I. Abstract of the Paper’s Core Research

This study employs the polyol method to synthesize 2.4-nm ultrafine PVP‑coated nickel nanoparticles, and develops a stable ultrasonic‑atomized AJP nickel ink by formulating an ethylene glycol/water dual‑solvent system. Using aerosol jet printing, micron‑scale nickel interconnects are directly written onto diverse substrates, including sapphire, AlN, LiNbO₃, and Kapton. Comprehensive experiments covering sintering, high‑temperature durability, redox stability, and graphene growth are systematically conducted. The key findings are as follows:

A stable nickel nanoparticle ink system compatible with aerosols

PVP surface modification suppresses particle agglomeration, while an ethylene glycol–water mixed solvent regulates the atomization and drying rates, enabling long‑term continuous printing; single‑track deposition yields uniform 40 μm nickel conductive traces, and multilayer stacking can produce thick film structures.

The high-temperature failure mechanism of nickel thin films has been fully elucidated.

In an argon atmosphere, the thin film exhibits stable electrical conductivity at 400–600°C; however, above 600°C, significant grain growth occurs, leading to spontaneous dewetting and the formation of discrete nickel islands, which disrupt the conductive pathways and cause an irreversible decrease in reflectivity. AFM/MFM characterization provides direct visualization of the magnetic domain distribution within these islands.

Nickel oxide can be reduced and remediated in the gas phase.

After high-temperature oxidation, a short‑time treatment at 600°C in a 10% H₂/Ar atmosphere reduces the nickel thin film via a nucleation–agglomeration mechanism, yielding a continuous conductive nickel layer; the degree of reduction in thick films is significantly higher than that in thin films.

High-precision, multi-substrate-compatible molding

Interdigital electrodes, grids, and custom fill patterns can be fabricated on piezoelectric AlN, lithium niobate, flexible PI, and sapphire substrates, with a minimum line width of 15 μm. A single system enables the fabrication of both planar and thick nickel‑filled structures.

Graphene with patterned morphology directly grown on an aerosol‑derived nickel substrate

The thickened, cross‑filled nickel pattern exhibits thermal stability up to 1000°C in graphene CVD growth; Raman characterization confirms the formation of multilayer graphene, enabling an integrated, lithography‑free patterning process via “aerosol‑printed nickel template–CVD graphene.”

II. Complete Preparation Process for Aerosol-Sprayed Nickel Thin Films

1) PVP‑coated nickel nanoparticles were synthesized via the polyol method, followed by centrifugation, washing, and drying.

2) An ink with a solid content of 100 mg/mL was prepared using a 1:9 glycol–water mixed solvent, and ultrasonic dispersion was employed to eliminate agglomeration.

3) Optomec AJ equipment, ultrasonic atomization, optimized sheath/gas‑carrier ratio, and a 100°C substrate temperature to facilitate solvent evaporation;

4) Prepare thick nickel substrates by either single-layer printing of fine electrodes or multi-layer cross‑filling as required.

5) Argon gas sintering at a temperature gradient of 450–700°C removes organic residues and promotes grain‑to‑grain electrical connectivity.

6) High-temperature durability testing / Hydrogen-atmosphere reduction annealing for repair / Transfer CVD furnace–grown patterned graphene.

III. Core Advantages of the Three‑Five Aerosol Process (Comparison with Lithography and Magnetron Sputtering)

Maskless digital direct writing, custom complex nickel patterns

CAD‑based free‑form design of interdigitated, grid‑like, and thick‑fill patterns—no photolithography masks or metal masks required—and custom graphene growth templates can be prototyped on the same day.

Fully compatible with multi-material substrates.

Piezoelectric ceramics, sapphire, flexible polyimide, and metal sheets can all be directly printed, overcoming the limitation of sputtering, which is restricted to planar hard wafers.

Thickness is precisely controllable, with a thin electrode and a thick graphene substrate formed as an integrated structure.

Single-layer fabrication of SAW sensor fine electrodes, with multilayer cross‑stacked thick nickel films that withstand graphene’s high‑temperature processing; a single ink formulation accommodates the requirements of both device types.

Low‑consumable, site‑specific deposition significantly reduces the use of precious nickel.

Nickel is deposited only in the functional areas, whereas sputtering over the entire substrate results in substantial waste of nano‑nickel, significantly reducing R&D costs in the laboratory.

