Thermal Conductivity Of Ceramics Research Articles

High-entropy (Ho0.2Y0.2Dy0.2Gd0.2Eu0.2)2Ti2O7/TiO2 composites with excellent mechanical and thermal properties

High-performance ceramics with low thermal conductivity, high mechanical properties, and idea thermal expansion coefficients have important applications in fields such as turbine blades and automotive engines. Currently, the thermal conductivity of ceramics has been significantly reduced by local doping/substitution or further high-entropy reconfiguration of the composition, but the mechanical properties, especially the fracture toughness, are insufficient and still need to be improved. In this work, based on the high-entropy titanate pyrochlore, TiO2 was introduced for composite toughening and the high-entropy (Ho0.2Y0.2Dy0.2Gd0.2Eu0.2)2Ti2O7-xTiO2 (x=0, 0.2, 0.4, 1.0 and 2.0) composites with high hardness (16.17 GPa), Young's modulus (289.3 GPa) and fracture toughness (3.612 MPa·m0.5), low thermal conductivity (1.22 W·m-1·K-1), and thermal expansion coefficients close to the substrate material (9.5×10-6/K) were successfully prepared by the solidification method. The fracture toughness of the composite toughened sample is 2.25 times higher than that before toughening, which exceeds most of the current low-thermal conductivity ceramics.

Guide of selecting substitutional elements for lower thermal conductive ceramics applied to thermal barrier coatings

Guide of selecting substitutional elements for lower thermal conductive ceramics applied to thermal barrier coatings

A new type of spinel ceramic for CBGA package

High thermal conductivity with HITCE are highly required for microwave dielectric ceramics for CBGA electronic packaging applications. In this work, AAl2O4-x wt. % (Li2CO3-Al2O3-SiO2) (A = Zn, Mg) ceramics with spinel structure were synthesized through a conventional solid-state reaction method. The coefficient of thermal expansion of the ceramics are much higher than spinel ceramics because of the addition of Li2CO3-Al2O3-SiO2 additive. Meanwhile, the high thermal conductivity ceramics are also obtained due to the high thermal conductivity of its spinel structure and dense microstructure. Especially, ZnAl2O4-5 wt. % (Li2CO3-Al2O3-SiO2) compound exhibits thermal conductivity of 13.531 W/mK, TCE value of 8.65ⅹ10−6 K−1 (30–490 ℃), dielectric constant of 8.40 and dielectric loss of 4.40ⅹ10−4. And MgAl2O4-10 wt. % (Li2CO3-Al2O3-SiO2) compound exhibits thermal conductivity of 16.8 W/mK, TCE value of 8.9ⅹ10−6 K−1 (30–500 ℃), dielectric constant of 7.89 and dielectric loss of 8.90ⅹ10−3. The excellent thermal and dielectric properties give it a great potential in Ceramic Ball Grid Array electronic packaging applications.

Experimental study on the effect of the thermal barrier coated (TBC) piston on combustion of gasoline compression ignition (GCI)

Gasoline compression ignition (GCI) has the advantage in achieving high thermal efficiency and low emissions simultaneously. However, the difficulty in realizing stable compression combustion at low load is a major problem that GCI faces and needs to be addressed. In addition, improving thermal efficiency of internal combustion engine is also an important research topic, especially for satisfying low carbon goals in the future. Therefore, to explore strategies to improve combustion stability and thermal efficiency of GCI combustion, the low thermal conductivity ceramics (MgZrO3) was coated on the top surface and combustion chamber of the piston to reduce the thermal conductivity, then achieving the purpose of increasing the in-cylinder temperature and reducing the heat transfer. The experiment was conducted under different loads conditions to investigate the effect of thermal barrier coated (TBC) piston on GCI combustion. The result shows that the indicated thermal efficiency of GCI can be increased by 6.4% with the TBC piston at low load, meanwhile the THC and CO emissions are also reduced effectively. Furthermore, employing TBC piston can significantly improve the combustion stability of low load GCI combustion and therefore has great potential to extend lower load operation. However, the effect of TBC piston on improving indicated thermal efficiency decreases as engine load increases, and the difference in combustion between the TBC piston and piston without TBC is fairly small under high load condition. Compared to piston without TBC, the indicated thermal efficiency of TBC piston is higher and change slightly as EGR rate increases. With the TBC piston, lower NOx and soot emissions can be obtained while the indicated thermal efficiency remains at high value by introducing high EGR rate.

