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Synthesis and Characterization of MgA1ON-BN refractories
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

In order to meet the need of metallurgical industry in the world, a new MgAlON-BN composite which can be used for example in special refractory nozzles, tubes and break rings for the continuous casting of steel was studied in the present thesis. The aim was to understand the mechanism of synthesis and their physicochemical properties during the application. Thus, the thermodynamic properties, synthesis process, mechanical properties, thermal shock behaviour, thermal diffusivity/conductivity as well as corrosion resistance to molten iron containing oxygen and molten slag of MgAlON and MgAlON-BN composites have been investigated.

The Gibbs energy of formation of MgAlON was estimated using the method proposed by Kaufman. The phase stability diagram of Mg-Al-O-N-B was investigated, and consequently the synthesis parameters were determined. MgAlON and MgAlON-BN composites were fabricated by hot-pressing method. The composites obtained this way were characterized by XRD, SEM, TEM and HREM analyses. A Matrix-flushing method was employed in the quantitative XRD analysis for the multi-component samples to understand the mechanism of synthesis.

The relationship between mechanical properties and microstructure of the composites was investigated. The experimental results indicated that BN addition has significant influence on the mechanical properties of the composites. These can be explained by the fact that BN has low Young’s modulus, density and non-reactive nature as well as considerable anisotropy of many properties such as thermal expansion, thermal diffusivity/conductivity. Thus, the addition of BN in MgAlON is likely to lead to the presence of microcracks caused by the mismatch of thermal expansion coefficient. The microcracks result in the enhancement of the strength at elevated temperature and thermal shock durability of the composites.

Effective thermal conductivities were evaluated from the present experimental results of thermal diffusivities, heat capacity and density. A model suitable for present composites has been derived based on Luo’s model. The predicted lines calculated by the model were in good agreement with experimental results.

The reactions between the composites and molten iron as well as the slag were investigated by ‘‘finger’’ experiments and sessile drop experiments. Both experimental results indicated that the BN addition has positive influence on the corrosion resistance. These are attributed to the excellent corrosion resistance of BN to molten iron and slag, such as the higher contact angle between BN substrate and liquid iron and molten slag compared with that obtained for pure MgAlON.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , xii, 43 p.
Keyword [en]
MgAlON-BN composites, Mechanical properties, Thermal shock durability, Thermal diffusivity/conductivity, Corrosion resistance.
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-3883ISBN: 91-7178-271-0 (print)OAI: oai:DiVA.org:kth-3883DiVA: diva2:9863
Public defence
2006-03-31, Salongen, KTHB, Osquars backe 31, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100929Available from: 2006-03-15 Created: 2006-03-15 Last updated: 2010-09-29Bibliographically approved
List of papers
1. Synthesis and characterization of MgAlON-BN composites
Open this publication in new window or tab >>Synthesis and characterization of MgAlON-BN composites
2007 (English)In: INTERNATIONAL JOURNAL OF MATERIALS RESEARCH, ISSN 1862-5282, Vol. 98, no 1, 64-71 p.Article in journal (Refereed) Published
Abstract [en]

In the present paper the Gibbs energy of formation of MgAlON was evaluated, and on this basis the phase stability diagram of Mg-Al-O-N-B was established. Dense magnesium aluminium oxynitride-boron nitride (MgAlON-BN) composites with 0-30 vol.% BN were synthesized by hot pressing in the temperature range 1750-1850 degrees C in the phase stability region. The phase compositions of the composites analyzed by X-ray diffraction indicated that the main phases were MgAlON and h-BN and no impurities were found. The microstructures of the composites analyzed by transmission electron microscopy, scanning electron microscopy, and high-resolution electron microscopy showed that the MgAlON did not react with BN, and the latter was distributed on the grain boundary of MgAlON homogeneously. Excessive BN (> 20 vol.%) resulted in a discontinuous microstructure for MgAlON. A matrix-flushing method was employed in the quantitative X-ray diffraction analysis for the multi-component MgAlON-BN composites. The results showed that the relative MgAlON content decreased in the process of sintering MgAlON-BN composites. Thermodynamic analysis showed that some Al2O3, AlN, and MgO escaped under the reaction conditions. The lattice parameters of MgAlON in the composites have also been evaluated.