Post‑printing low‑temperature thermal treatment in staged steps, with no damage to the flexible substrate.

During printing, the substrate temperature is only 100°C, and subsequent sintering employs staged heating as needed, allowing flexible substrates such as Kapton to retain their full mechanical properties.

IV. Five Core Technological Innovations of This Study

Innovation 1: Specialized PVP-Coated Nickel Nanoparticle Aerosol Ink

Ultrafine 2.4 nm nickel particles, combined with a dual-solvent system, address nozzle clogging and coffee-ring defects, enabling the batch printing of micron-scale, uniform conductive traces.

Innovation 2: A Comprehensive Theoretical Framework for High-Temperature Failure and Hydrogen Reduction in Nickel Thin Films

Elucidate the failure mechanisms of wetting islands at temperatures above 600°C; propose a nickel oxide nucleation–agglomeration–reduction model, distinguishing between the reduction behaviors of thick and thin films, thereby establishing a standardized process window for high-temperature device thermal treatment.

Innovative 3‑D aerosol‑assisted thick nickel templates enable lithography‑free patterning of graphene.

The thickened cross‑filled nickel structure withstands CVD temperatures as high as 1,000°C, eliminating the need for photolithographic etching of nickel films; graphene can be directly grown by patterning in a single step, thereby simplifying the fabrication process of two‑dimensional devices.

Innovative Integrated Multi-Substrate High-Temperature Piezoelectric Sensor Electrode Solution

Direct-write high-precision interdigital transducers on AlN and LiNbO₃ piezoelectric substrates, compatible with high-temperature SAW sensors, replacing the costly vacuum evaporation process.

Innovation 5: A Fully Additive Manufacturing Closed-Loop Process—AJP Nickel Printing + CVD Graphene

All components, from the conductive electrode to the substrate for two-dimensional material growth, are fabricated via additive manufacturing, eliminating reliance on cleanroom lithography and sputtering equipment.

V. Current Industry Challenges and Corresponding Solutions

Challenge 1: Conventional photolithography and sputtering nickel processes are time-consuming and rely on costly cleanrooms.

✅ Maskless direct writing of aerosols enables pattern fabrication on a desktop platform, eliminating the need for multiple process steps such as photolithography, evaporation, and lift-off.

Challenge 2: At high temperatures, nickel thin films tend to delaminate and form isolated islands, thereby limiting their use in high-temperature devices.

✅ A safe sintering window of 400–600°C has been defined; a multilayer, thickened nickel film enhances high-temperature stability, enabling graphene growth at 1000°C.

Challenge 3: After high-temperature oxidation, the nickel thin film cannot be repaired, resulting in direct scrapping of the sample.

✅ Short-term thermal treatment of low-concentration hydrogen–argon mixtures reduces nickel oxide back to a continuous conductive nickel layer, enabling the sample to be reused.

Challenge 4: Graphene patterning requires photolithography and etching of the nickel substrate, resulting in a complex fabrication process.

✅ Aerosol‑direct printing of nickel templates with target patterns, followed by in‑situ CVD growth of patterned graphene, eliminating the need for etching.

Challenge 5: The sputtering process struggles to deposit nickel on flexible, non‑planar piezoelectric substrates.

✅ Aerosol-based non-contact jet deposition enables uniform coating on curved surfaces, flexible films, and piezoelectric ceramics.

VI. Future Diversified Industrial Application Scenarios

1 High-Temperature SAW Piezoelectric Sensor for Extreme Environments

Aviation and nuclear power high-temperature monitoring: AlN-based surface acoustic wave devices with low-cost, mass‑production of aerosol‑deposited nickel interdigital transducers.

2 Graphene-based optoelectronic devices and gas sensors

Custom-patterned graphene growth on nickel substrates, enabling rapid prototyping of flexible optoelectronic and highly sensitive gas-sensing chips.

3 Flexible magnetic sensing and electromagnetic shielding circuits

Kapton‑based printed nickel‑conductive/magnetic mesh, for wearable electromagnetic monitoring and lightweight shielding films.

4 MEMS micro-heating and electrode-integrated chip

Micron-scale nickel interconnects for sapphire-based microdevices, and additive manufacturing of miniaturized microelectromechanical systems.