Large‐scale plasma grafts voltage stabilizer on hexagonal boron nitride for improving electrical insulation and thermal conductivity of epoxy composite

AbstractBetter electrical insulation and thermal management are both urgently required in integrated power semiconductors. Electrical insulation epoxy encapsulation suffers from poor heat conduction, which has increasingly become a bottleneck of power semiconductors integration. Although incorporating high thermal conductivity ceramics, such as hexagonal boron nitride (hBN), aluminium nitride etc. into epoxy promotes the thermal conductivity, the eco‐friendly scalable fabrication of these composites with sufficient electrical breakdown strength remains a formidable challenge. Suitable voltage stabilizers are known to provide additional benefits to breakdown strength. Herein, a high‐throughput approach combining plasma with roll‐to‐roll was developed. The voltage stabilizer (acetophenone) was grafted on interfaces between hBN and epoxy matrix through plasma. The high‐energy electrons are consumed by the grafted interface, which leads to the significant suppression of partial discharge in Epoxy/hBN. Meanwhile, interfacial phonon scattering is repaired by grafting. Therefore, the epoxy composite concurrently exhibits improved breakdown strength (by 27.4%) and thermal conductivity (by 142.9%) at about 11.9 wt.% filler content, outperforming the pure epoxy. Consequently, a promising modification strategy for mass production is provided for the encapsulation materials in various high‐power‐density semiconductor devices.

High-entropy carbide ceramics with refined microstructure and enhanced thermal conductivity by the addition of graphite

Aiming at the refined microstructure and enhanced thermal conductivity of high-entropy carbide (HEC) ceramics for high-temperature applications, the addition effect of graphite was comprehensively investigated in this study. HEC ceramics incorporated with different contents of graphite were solidified by spark plasma sintering (SPS) using self-synthesized high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C powder and graphite as starting materials. The results demonstrate that the incorporated graphite removed the oxygen impurity in the mixed powders, decreased the oxygen content and increased the lattice parameter of the HEC phase, and improved the densification behavior of HEC ceramics. On the other hand, the addition of graphite brings a refinement of HEC grains and improves the mechanical properties. More importantly, the thermal conductivity of the HEC ceramics was significantly increased owing to the removing effect of oxide impurity by the added graphite. It is considered that the lattice "purified" HEC grains with low oxygen content contribute to the improvement in thermal conductivity of the ceramics.

Fabrication and optical characterizations of hot-pressed transparent SrF2/Nd:SrF2/SrF2 composite ceramic

A novel transparent SrF2/Nd:SrF2/SrF2 composite ceramic with sandwich-like laminar configuration was designed and successfully fabricated by the hot-pressed sintering method. The composite ceramic possesses an apparent transition interfacial (about 200 μm in thickness) between SrF2 and Nd:SrF2 layers, formed by non-uniform distribution of raw powders and the diffusion of Nd ions during the high temperature sintering. The average grain sizes of SrF2 and Nd:SrF2 layers are about 262.1 μm and 28.6 μm, respectively. For a 2-mm thick transparent SrF2/Nd:SrF2/SrF2 composite ceramic hot-pressed at 900 °C for 2 h, the transmittance at 500 nm and 1200 nm are about 49.6 % and 62.3 %, respectively. The microstructure, emission spectra and thermal conductivities of ceramics are also detected and studied.

Comparative study of fluoride and non-fluoride additives in high thermal conductive silicon nitride ceramics fabricated by spark plasma sintering and post-sintering heat treatment

High thermal conductivity silicon nitride (Si3N4) ceramics fabricated using fluoride and non-fluoride additives were comparatively investigated in this study for the first time. Three series of sintering aids, namely, MgF2-Y2O3, MgSiN2-Y2O3 and MgO-Y2O3 were used to fabricate Si3N4 ceramics by spark plasma sintering (SPS) at 1650 ℃ for 5 min and post-sintering heat treatment at 1900 ℃ for 4 h. Microstructure, thermal conductivity, and mechanical properties of Si3N4 ceramics were comparative investigated. Results indicate that MgF2 as sintering additive for Si3N4 ceramics is able to effectively reduce the temperature of liquid phase in SPS process. The proportion of large grains is about 34% in fluoride-doped sample, which is more than those samples using MgSiN2 and MgO as sintering aids (24% and 25%, respectively). Thermal conductivity of ceramics is 76, 64, and 60 Wm−1 K−1 for samples with MgF2, MgSiN2 and MgO, respectively. Fluoride-doped sample displayed good mechanical properties including bending strength (857.6 MPa), hardness (14.9 GPa), and fracture toughness (7.7 MPa m1/2).