Keyword
MgAlON-BN, thermodynamic analysis, composites, quantitative analysis
National Category
Information Science
Identifiers
urn:nbn:se:kth:diva-5477 (URN)000244175800012 ()2-s2.0-33847062418 (Scopus ID)
Note
Uppdaterad från "in press" till published: 20100929. QC 20100929Available from: 2006-03-15 Created: 2006-03-15 Last updated: 2010-09-29Bibliographically approved
2. Mechanical Properties and Microstructures of hot-pressed MgAlON-BN Composites
Open this publication in new window or tab >>Mechanical Properties and Microstructures of hot-pressed MgAlON-BN Composites
2007 (English)In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 27, no 1, 319-326 p.Article in journal (Refereed) Published
Abstract [en]

The relationship between the mechanical properties and microstructure of hot-pressed MgAlON-BN composite materials was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution electron microscopy (HREM) and X-ray diffraction (XRD) techniques. The phase compositions of hot-pressed samples prepared from starting mixtures of Al2O3, AlN, MgO and h-BN consisted of MgAlON phases as a matrix and BN phases as the second phase. The density, bending strength at room temperature, fracture toughness and Vickers hardness were measured. The results indicated that the density, strength and Vickers hardness decrease with increasing h-BN content due to the non-reactive nature and layered structure of h-BN. The fracture toughness, however increased with increasing h-BN addition, reaching a maximum of 3.64 MPa m(0.5); it decreased with further increase of BN content. The increase of fracture toughness was attributed to the presence of microcracks and the decrease was considered to be the discontinuous microstructure of the MgAlON phases. Temperature dependence of bending strength remained constant at low temperature, followed by an increase at 800 degrees C and then, dropped quickly. The increase in the bending strength of the composite was attributed to the decrease of residual stress and to the interwoven microstructure of the composites which prevented grain boundary slip and reduced the attenuation rate of high temperature strength. The machinability of the composites was examined. The results indicate that the composite materials with BN content more than 15 vol.% exhibit excellent machinability and could be drilled using conventional hard metal alloy drills

Keyword
mechanical properties, microstructure, hot pressing, composites, refractories, BN, MgAlON
National Category
Information Science
Identifiers
urn:nbn:se:kth:diva-5478 (URN)10.1016/j.jeurceramsoc.2006.04.184 (DOI)000242565400044 ()2-s2.0-33751066193 (Scopus ID)
Note
QC 20100929Available from: 2006-03-15 Created: 2006-03-15 Last updated: 2017-11-21Bibliographically approved
3. Thermal shock Behavior of MgAlON and MgAlON-BN Composites
Open this publication in new window or tab >>Thermal shock Behavior of MgAlON and MgAlON-BN Composites
(English)Article in journal (Refereed) In press
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-5479 (URN)
Note
QC 20100929Available from: 2006-03-15 Created: 2006-03-15 Last updated: 2010-09-29Bibliographically approved
4. Investigation of Wetting Characteristics of Liquid Iron on Dense MgAlON-based Ceramics by X-Ray Sessile Drop Technique
Open this publication in new window or tab >>Investigation of Wetting Characteristics of Liquid Iron on Dense MgAlON-based Ceramics by X-Ray Sessile Drop Technique
2006 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 37, no 3, 421-429 p.Article in journal (Refereed) Published
Abstract [en]

The wetting characteristics of liquid iron on dense MoAlON-based composite ceramics were investigated using X-ray sessile drop technique. The contact angles were measured on substrates of different composites as functions of temperature and varying partial pressures of oxygen. The results with pure argon gas showed that contact angles kept almost constant in the temperature range 1823 to 1873 K. The contact angle was found to show a slight increase with increasing boron nitride (BN) content in MgAlON-BN composites. These are attributed to the higher contact angle between BN substrate and liquid iron drop compared with that obtained for MgAlON substrate. When the CO-CO2-Ar gas mixtures were introduced into the system, the contact angle showed an initial quick decrease followed by a slow decrease and then a period of nearly constant contact angle at a given temperature corresponding to the steady-state condition. Even in this case, BN seemed to cause an increase in the equilibrium contact angle. The equilibrium contact angle was found to decrease with increasing temperature. XRD results indicated that the substrate was oxidized and the oxidation products combined with FeO formed by the oxidation of the iron drop to form FeAl2O4 and Mg1-xFexO. These were likely to form a ternary FeO-Al2O3-MgO slag or a quaternary slag by combining with B2O3. An interesting observation is that the iron drop moved away from the original site, probably due to the Marangoni effect.

