The Increasing Importance of the Thermal Conductivity of Ceramics Ceramics Expo 2017 Richard Clark - Senior Technical Specialist richard.clark@morganplc.com www.morganadvancedmaterials.com
Contents Overview of Morgan Advanced Materials Thermal Conductivity Definitions and measurement Thermal conductivity of traditional ceramics New materials widening the thermal conductivity range Applications and market prospects 2
Morgan Advanced Materials Founded in England in 1856 Ticker on LSE: MGAM 2016 revenue: GBP989.2 (2015 GBP911.8 million) 6 Global Business Units Global Business Units Thermal Ceramics Molten Metal Systems Electrical Carbon Seals and Bearings Technical Ceramics Composites and Defense Systems 3
Definitions and measurement of thermal conductivity Thermal conductivity, a transport property frequently referred to as λ, is a measurement of the ability of a material to conduct heat, considered to equate to the time rate of heat flow under steady conditions through unit area per unit temperature gradient in the direction perpendicular to the area. With many assumptions, including λ being constant, conductive heat flow is governed by Fourier s law: Q= - λa(dt/dx) where Q is heat flow, λ is thermal conductivity, A is area, dt is temperature difference and dx is thickness. Measurement methods Steady-state (typically λ < 5 Wm -1 K -1 and temperature <650 C except guarded-comparative-longitudinal) Dynamic (transient) Heat-flow meter Guarded heat-flow meter Guarded hot-plate Guarded-comparativelongitudinal heat flow Hot wire Laser flash (diffusivity) Easy, accurate, quick primarily for insulation Slightly higher temperature measurement range Wider temperature range, absolute method λ to 200 Wm -1 K -1 and temperature up to 1000 C λ to 20 Wm -1 K -1 and temperature to 2000+ C λ to 2000+ Wm -1 K -1 and temperature to 2000+ C ASTM C518-15 ASTM E1530-11(2016) ASTM C177-13 ASTM E1225-13 ASTM C1113 / C1113M-09 (2013) ASTM E1461-13 4
Wide range of thermal conductivity in traditional ceramics Thermal conductivity (W/m/K) 10000 1000 100 10 1 0.1 94% alumina λ is about ½ that of 99.5% alumina Air Zirconia Fused silica Silicon nitride CVD silicon carbide Aluminum nitride Alumina Sapphire Beryllium oxide Copper Diamond AlN with low lattice oxygen % can have λ approaching that of copper Many factors control the thermal conductivity of ceramics: Grain size and boundaries; Bonding; Purity; Type and structure of impurities; Porosity Main mechanism for heat transfer in ceramics is atomic/lattice vibration 0.01 0.001 Bounded by fused silica at 1.4 Wm -1 K -1 and diamond at 2200 Wm -1 K -1 for traditional ceramics 5
Recent areas of interest in thermal conductivity (high) Thermal conductivity (W/m/K) 10000 1000 100 10 1 0.1 0.01 0.001 Air Zirconia Fused silica Silicon nitride CVD silicon carbide Aluminum nitride Alumina Sapphire Beryllium oxide Copper Diamond Cubic Boron Arsenide: newly determined to have thermal conductivity similar to that of diamond As an electrical insulator this would make it suitable for passive cooling in microelectronics Graphene: For unstrained graphene λ can be up to 5,450 Wm -1 K -1 (theoretical, experimentally shown to 5,300 Wm -1 K -1 ) For strained graphene λ diverges based on sample size (length in direction of heat flow) so appears to violate many beliefs of λ being an intrinsic material property (and Fourier s law) 2D h-bn: for electronics substrates and packaging 6
Recent areas of interest in thermal conductivity (low) Thermal conductivity (W/m/K) 10000 1000 100 10 1 0.1 0.01 0.001 Air Zirconia Fused silica Silicon nitride CVD silicon carbide Aluminum nitride Alumina Sapphire Beryllium oxide Copper Diamond Silica aerogel λ = 0.0134 Wm -1 K -1 (compared to air at 0.026 Wm -1 K -1 at room temperature) Graphene aerogel λ = 0.053 Wm -1 K -1 (at room temperature and as much as an order of magnitude lower at low temperatures(*)) (Silica based) microporous materials with bulk thermal conductivity ½ that of calm air at operating temperatures for insulation * Xie et al, Carbon Volume 98, March 2016, Pages 381 390 7
Types of high temperature insulation fibers Amorphous High Temperature Insulation Crystalline ASW/RCF invented 1942 PCW invented 1969 AES invented 1986 (Microporous invented 1958) Alkaline Earth Silicate (AES) Aluminosilicate (ASW / RCF) Polycrystalline (PCW) Melt-spun CaO, MgO, SiO 2, ZrO 2 Superwool Plus, Superwool HT Melt-spun / blown Al 2 O 3, SiO 2, (ZrO 2 ) Kaowool Sol-gel Al 2 O 3 Alphawool Key Advantages of Superwool over RCF: Low bio-persistence Low shrinkage up to classification temperature Low thermal conductivity 8
Thermal conductivity comparison for fiber materials Thermal conductivity comparison of WDS microporous insulation products vs. other types of insulating products 9
Applications where thermal conductivity is key High Thermal Conductivity Microelectronics packaging (passive cooling) (e.g. BeO, AlN, diamond) Lasers/photonics (e.g. BeO and AlN) (also YAG in solidstate lasers) Aero/defense (e.g. diborides) Low thermal conductivity Kiln walls (insulation e.g. RCF, AES) Aerospace (insulation e.g. silica aerogel) Gas turbines (Thermal Barrier Coatings e.g. 7YSZ) 10
Drivers increasing the need for ceramics higher temperatures are behind many material changes Application Electronics / semiconductor Automotive Energy / Power Industrial CPI Medical Military / Defense Aerospace Drivers Higher temperature / increased need for precision Lower vehicle weight (ICE and xev) / higher engine temperature Higher temperature / need for improved efficiency Higher temperature / higher wear environment Higher temperature / higher pressure / high corrosion High corrosion / innovation (new inventions) Higher temperature / lower weight Higher temperature / lower weight Largest market segment is electronics. Oxide ceramics are about 60% of the total market and alumina is more than half of that. Market size strongly dependent on definition! 11
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