Comparative study of oxide and non-oxide additives in high thermal conductive and high strength Si3N4 ceramics

A comparative study between oxide and non-oxide sintering additive was conducted to examine the thermal and mechanical properties of Si3N4 ceramics. The effects of additive types (oxide or non-oxide) on microstructure, thermal conductivity and mechanical strength were investigated. Replacement of oxide additives by non-oxide additives induced a decrease in the secondary phase and lattice oxygen contents of sintered samples. The use of non-oxide sintering additives increased the thermal conductivity of Si3N4 ceramics; however, the sintering additive had less impact on the ceramic flexural strength. The measured strengths of all samples ranged from 822MPa to 916MPa. Therefore, the thermal conductivity of ceramics incorporating non-oxide sintering additives increased while maintaining flexural strength; these materials possessed a flexural strength of 862MPa and a thermal conductivity of 101.5W/mK.

The Direct plating copper (DPC) ceramic material on Al2O3/AlN or LTCC (Low-temperature co-fired ceramic) substrates

RESEARCH BACKGROUND: The now used ceramic substrate or sub-mounted are normally based on Ag-printed, direct bonding copper (DBC) ceramic or LTCC (Low temperature co-fired ceramic)/HTCC (High temperature co-fired ceramic) technology.Due to the limit of the screen-printed process, the resolution and conducting material thickness the Ag-printed, LTCC and HTCC substrate are poor. The poor resolutions make these materials difficult to use in high density and flip-chip device design. And the related thinner conducting material (normally <20um) limits the power rating of the design.DBC is now widely applied in power circuit design, however, duo to the copper lamination process requirement, more than 300um in thickness of copper layer is needed. Any lower copper thickness design should have an extra costly grind to reach. Also, the DBC material is difficult to provide to the multilayer trace design. OUR GOAL: We want to provide a solution with multilayer ceramic substrate for high power and high device density applications. Besides, the material properties, the adhesion of the metal/ceramic also be considered. Following are the material characteristics required for the development:A low electrical resistance material: Copper.A thick trace material thickness of more than 3 oz.A high thermal conductivity and stability ceramics with via-holes for TSV plating (Drilled Al2O3/AlN substrate ) or non- shrinking LTCC materialHigh metal trace resolution whose line width and space could be only 50 umWell metal/ceramic adhesion uniformity and strength is required: The voids between metal/ceramic < 1%; The adhesion strength> 2 kg/2*2mm2. METHODS & RESULTS: Metal trace plating: For high resolution and lower material electrical resistance request of the trace metal, we introduce electrical casting direct-plating copper (DPC) technology. The first copper is sputtered on the ceramic substrate using Ti as combined/buffer layer between copper and ceramic to provide good adhesion strength and stability. The second copper is made by electrical casting process to increase its thickness to 3 to 5 oz. (100~150um). The key technology of the metal trace plating is the material control of the sputter layers and the second copper layer stress release during plating. Multilayer Ceramic substrates: For double layers design, we use sintered Al2O3 or AlN substrates with electrical conducting via-holes design. The via-holes are made by laser drilling. And the conducting of the front and back side is connected by the following plating process. The key technology of this process is the stability of the via-holes. We have to make sure the via-holes cleaning, impurity removing and material variation during high temperature laser drilled is well controlled. For the more than three layers design, the non-shrinking LTCC is used. The dimension mismatch of the non-shrinking LTCC can controlled less than 100um., much better than that of normal LTCC/HTCC. By the correction of the following DPC process, the tolerance of the metal trace can be controlled < 30 um. The key technology of this process is the non-shrinking LTCC technology and the adhesion of the DPC metal on LTCC material.