5 University-level R&D Platform for Two-Dimensional Materials and High-Temperature Microelectronics

No lithography or sputtering production line is required; aerosol-based methods enable rapid fabrication of nickel electrodes and graphene growth templates, significantly reducing investment in research equipment.

Image and Text Guide

Figure 1. Synthesis and characterization of PVP-capped Ni nanoparticles.

Figure 1(a): XRD—FCC Ni peaks at 45°, 52°, 77°, and 93° for (111), (200), (220), and (311), respectively; Scherrer analysis yields a crystallite size of 2.4 nm. (b) Wide-field TEM—soft agglomerates measuring 100–300 nm. (c) High-resolution image of the soft agglomerates. (d) Hard agglomerates measuring 20–50 nm. (e) Lattice fringes with spacings of 2–5 nm. (f) SAED pattern showing polycrystalline rings, confirming the FCC Ni phase.

Figure 2. TGA, magnetic response, sintering-induced phase transition, and ink rheology of Ni nanoparticles

Figure 2(a): TGA shows a total weight loss of 14.1% up to 1000 °C: glycol solvent below 175 °C, and PVP polymer between 175 and 550 °C; (b): permanent magnets adsorb Ni particles; (c): XRD after sintering at 700 °C reveals the presence of NiO (220), while annealing in an Ar atmosphere at 700 °C for 1 hour yields phase‑pure Ni; (d): grain size increases from 2.4 nm to 54.6 nm at 700 °C and to 68.2 nm at 1000 °C; (e): physical sample of the ink; (f): droplet of the ink, with a surface tension γ = 61.2 mN/m; (g): contact angles decrease over time at 25, 50, and 100 °C, and heating accelerates both ink wetting and rapid drying.

Figure 3. Mass characterization of AJP-printed Ni interdigitated electrodes on an AlN substrate.

Figure 3(a): Stitched 5× micrograph of the Ni interdigitated electrode on AlN; (b): 2D height map obtained with a profilometer; (c, d): Magnified view of the C‑DIC electrode fingers; (e): Z‑direction cross‑section along the scan line, with a ridge height of approximately 350–400 nm; (f): 10× micrograph of the Ni wire array; (g): Scatter plot of wire thickness versus wire width; (h): The average wire width of 16 wires is 40.76 ± 1.24 μm, demonstrating excellent repeatability.

Figure 4. Failure mechanism of the Ni four-point probe structure during high-temperature sintering

Figure 4(a,b): After sintering at 600 °C, four‑point probe measurements—optical image plus profilometer cross‑section—with a line height of ~250 nm; (c,d): After sintering at 650 °C, the electrodes begin to darken, and the line height is ~150 nm; (e): C‑DIC at 800 °C—numerous dewetting Ni islands appear on the surface; (f): AFM 3D topography; (g): MFM magnetic‑force microscopy image—micrometer‑scale magnetic fields emerge between the Ni islands, indicating that the continuous film has fragmented into discontinuous magnetic domains, leading to loss of electrical functionality.

 

Figure 5. Reduction mechanism of the oxidized Ni film in forming gas and its sintering resistance.

Figure 5(a,b): Partial reduction of Ni at 600 °C for 5 min in 10% H₂/Ar, measured by four‑point probe: C‑DIC and phase‑contrast imaging under the same field of view reveal distinct regions—bright for the reduced areas and dark for the un-reduced areas—with island‑like reduction nuclei coalescing into a continuous film; (c,d) evolution of the surface microstructure after sintering at 450 °C versus 900 °C; (e) 3D AFM topography at 450 °C, still maintaining continuity; (f) electrical resistance as a function of sintering temperature: ~3,500 Ω at 450 °C, dropping to ~80–100 Ω at 500/550 °C, and further decreasing to ~40 Ω after 5 min in forming gas at 600 °C.

Figure 6. CVD-grown patterned multilayer graphene on a Ni scaffold

Figure 6(a): AJP‑printed 1 cm² Ni pattern of a bronco head on sapphire (thick bronco); (b,c): C‑DIC images showing edge details with steep steps and minimal overspray; (d): Raman spectrum: D peak at 1350 cm⁻¹, G peak at 1590 cm⁻¹, and 2D peak at 2700 cm⁻¹, confirming multilayer graphene; (e): Intensity map of I(2D)/I(G); (f): Interdefect distance map calculated using the Cancado relationship.