Arc Melting: A Route for High-Temperature Ceramic Composites, Doping, and Powder

SynthesisCore faculty: Richard A. Haber

Research associate: Chawon Hwang & Jun Du

Graduate student: Qirong “Bruce” Yang

June 24th, 2020

CCOMC Meeting

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• Arc melting overview• What’s arc melting

• Rutgers units

• Arc melter as a versatile research tool:• Part I: Doping and powder synthesis

• Al-doping

• Si-doping

• Part II: High temperature ceramics• Synthesis of compounds

• UTHC composites

• Eutectic composites

Agenda

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Rutgers arc melter

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• Electric arc: A continuous high-density electrical discharge through an ionized medium (air or Ar etc.) caused by a dielectric breakdown.

• Temperature: 3,000-20,000 °C (arc flash)

• Operating temperature: >3,500 °C but depends on current density

What is arc melting?

Cathode(W stinger)

Anode(water-cooled copper hearth)

100 - 800 Amp

Sample

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Our lab-scale arc melters

Arcast arc melterCurrent: < 800 AmpW tip: 9.5 mm ØCapacity: < 30 g ceramics

< 100 g metalsAttachments: casting molds

vacuum suctionE/M stirring

Centorr arc melterCurrent: < 350 AmpW tip: 1.5mm ØCapacity: < 1 g ceramics

< 10 g metals

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Part I: Arc melting for doping and powder synthesis

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Why doping boron carbide

To demonstrate that arc melting can be a viable way to dope ceramic: boron carbide as the model material

Two issues with consolidated boron carbide ceramics:1. Suffers from brittleness due to the lack of dislocation movement2. Susceptible to stress-induced amorphization, a phenomenon believed

to cause boron carbide’s unexpected glass-like fracture behavior under high stress impacts

Solution:1. Al-doping can enable dislocation movement2. Si-doping can mitigate amorphization

Armor ceramics

Water nozzle

Abrasive media

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Why arc melting for Si-doping

Method Processing time (hr) Specimen scale Microstructure control

High energy ball milling 10+ µm NA

Nano-rod growth 10+ nano NA

Diffusion couple 4-6 100s µm No

Reaction hot pressing 4-6 mm-cm Little to none

Existing methods for Si-doping

25 µm

Si-doping through rxn hot pressing

• Rapid grain growth due to liquid

phase sintering

• Decreased in mechanical properties

• Need a scalable process that offers

microstructure control

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How to dope boron carbide

Pre-reacted powdersPre-reacted feedstock Final microstructure

• Can control the melt composition → powder with a variety of chemistry• Powder is already pre-reacted → less sintering time → smaller gains • Control powders size → control microstructure• Mimics the existing industrial process for producing boron carbide powder → scalability

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Particle after milling

10 min 30 min

60 min 120 min

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Si-doping mitigates amorphization

20 µm

Si kα

• Si-doped boron carbide bulk samples processed using the arc melting approach• 48% reduction in amorphization intensity• Comparable hardness to a commercial grade B4C

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Structural change due to Al-doping

• Larger unit cell; indicated by downshift in the XRD pattern• Change in the chain structure

(BxC) X-Al-X X = C or B

(B11C) C-B-C linear

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Dislocation movement

• Al-doping enabled dislocation movement in boron carbide!• New deformation mechanism to dissipate strain energy

Dislocations in stainless steel

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Icosahedral slip under shear deformation

• Breaking and reconfiguring bonds allow planes of icosahedra to slip without breaking

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Challenges

• Difficult to obtain 100s µm feed

• Minimize contamination from the particle refinement process → avoid using foreign grinding media

• Prevent oxidation on the milled powders → mill in solvent and dry in vacuum ovens

• Streamline the process to lower the cost

• Cheaper Si and B source

Jaw crusher(Steel)

0.5-3 mm

Netzsch(polymer)

<1 µm

Melted feedstock

3-4 cm

Attritor(SiC)

<10 µm

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Part II: Arc melting for high-temperature ceramics

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Advantages of arc melting

Advantages:• Very high processing temperature• Fast! L → S, no sintering • Can use powder or chucks• No powder mixing• Easy mixing in liquid phase• Casting/molding complex shapes

Limitations:• Batch size (industrial-scale do exist)• Cavities from L → S• Materials should be “stable” under arc• Can’t control temperature

TaC HfC

ZrC WHfB

2TaN H

fNZrB

2TiB

2TiC Ta

TaB2

TiNZrN S

iCB4C

2000

2200

2400

2600

2800

3000

3200

3400

3600

3800

4000

Meltin

g t

em

pera

ture

(oC

)

Materials

Arc melting

Conventional sintering

Fahrenholtz et al. 2014

UHTC

✔Generally applicable for metals and ceramics with high melting temperature

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Synthesis of compounds from elements

Ta

X

Raw materials After melting

Ir

Ir

Hf

Y

TmeltTa: ~3020 °CIr: ~2460 °CHf: ~2230 °CX: ~2000 °CY: ~1500 °C

Arc melting provides a fast avenue to process compounds from elements with high melting point

Could be used for high entropy alloys such as (Hf, Ti, Zr, V, Nb, Ta) diborides and other nitride/carbide systems

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UHTC composites

UHTC application:

B4CTiB2ZrB2

UHTC composites designs• + SiC → improve oxidation resistance• + SiC → increase flexural strength• + B4C→ increase hardness• + HfB2 → increase toughness• + ZrC → improve thermal conductivity• other UHTC → fine tune properties

SiC

Fast processing time for cheap!

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Ceramic eutectic systems

TiB2-SiC

ZrB2-SiC ZrB2-B4C-SiC

TiB2-B4C TiB2-B4C-SiC

SiC-B4C

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ZrB2–B4C eutectic systemWhy eutectic?1. Fine microstructure → excellent mechanical properties2. Secondary phases → improves toughness; alter fracture behavior3. Microstructure texturing → anisotropy

Yang et al, Mater. Chara. (2019) 155, pp. 109797

• Hardness and toughness anisotropy due to microstructure texturing• Electronic and physical properties could also experience anisotropy

ZrB2

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Arc melting presents itself as a versatile research equipment

• Si- and Al-doped powders and bulk materials

• Synthesis of compounds from high melting temperature elements (Ta, Ir, Hf)

• UHTC composites

• Eutectic composites

Key takeaways

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Acknowledgements

Thank you for your attentionQuestions?

Qirong “Bruce” Yangqy48@scarletmail.rutgers.edu