1. Adviser: Dr. Shengmin Guo
Electromagnetic Interference (EMI) Shielding Effectiveness of High Entropy AlCoCrFeNi Alloy
Yi Zhang
Adviser: Dr. Shengmin Guo
Good morning every body. Today, we’ll talk about the electromagnetic interference shielding effectiveness of high entropy AlCoCrFeNi alloy. Besides the intrinsic properties of HEA, the particle size and shape are the important for the EMI shielding effectiveness. How to produce micro or nano High entropy alloy particle effectively attracted our attention.
LOUISIANA STATE UNIVERSITY
College of Engineering
Department of Mechanical &
Industrial Engineering

2. Fabrication of High Entropy Alloy
Arc melt casting: most of the bulk alloys were prepared by arc melt casting. However, the arc melt process can only prepare samples with limited shapes and sizes, which will definitely restrict the applications of high entropy alloys.
High energy ball milling (HEM) is a widely used solid state processing route for synthesis of advanced material, which can easily lead to the formation of nanocrystalline and increase the properties and application scope of high entropy alloys. It can produce large- scale, micro- and nano-sized material, which are high in purity and uniform in composition.
Many high-entropy alloy systems have been exploited recently, and most of the bulk alloys were prepared by arc melt casting. However, the arc melt casting process can only prepare samples with limited shapes and sizes, which will definitely restrict the applications of high-entropy alloys. The high energy ball milling is a widely used solid state processing route for synthesis of advanced materials, which can easily lead to the formation of nanocrystalline and will definitely increase the properties and application scope of high-entropy alloys.

3. Fabrication of Large HEA and Small HEA Particles
The powder of Al, Co, Cr, Fe and Ni with the same atomic percentage were mixed together and then put into stainless steel jar and mill together with stainless steel balls
Large HEA particles (HEAL) were obtained after ball milling for 10 hours.
After then, 5 hours wet ball milling was used to reduce the size of large HEA particle
The powder of Al, Co, Cr, Fe and Ni with the same atomic percentage were mixed together, and then put into stainless steel jarmill together with stainless steel balls. The large HEA particles were obtained after ball milling for 10 hours. After then, 5 hours wet ball milling was used to reduce the size of large particle. The milling process and handling of raw materials were carried out in an argon gas environment inside a glove box to protect the particles from oxidation.
Schematic diagram of the high energy ball milling synthesis mechanism for HEA particles

4. Lattice constant of small particle was 0.289 nm
XRD peaks of Al, Cr, Fe, Co and Ni can be traced in the premixed powders as shown in Figure a
The crystalline structure of large and small HEA particle were confirmed to be BCC phase
Lattice constant of large HEA was determined as nm by the Bragg’s law
Lattice constant of small particle was nm
The XRD peaks of Al, Cr, Fe, Co and Ni can be traced in the premixed powders as shown in Figure a. The crystalline structure of large HEA particle were confirmed to be BCC phase, see the appropriate indices identified. The lattice constant of large HEA was determined as nm by the Bragg’s law. From Figure b, the pattern of the small HEA particle were also proved to be BCC phase and the lattice constant was nm.
XRD spectra of (a) premixed metal powder and (b) the large (bottom spectrum) and the small (top spectrum) high entropy AlCoCrFeNi alloy particles

5. Figure a, it can been seen that the large HEA particles are non- agglomerated. Figure b demonstrate that the particle is quite equiaxed.
Figure 3c; small HEA consist of non-agglomerated particles with polygonal shapes and sharp edges, even some of them are flake
From Figure a, it can been seen that the large HEA particles are non-agglomerated and they have wide size distribution from nanometers to larger than 20μm. Figure b is a representative of the large HEA particles, it also demonstrate that the particle is quite equiaxed.
The small particles were shown in Figure 3c; they consist of non-agglomerated particles with polygonal shapes and sharp edges, even some of them are flake. Figure d shows that some small particles is flake. From Figure a and c, it can been seen that the additional wet ball milling is effective for the changing in particle size and morphology.
SEM micrographs of (a) large and (c) small high entropy AlCoCrFeNi alloy particles, (b) represent the large particle in (a), and (d) represent the small particle in (c).

6. Composition (at %) analysis results obtained from EDS for large and small HEA particle
Element Al Cr Fe Co Ni
HEAL(%)
19.95
17.77
21.58
21.62
19.07
HEAS(%)
18.89
18.75
22.84
20.30
19.24
To further explore the chemical composition, the EDS point measurement was used and the results were listed in this Table. The point 1 and point 2 are randomly chosen in the large particle and small particle, respectively. From this Table , the concentration of Al, Cr, Fe, Co and Ni for point 1are close to 1:1:1:1:1, the chemical composition of the large particle is agree with the percentage of pre-mixed powder, which suggesting the large particles is homogeneous in chemical composition. For point 2, the elemental proportion are still close to the results of point 1, which indicate the small particle is also homogeneous in chemical composition as the large.
The concentration of Al, Cr, Fe, Co and Ni for point 1are close to 1:1:1:1:1, the chemical composition of the large particle is agree with the percentage of pre-mixed powder, which suggesting the large particles is homogeneous in chemical composition.
For small HEA particle, the elemental proportion are still close to the results of large HEA particle, which indicate the small particle is also homogeneous in chemical composition as the large particle.