Keyword
spinel, wettability, resistance, surfaces, al2o3, mgo
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-5480 (URN)10.1007/s11663-006-0027-6 (DOI)000237948700011 ()2-s2.0-33845676556 (Scopus ID)
Note
Uppdaterad från "In press" till published: 20100929. QC 20100929Available from: 2006-03-15 Created: 2006-03-15 Last updated: 2017-11-21Bibliographically approved
5. Thermal diffusivity/conductivity of MgAlON-BN Composites
Open this publication in new window or tab >>Thermal diffusivity/conductivity of MgAlON-BN Composites
2006 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 37, no 4, 615-621 p.Article in journal (Refereed) Published
Abstract [en]

Thermal diffusivity and heat capacity of MgAlON and MgAlON-BN composites were measured in the temperature range of 25 degrees C to 1300 degrees C using a laser flash technique and a differential scanning calorimeter (DSC) technique, respectively. Based on these measurements, effective thermal conductivity of the composites was calculated using the values measured earlier in the same substance. The experimental effective thermal conductivity results of the composites containing different BN contents were found to show the similar trend, which decreased rapidly with increasing temperature below 900 degrees C followed by a slow decrease with further increasing temperature. This can be explained by the fact that thermal conduction in both components, MgAlON and BN, was dominated by phonons. The phonon mean free path decreased with increasing temperature, limited by the characteristic length between two neighboring atoms. The BN addition has significant influence on the effective thermal conductivity. The effective thermal conductivity of the composites containing BN exhibited a small degree of anisotropy with respect to preferred orientation of the BN phase. The degree of anisotropy of the composites increased with increasing BN content, which is particularly pronounced at the higher BN additions. An equation suitable for the present composites has been derived based on Luo's model. The model was slightly modified in the present article. The predicted values calculated by the model were in good agreement with experimental results.

Keyword
conductivity, resistance, ceramics
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-5481 (URN)10.1007/s11663-006-0045-4 (DOI)000239590300012 ()2-s2.0-33747452376 (Scopus ID)
Note
Uppdaterad från "In press" till published: 20100929. QC 20100929Available from: 2006-03-15 Created: 2006-03-15 Last updated: 2017-11-21Bibliographically approved
6. Reactions between MgAlON-BN composites and CaO-SiO2-Al2O3-MgO-''FeO'' slag
Open this publication in new window or tab >>Reactions between MgAlON-BN composites and CaO-SiO2-Al2O3-MgO-''FeO'' slag
2007 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 38, no 2, 231-241 p.Article in journal (Refereed) Published
Abstract [en]

The reactions between MgAlON and MgAlON-BN composites and synthetic CaO-SiO2-Al2O3MgO-"FeO" slag at different temperatures were investigated under stagnant condition using the "finger" experiment as well as X-ray sessile drop methods. The corrosion rate was denoted by the radius difference between the initial radius of the specimen rod and the unreacted core at definite time intervals. The experimental results showed that the radius difference is linear with the square root of time. This indicated that the diffusion of ions in the slag through the product layer is the rate-determining step. One product layer was observed between the unreacted core and slag. X-ray images showed that gas bubbles were generated during the slag penetration. The slag penetration process depends strongly on the temperature. The apparent activation energy was evaluated to be 376.4 kJ/mol. The FeO addition into slag increased the slag corrosion rate. This is most probably due to the decrease of the slag viscosity, because the Fe+2 generally behaves as a network breaker. Furthermore, MgAlON and boron nitride (BN) can be oxidized by FeO, which also increased the slag corrosion rate. The slag corrosion rate decreased with increasing BN content. This can be explained by the fact that the grain boundary interfacial energy decreases with the increase of BN content and nonwetting of BN by molten slag compared to pure MgAlON.

Keyword
corrosion, melts, refractories, viscosities, resistance, spinel
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-5482 (URN)10.1007/s11663-007-9038-1 (DOI)000247008600009 ()2-s2.0-34250892133 (Scopus ID)
Note
Uppdaterad från "In press" till published: 20100929. QC 20100929Available from: 2006-03-15 Created: 2006-03-15 Last updated: 2017-11-21Bibliographically approved

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Citation style
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