriers

“…While a chemical deposition process on ordered graphite is often hindered by the low specific surface area of the substrate, the profusion of defects amidst the graphite layers and the grooved surface topology of HR carbon microfibers provided a well-suited surface for ALD. ,, The isotropic growth of the film from the edges of the closely packed grooves enabled a total coverage with a relatively low number of deposition cycles. The wide distribution in groove widths could not be argued to imply different kinetics of film radial growth, as the layer observed on the cross-sectional images was found homogeneous in thickness without apparent keyholes .…”

“…51,52 This thermal treatment yielded the total degradation of the carbon microfiber, consistently with previous reports. 1,10,11 This study focused on the structural soundness following thermal oxidation of the aluminum oxide thin film at the microfiber edges, as showcased in Figure 4d−k. The edges of carbon microfibers covered by 200 cycles of amorphous aluminum hydroxide deposition and following thermal oxidation are presented in Figure 4d, f, h. The film was observed to withstand the thermal cycle well, unbindingly from the original shape of the transversal section, as here shown for a variety of near-circular microfiber edges in Figure 4d. An enlarged view of a single microfiber edge shown in Figure 4f confirmed previous observations of conformal deposition, as seen from the preservation of the amorphous carbon surface aspect, with significantly smoother texture than the microfiber surface grooves.…”

“…1,10,11,15−17 To answer the multipart problem of deposition of an efficient refractory OBL on a carbon surface exhibiting a sharp interface with the carbon microfiber and with thermal expansion coefficients in a compatible range and a density light enough to limit the loss in elongation modulus, several compositions of thin films have been explored, namely, vitreous materials such as boron, 18 zirconium, 7,18 or silicon, 2,9 and light metal oxides such as alumina or titania. 1,10,12,19 Deposition of the latter two was widely reported feasible by low temperature ALD, which is here crucial considering the sensitivity of the carbon microfibers to thermal oxidation. 12,20 Furthermore, the use of partial exposure mode in ALD (outlet valve of the reactor shut during precursor pulses), which favors the diffusion of precursors within the bulk of the substrate, allows for in-depth gradient-free film coverage of the 3D substrate, as opposed to film coverage solely on the microfibers exposed at the surface of the woven fabric.…”

“…21 While efficient protection over 1275 K in air was reported for 100 μm thick coatings applied on C/C composites, 2,22 the performance of submicrometer thick films applied on carbon microfibers was not found to be significant. 1,10,12,19 Furthermore, investigations concerning the oxygen diffusion pathway at the fiber−film interface and potential phase transitions within the film as a function of temperature were found to be scarcely reported. As such, this article reports the investigation of the crystallization of aluminum hydroxide thin films following conformal deposition on carbon microfibers within a woven fabric.…”

“…Indeed, carbon microfibers are not subjected to creep with the increase of temperature, and exhibit even higher elongation modulus, contrary to ceramic microfibers such as SiC or boron-doped alumina . Thus, carbon microfibers make a promising material alternative for applications in air at high temperature, provided that they are shielded from oxidation over 670 K. Among the various methods reported for the deposition of a refractory oxygen diffusion barrier layer (OBL), such as chemical vapor deposition (CVD), sol-dipping, , or plasma electrolysis, , atomic layer deposition (ALD) was often preferred for the conformal aspect of the deposited film. ,− Indeed, ALD relies on the sequential injection of two coreactants, thus allowing for the deposition of a conformal layer through self-limitation of the half-reactions, constrained in thickness to one atomic layer of the deposited elements. ,,,, Owing to the gas phase process that is ALD, the conformal deposition was also successfully reported on three-dimensional substrates and textured surfaces. ,,,− To answer the multipart problem of deposition of an efficient refractory OBL on a carbon surface exhibiting a sharp interface with the carbon microfiber and with thermal expansion coefficients in a compatible range and a density light enough to limit the loss in elongation modulus, several compositions of thin films have been explored, namely, vitreous materials such as boron, zirconium, , or silicon, , and light metal oxides such as alumina or titania. ,,, Deposition of the latter two was widely reported feasible by low temperature ALD, which is here crucial considering the sensitivity of the carbon microfibers to thermal oxidation. , Furthermore, the use of partial exposure mode in ALD (outlet valve of the reactor shut during precursor pulses), which favors the diffusion of precursors within the bulk of the substrate, allows for in-depth gradient-free film coverage of the 3D substrate, as opposed to film coverage solely on the microfibers exposed at the surface of the woven fabric …”