Research Papers: Heat and Mass Transfer

Buckling of Magnetically Formed Filler Fiber Columns Under Compression Increases Thermal Resistance of Soft Polymer Composites

[+] Author and Article Information
Matthew Ralphs, Chandler Scheitlin

School for Engineering of Matter,
Transport and Energy,
Arizona State University,
Tempe, AZ 85287

Robert Y. Wang

School for Engineering of Matter,
Transport and Energy,
Arizona State University,
Tempe, AZ 85287
e-mail: rywang@asu.edu

Konrad Rykaczewski

School for Engineering of Matter,
Transport and Energy,
Arizona State University,
Tempe, AZ 85287
e-mail: konradr@asu.edu

1Corresponding authors.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received June 18, 2018; final manuscript received August 29, 2018; published online October 24, 2018. Assoc. Editor: Ravi Prasher.

J. Heat Transfer 141(1), 012001 (Oct 24, 2018) (8 pages) Paper No: HT-18-1401; doi: 10.1115/1.4041539 History: Received June 18, 2018; Revised August 29, 2018

Thermally conductive soft composites are in high demand, and aligning the fill material is a potential method of enhancing their thermal performance. In particular, magnetic alignment of nickel particles has previously been demonstrated as an easy and effective way to improve directional thermal conductivity of such composites. However, the effect of compression on the thermal performance of these materials has not yet been investigated. This work investigates the thermal performance of magnetically aligned nickel fibers in a soft polymer matrix under compression. The fibers orient themselves in the direction of the applied magnetic field and align into columns, resulting in a 3× increase in directional thermal conductivity over unaligned composites at a volume fraction of 0.15. Nevertheless, these aligned fiber columns buckle under strain resulting in an increase in the composite thermal resistance. These results highlight potential pitfalls of magnetic filler alignment when designing soft composites for applications where strain is expected such as thermal management of electronics.

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Grahic Jump Location
Fig. 1

(a) Scanning electron microscopy image of the nickel fibers used in these composites and (b) illustration showing the molding and magnetic alignment of the nickel fiber–polymer composites including a schematic of the applied magnetic field

Grahic Jump Location
Fig. 2

Illustration of the Poisson's effect on the orientation of fiber buckling, which causes the fibers to buckle outward in the radial direction

Grahic Jump Location
Fig. 3

Optical microscope images of the aligned (top to bottom) nickel fiber–polymer composites under zero compressive strain at nickel fiber volume fractions of 0.05 (a), 0.15 (c), 0.25 (e), and 0.35 (g) along with fiber alignment measurements ((b), (d), (f), and (h)), respectively. White lines in (a) highlight aligned fiber column axis.

Grahic Jump Location
Fig. 4

(a) Fill volume fraction, ϕ, versus the thermal conductivity enhancement over that of the matrix material for aligned (ǁ) and unaligned nickel particles, nickel fibers, and nickel platelets. Data for the nickel particles* and nickel platelets** are taken from Refs. [33] and [34], respectively. (b) Measured thermal conductivity for aligned and unaligned nickel fibers in the polymer matrix at various ϕ. Nickel fiber measurements are at low strain (ϵ<0.05).

Grahic Jump Location
Fig. 5

Measured (points) and theoretical (lines) values for (a) thermal conductivity and (b) thermal resistance at a nickel fiber volume fraction of 0.05 as compressive strain increases; and (c) a simple resistance network at the junction between two aligned nickel fibers in the polymer matrix. Inset (a) (bottom) shows a schematic illustrating the transition from parallel to series heat conduction (see Fig. S4 which is available under the “Supplemental Materials” tab for this paper on the ASME Digital Collection, for a more detailed schematic) that is the basis for the theoretical model for the dashed line that more closely fits the measured values in both (a) and (b) and inset (b) shows optical microscope images supporting the schematic in inset (a) (bottom). Inset (a) (top) shows a schematic for the theoretical model on which the line well above the measured values in (a) and well below the measured values in (b) is based.

Grahic Jump Location
Fig. 6

(a) Measured thermal conductivity, (b) ratio of compressed thermal conductivity to uncompressed thermal conductivity, (c) measured thermal resistance, and (d) ratio of compressed thermal resistance to uncompressed thermal resistance for various nickel fiber volume fractions and increasing compressive strain, ϵ



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