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Sci Eng Compos Mater 18 (2011): 117–125 © 2011 by Walter de Gruyter • Berlin • Boston. DOI 10.1515/SECM.2011.021
Novel fabrication process of AlN ceramic matrix composites at low temperatures
Hesam Nasery, Martin Pugh and Mamoun Medraj* Department of Mechanical and Industrial Engineering, Concordia University, 1515 St. Catherine St. West, EV4.139, Montreal, Quebec, Canada, H3G 1M8, e-mail:
[email protected] *Corresponding author
Abstract A novel processing method to fabricate AlN-MgO-MgAl2O4 composites has been developed, using non-sintered, porous, aluminum nitride (AlN) preforms infiltrated with a magnesium alloy in two directions, downward and upward, at relatively low temperatures (650°C, 800°C, 950°C). Microstructural, phase, and chemical analyses show that at 950°C and 135 min holding time, a continuous network of ceramic phases can be achieved successfully. Magnesium oxide and spinel phases (MgAl2O4) are formed in-situ, when nitrogen gas is used. Due to the formation of a magnesium nitride layer on the surface of the non-sintered aluminum nitride, the infiltration mechanism proceeds effectively. No metallic phases are observed in the samples processed at 800°C and higher: these samples show high electrical resistivity ranging from 6.73×108 to 2.10×1011 Ω⋅cm. Thermal diffusivity, heat capacity and density have been measured using the nano-flash method, differential scanning calorimetry and Archimedes technique, respectively. The effects of residual porosity and holding time on the thermal conductivity have been studied. Maximum thermal conductivity and density at room temperature are 96 W/(mK) and 2.45 g/cm3, respectively. Keywords: aluminum nitride; metal composite; physical properties; spontaneous infiltration.
1. Introduction In recent years, the demand for high thermal conductivity materials to develop high-density integrated packages has increased. Although aluminum nitride (AlN) was first synthesized in 1877, it was not used for high thermal conductivity applications until its capability in the thermo-mechanical and high performance electronic industry was realized (mid 1980s). Furthermore, it has attracted much attention over the past few decades, by electronic industries, due to other significant properties such as low dielectric constant, high electrical resistivity, coefficient of thermal expansion (CTE) close to that of silicon chips, and non-toxicity. Therefore, it has the
potential of operating in electrical devices at elevated temperatures [1–4]. Interest in AlN increased when it was clarified that it is a good phonon heat conductor and after its thermal conductivity was reported as 320 W/(mK) for single crystal AlN [5–7]. Currently, alumina and SiC are dominant among substrate materials and in a few applications (beryllia) BeO is used [4]. The most important electrical and mechanical properties of AlN compared with those of BeO, Al2O3, and SiC are shown in Table 1. A wide range of thermal conductivities for crystalline AlN from 50 W/(mK) to 270 W/(mK) has been reported [3, 18]. This discrepancy can be due to the presence of impurities and porosity [6, 13, 16]. Oxygen has been found to be the most important impurity. The most important obstacle for commercializing aluminum nitride, apart from cost, is the lack of reproducibility in thermal conductivity and the adhesion of metallization layers. Full densification of AlN is difficult, due to its high covalent bonding and oxygen impurities [8, 19]. Various parameters such as sintering conditions, holding temperature, holding time, and the quantity of sintering additives are required in making high thermal conductivity and dense AlN [20]. The different values of AlN thermal conductivity reported in the literature are summarized in Table 2. Results show that the highest values of thermal conductivity are achieved for AlN sintered at very high temperature (up to 1950°C) and after a long sintering time (up to 100 h). Hence, it is an expensive material. The aim of this work is to produce a dense AlN composite with appropriate thermal properties comparable to that of sintered aluminum nitride ceramic, but fabricated at relatively low temperatures (650°C, 800°C, 950°C). It focuses on spontaneous liquid infiltration of molten magnesium alloy into porous AlN preforms using in-situ reaction in a nitrogen atmosphere. The combination of the thermal properties of the AlN ceramics with the advantages of magnesium alloys, such as low melting point and high affinity for oxygen, is fascinating for some appropriate applications. Fabricating AlN-MgOMgAl2O4 composites at low temperatures (1014 140–170 8.9 No
3.89 304–314 23–27 – Opaque >1014 100 8.5 No
2.90 245 12 – Opaque >1014 100 6.5 Yes
3.217 – 24 – – >1014 0.7 40 No
[8–10] [8, 11–13] [3, 14, 15] [12] [13] [1, 16, 17] [1, 16, 17] [1, 16, 17] [18]
Table 2 Thermal conductivity of aluminum nitride. Thermal conductivity of AlN at room temperature W/(mK)
Additives
Remark
References
160–270 272 160 155 245 114–194 175 180
Y2O3 Y2O3 No additives Y2O3 Y2O3 SiO2 and Y2O3 CaO CaO Al2O3 Y2O3
Sintering at 1750°C–1950°C (1 h) Sintering at 1900°C (100 h) under nitrogen atmosphere Hot pressing at 1800°C Hot pressing at 1900°C Sintering at 1850°C (30 min) and annealing at 1850°C (100 h) Sintering at 1825°C–1860°C (1 h) Sintering at 1800°C (1 h) under nitrogen atmosphere Sintering 1500°C–1900°C (1 h)