Novel organic–inorganic composites with high thermal conductivity for electronic packaging applications: A key issue review

Significant progress has been made recently in developing the organic–inorganic composites with high thermal conductivity, low dielectric constant, and dielectric loss, for applications in the electronic packaging and substrates. Many studies have shown that some polymers filled with high thermal conductivity and low dielectric loss ceramics are suitable for electronic packaging for device encapsulation. Until now, extensive attentions have been paid to the preparation of polymeric composites with high thermal conductivity and low dielectric loss for the application in electronic packaging. In contrast, the thermal conductivities of these dielectric materials are still not high enough and that might restrict their serviceable range. Herein, we briefly reviewed recent progress in this field and introduced a kind of novel composites with surface insulation modified metal aluminum cores to form multilayer coating structures as fillers in polyimide matrix for electronic applications. This structure can significantly improve the thermal conductivity and dielectric properties of composites and give some insights into the effects of modified fillers of composite materials. Such multilayer core–shell structures should have great potentials for the improvement of nanoparticle‐based fillers and applications of electronic packaging. POLYM. COMPOS., 38:803–813, 2017. © 2015 Society of Plastics Engineers

Review on Ceramic Application in Automotive Turbocharged Engines

Research on the use of thermal barrier coatings in internal combustion engine had contributed in achieving higher thermal efficiency, improved combustion and reduced emissions of the engine. Low thermal conductivity ceramics can be used to control the temperature distribution and heat flow in high temperature structural components due to its inherent thermal insulation properties. For this reason much has been and is being done on the study and development of ceramics for use in automotive engine components working under severe temperature conditions and heavy loads due to their inherent thermal and mechanical properties. The objective of the study is to review the contributions of structural ceramics in the development and improvement of some of the major automotive engine components working under severe conditions of temperature. It is expected that the study will serve as a useful guide for the selection of materials which can withstand severe conditions of temperature and heavy loads for a novel turbocharger and turbocharged engine applications.

The effect of temper on the thermal conductivity of traditional ceramics: Nature, percentage and granulometry

Abstract Traditional ceramics were commonly produced using a mixture of clay and temper materials, which were added in different percentage according to the craftsman purposes. The present study aims to examine up to which extent some technological parameters (nature, granulometry and percentage of the temper and firing temperature) affect the thermal conductivity of traditional ceramics. With this purpose a kaolinitic clay was tempered either with quartz or limestone belonging to two different granulometric distributions in percentage of 5%, 15% and 25%, and fired at 500, 750 and 1000 °C. Moreover the dependence on firing temperature was studied. Thermal conductivity was measured with a modified Lee's disks apparatus in a temperature range from 120 to 370 °C. It was found that quartz-tempered ceramics are more conductive than the fired non-tempered clay, while limestone-tempered sample are less conductive. Mineralogical and microstructural data are also provided and the influence of the α – β quartz-phase transition on the thermal conductivity of ceramics is discussed.

Sintering Behavior and Properties of Si<sub>3</sub>N<sub>4</sub> Ceramics with Yttrium Chloride Additive

The big gap between the thermal conductivity of Si3N4 ceramics and the theoretical value of beta-Si3N4 single crystal implies that the low thermal conductive oxide grain-boundary phases in the ceramics possibly have a great influence on the thermal conductivity of the ceramics. In this work, yttrium chloride (YOCl) is introduced as sintering additive instead of the commonly-used Y2O3, in order to decrease the amount of the grain-boundary oxide; and the influence of YOCl on the sintering and the properties of Si3N4 ceramics was studied. Results show that YOCl additive can react with Si3N4 powders to form similar Y-Si-O-N grain-boundary phases just like Y2O3 while the Cl disappears during sintering. The thermal conductivity of the ceramics sintered with YOCl additive (85 W/m•K) increases about 15% as comparing with the ceramics sintered with Y2O3 additive, even though the relative density of the former reaches merely 97%. The microstructure of the sample sintered at different temperatures was investigated in the work, for elucidating the sintering process and the relationship between the properties and the microstructure of the YOCl added ceramics.

High thermal conductivity ceramics from combustion synthesized AlN powder through microwave sintering and reheating

Combustion synthesized AlN powders were investigated for use as starting material for obtaining high thermal conductivity specimens by microwave sintering and reheating. Microwave sintering and reheating were carried out in a TE103 single mode cavity with an adjustable microwave power in the range 0–3 kW at 2.45 GHz. Densification was found to be a primary requirement to obtain a high thermal conductivity AlN. AlN powders with good sinterability are required to achieve a high densification. AlN powder with D 50 of ∼6 μm was found to have a poor sinterability that with D 50 of ∼3 μm or finer, a good sinterability. Oxygen content was found to be another important factor determining the thermal conductivity. The thermal conductivity can be enhanced from ∼130 to ∼155 W/m K when the oxygen content is reduced from 2.3 to 1.4 wt %. The thermal conductivity can be significantly improved by microwave reheating of the sintered specimen under a reducing atmosphere. This is considered to be due to enhanced removal of the second phases by the reducing atmosphere.