7. Range of large HEA particle size is broad: from 500 nm to 20 μm.
The average particle size: 7μm
The small HEA particle exhibited more narrow distribution, the range is 100 nm-7μm
The average particle size: 7μm
Figure a and b are the statistical size distribution of large and small particle respectively. From the Figure a, it can been seen that the range of large particle size is broad, which is from 500 nm to 20 μm. However, the small particles exhibited more narrow distribution, the range is 100 nm-7μm. The average size of the large particles are 7μm, and the small particles are 3μm, which demonstrated that the wet ball milling is effective in reducing the particle size.
The statistical size distribution of the (a) large and (b) small HEA particles

8. To further investigate the crystal structure of the small particles, the TEM and SAD patterns are taken on one single randomly selected small particle.
To further investigate the crystal structure of the small particles, the TEM and SAD patterns are taken on one single randomly selected small particle. Figure a is the TEM image under a relatively lower magnification, a corresponding selected-area electron diffraction (SAD) pattern taken on the whole particle. The SAD pattern is indexed to BCC [ 1 11] zone axis patterns, which confirms the XRD results.
TEM image of the smaller HEAs particle, and (b) High-resolution TEM images from the marked region (Region A). The inset in (5a) is the corresponding ED pattern along the [ 1 11] zone axis.
The SAD pattern is indexed to BCC phase, which confirms the XRD results.

9. Fabrication of HEA/Epoxy Composites
Epoxy resin monomer
HEA
Mixture
High mechanical stirring
sonication
Curing agent
Low mechanical stirring
Sonication in ice-water bath
Uncured composite
Solidified specimen
Poured into molds
Curing 24 hours
Large particles or small particles were firstly dispersed into the epoxy resin monomers, then the suspension was mechanically stirred for one hour at high speed for uniform dispersion then , one additional hour of sonication, was essential to remove the bubbles, which were trapped in the early processes. The curing agent was then added to this mixture with the volume ration of 1 to 1. Low-speed mechanical stirring was conducted for 20 minutes to mix the epoxy and curing agent well. Low-speed stirring was adopted to avoid the entrainment of new gas bubbles. Then sonication was performed in an ice-water bath to remove the entrained bubbles and lead to a homogeneous mixture. Finally, the uncured composite was carefully poured into mold and cured at room temperature for at least 24 h to ensure the formation of solidified specimens.

10. EMI Shielding Effectiveness
The total EMI SET of HEAL/epoxy composite is about 7 dB and the maximum value is only 8.44 dB at 29.0 GHz
The HEAS/epoxy composite shows excellent microwave shielding properties
The maximum SET is 20 dB at 35.7 GHz, suggesting that over 99 % EM wave is shielded
The SET value is more than 15 dB in the whole GHz range, corresponding to more than 97 % EM wave is shielded, which is considered an adequate level of shielding for many applications
EMI SET of HEAL/epoxy composite, HEAS/epoxy composite at GHz

11. Shielding Mechanism
It is obvious that the absorption is much more than the reflection in the whole frequency for both HEAL and HEAS
It can be seen that SEA of the HEAL/epoxy composite is low (5.4±1.4), while the SEA of the HEAS/epoxy composite is over 10.4 in the whole range of GHz and has a maximum value of 18 dB at 35.2 GHz.
The improvement of SER is not obvious from HEAL/epoxy to HEAS/epoxy composites, it just improve from 2.1 to 3.6 dB
The reduced size enhances the total EMI shielding mainly via the enhanced electromagnetic wave absorption performance.
The absorption SEA dB and the reflection SER dB of HEAL/epoxy composite and HEAS/epoxy composite at GHz

12. Conclusion Epoxy resin HEA Mixture monomer Uncured composite
AlCoCrFeNi high entropy alloy (HEA) are synthesized by high energy ball milling, and the additional wet ball milling optimize the particle size and shape.
The FE-SEM estimate that the quit equiaxed HEAL particle distribute in the range of μm, while the polygonal HEAS particle is in the size range of μm.
Comparing with the HEAL, the HEAS are found to have more excellent microwave shielding effectiveness over the tested frequency range of 26.5 to 40 GHz due to the smaller particle size and the flake-like shape.
The HEAS sample has a maximum total shielding (SET) of 20 dB, compared to 8.44 dB of the HEAL sample, mainly via the enhancement of absorption (SEA) from 6.8 dB of HEAL to 18 of HEAS.
Epoxy resin
monomer
HEA
Mixture
High mechanical stirring
sonication
Curing agent
Low mechanical stirring
Sonication in ice-water bath
Uncured composite
Solidified specimen
Poured into molds
Curing 24 hours

13. Thanks for your time!