Date: Thursday, Feb 16th
Presenter: Ravi Eluri, Microproducts Breakthrough Institute, OSU
Microchannel processing technology (MPT) offers several advantages to the field of nanomanufacturing: enhanced heat and mass transfer due to reduced liquid layer thickness, improved mixing in very short time intervals owing to shorter diffusional distances and narrower particle size distribution via homogeneous nucleation. However, various challenges are involved in fabrication of these microchannel devices. With the use of size dependent properties of nanomaterials some of the fabrication challenges can be resolved. This study is segmented in several sections to address the solutions in logical way. First section discusses the proof of concept for nanoparticle-assisted diffusion brazing at lower temperatures. In this, we investigated the effect of braze filler particle size (~5nm and ~50nm) and various processing parameters (heating rates: 5C/min and 25C/min) on vacuum diffusion brazing (550C and 570C) of thin foils of 3003 Al using a Ag nanoparticle (AgNPs) interlayer. A tensile strength of 69.7 MPa was achieved for a sample brazed at 570C for 30 min under 1MPa with an interlayer of approximately 7 micrometers in thickness. Further decrease in brazing temperature (500C) was achieved by sputtering a 1µm layer of Cu onto Al specimens to get a tensile strength of 65.8 MPa. For the second part, we have synthesized nickel nanoparticles (NiNPs) of sub 10 nm for diffusion brazing of 316L stainless steel. Depending of the synthesis technique, solvent used and reducing agent incorporated, the average particle size of NiNPs varied: ~5.4 nm from conventional heating with hydrazine, ~ 4 nm from conventional heating with NaPH2O2 and ~ 4 nm from microwave heating with NaPH2O2. Continuous flow synthesis using hydrazine reducing agent in a µ-T mixer (channel 521 micrometers) resulted in narrower particle size distribution with a resident time of 10 s. For the following section, we have investigated several deposition techniques to control film thickness and uniformity to diffusion bond 316L stainless steel (SS) microchannel device. Using in-house prepared NiNPs, a hermetic joint up to 70 psi (tested pressure) was obtained in 316L SS substrates under brazing conditions of 800C, 1 MPa and 30 min. In the final section, a in-house designed scale-up micromixer was designed to get uniform resident time and synthesized ~5nm CdS NPs in the order of 9 lit/hr.
Involved characterization techniques: FT-IR, XRD, SEM, TED, EDS, EPMA, DSC, Mass spectrometry, and lap-shear testing.