Particle Injection in Direct Current Air Plasma Spray: Salient Observations and Optimization Strategies

External injection of high-melting point low thermal conductivity ceramics orthogonal to a typical direct current thermal plasma jet plays a vital role in determining the in-flight state of the particles and the process downstream. The interactions between low density ceramic particles and high temperature plasma jet is quite complex, which influences the spray process and associated deposition. Detailed in-flight particle diagnostics as well as spray stream visualization have significantly enhanced our capability to diagnose and control the process. In this paper we present some salient observations on the role of key variables on particle injection. A number of experiments were conducted using a 7MB torch (Sulzer Metco, Westbury, NY) with both Ar–H2 and N2–H2 plasma gases, where the carrier gas flow to inject Yttria Stabilized Zirconia (YSZ) was varied systematically and the resulting in-flight particle state was captured using an array of particle and spray stream sensors arranged in a 3D set-up. A notable observation is the existence of a “sweet-spot” in the plasma jet where the particle temperatures and velocities achieved a maximum. This sweet-spot can be characterized by the plume position (location of centroid of the spray stream) rather than carrier gas flow rate and is independent of primary gas flows and other process/material conditions. This result suggests a possible approach to optimize particle injection independent of plasma-forming-torch-parameters. Controlling particle injection at this sweet-spot has shown to benefit the overall process efficiency (in terms of melting) and process reliability (both in-flight measurement and coating build-up) with concomitant application benefits.

Low Thermal Conductivity Ceramics for Thermal Barrier Coatings

This paper summarizes the basic properties of a series of rare-earth zirconate ceramics (Gd2Zr2O7, Sm2Zr2O7, Dy2Zr2O7, Er2Zr2O7 and Yb2Zr2O7). The phases and microstructures were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Thermal properties of these materials were determined. The results indicated that Sm2Zr2O7 rare-earth zirconate ceramics have the lower thermal conductivity and the highest thermal expansion coefficient than other rare-earth zirconate ceramics. The dielectric constant decreases with the increase of atomic number.

Study on Thermal Conductivity of Spark-Plasma-Sintered Silicon Nitride Ceramics

In the study, the spark plasma sintering (SPS) method was adopted to fabricate Si3N4 ceramics with Y2O3-MgO additives. Specimens with different grain dimensions, grain shapes, α/β phase ratios, densities were obtained by changing the heating rate and dwell time of SPS. The relationship between the microstructure and the thermal conductivity was studied. Results show that the heating rate and the dwell time have great influence on the microstructure and properties of Si3N4 ceramics. Both equiaxed and columnar β- Si3N4 grains are formed during sintering, but the thermal conductivity of Si3N4 ceramics is affected only by columnar grains. The thermal conductivity of the ceramics increases together with the formation and the growth of the β- Si3N4 columnar grains.

Thermal conductivity of ceramics during irradiation

Thermal conductivity is one of the most important parameters for the design of fusion reactors using ceramic materials. Thermal diffusivity of ceramics decreases by phonon scattering and specific heat increases slightly as the temperature rises to 1300 K. The amount of point defects during neutron irradiation changes with the irradiation temperature. In this work, α -Al 2O 3, AlN, β -Si 3N 4 and β -SiC specimens were irradiated in the experimental fast reactor JOYO at 775, 835 and 1039 K to high neutron fluence. Thermal diffusivity was measured at elevated temperatures in vacuum via the laser flash method. Using an approximation function, the thermal diffusivity at the irradiation temperature was determined for each specimen, and the one during irradiation was estimated with additional assumptions. Thermal diffusivity of ceramics during irradiation was found to be almost temperature independent, and the thermal conductivity has a slight increase with temperature.

About the validity of analytical approximations for temperature and stress for actively cooled windows during thermal loading

The verification of analytical approximations for temperature and stresses during thermal loading is done for ceramic edge-cooled windows for the stellarator W7-X by comparison with more accurate numerical calculations. Numerical calculations show that a steady state temperature and stress approximations assuming edge-cooled circular plates can be applied only in the case when radiative cooling from a surface is neglected. The prediction for poor thermal conductivity ceramics under high heat flux load based on simple analytical equations can result in considerable mistakes in the temperature and, consequently, stress values. Even the prediction of the qualitative tendency of temperature and stress behaviour as a function of the window size can be wrong.