[21] [3] [6] [7] [4] [1, 2] [18] [8]
powder is 4–5 µm dry-pressable grade, Accumet Materials Co. (Ossining, NY, USA). It contains 5 wt% yttria, 1 wt% oxygen, 0.08 wt% carbon, 50 ppm iron, 40 ppm silicon and 80 ppm other impurities. Preforms of AlN are prepared from the same powder by hydraulic pressing to form green discs 25.4 mm in diameter and between 6 and 10 mm in height. Molten magnesium alloy (AZ91E) is employed for infiltration. In order to control the reaction atmosphere, the boron nitride crucible is contained in a steel chamber with the necessary pipes to introduce nitrogen gas (about 1 cm3/min). With. the purpose of achieving full infiltration, simultaneous downward and upward infiltrations have been employed. The boron nitride crucible containing the aluminum nitride preform and the two pieces of magnesium alloy (AZ91E), one on the top and one on the bottom of the preform, are placed in the furnace as illustrated in Figure 1. Experiments have been performed at three
Outlet Furnace
BN crucible
Temp (°C)
Inlet
different holding temperatures (650°C, 800°C, 950°C) and four different holding times (25, 60, 90, and 135 min). Figure 2 shows the temperature profile applied in these experiments. Sectioned and polished infiltrated samples have been examined using scanning electron microscopy (SEM) JEOL JSM-840A (Akishima, Tokyo, Japan) and energy dispersive spectroscopy (EDS) analysis. Phase identification is performed by X-ray diffraction (XRD) APD 1700, Phillips (Amsterdam, The Netherlands). In order to measure electrical resistivity at room temperature, an Agilent 4339B (Santa Clara, CA, USA) high-resistance device is used. To calculate thermal conductivity of samples, measurements of thermal diffusivity and heat capacity were made using the laser nanoflash method NETZSCH, LFA447, (Burlington, VT, USA) and differential scanning calorimetry (TA instruments-Q10), respectively.
T
Holding time, ∆t
t1
AIN
t2
Room temperature
Time (h)
Figure 1 Schematic set-up for upward and downward infiltration of liquid Mg alloys into AlN preform inside the BN crucible.
Figure 2 Experimental temperature profile with 4°C/min heating rate: ∆t=t2-t1 (25, 60, 90, and 135 min), T=650°C, 800°C, and 950°C.
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H. Nasery et al.: Novel fabrication process of AIN ceramic matrix composites
3. Results and discussion 3.1. Morphology and microstructural analysis
SEM micrographs (Figure 3) of polished samples infiltrated at 650°C, 800°C, and 950°C reveal that increasing the temperature and holding time result in fewer pores and cracks and consequently, continuous network of ceramic phases could be achieved at 950°C for 135 min as can be seen in Figure 3C. The dark regions of all samples are composed of aluminum and nitrogen. Metallic phases such as magnesium, aluminum, and gamma-phase, have been detected in samples infiltrated at 650°C by EDS and XRD (Figure 4). Therefore, the existence of metals in the final compositions (Figure 4A) indicates incomplete oxidation and nitridation and the ceramic matrix composite formed is unsuitable for electronic applications. In samples infiltrated at 800°C and 950°C, no un-reacted metal has been observed and bright spots have been distinguished as magnesium oxide and spinel (MgAl2O4)
phases by EDS (Figure 3B and C) and XRD (Figure 4B and D). In samples infiltrated at 800°C and 950°C, a continuous network of magnesium oxide, aluminum nitride, and spinel (MgAl2O4) exists. The XRD results and the EDS patterns are in good agreement. Yttrium appeared in the analysis of some spots, because the AlN powder contained some yttria (5 wt%). In samples infiltrated at 650°C, the XRD peaks of aluminum and magnesium are detected at all holding times. Gamma-phase (Mg17Al12) peaks have been observed only in samples with holding times of 90, and 135 min. However, these peaks are not as strong as those of aluminum and magnesium (Figure 4A). 3.2. Infiltration mechanism
The successful infiltration of magnesium alloy into AlN preforms is attributed to formation of a thin layer of magnesium nitride (Mg3N2), which forms on the surface of the AlN particles. As magnesium volatizes and reacts with oxygen and nitrogen, magnesium oxide and magnesium nitride form. Magnesium vapor is
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Figure 3
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SEM micrographs of samples infiltrated at: a) 650°C for 135 min, b) 800°C for 90 min, c) 950°C for 135 min.
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H. Nasery et al.: Novel fabrication process of AIN ceramic matrix composites
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Figure 4
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XRD patterns for infiltration at (A) 650°C, (B) 800°C, and (C) 950°C for different holding times.
very active because of its high pressure. Therefore, magnesium nitride particles form first by the reaction between vaporized magnesium and nitrogen gas. When magnesium nitride particles are reacted with the molten magnesium alloy (AZ91E),
a substitution reaction takes place between magnesium nitride and aluminum to form aluminum nitride according to: Mg3N2+2Al → 2AlN+3Mg
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(1)
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H. Nasery et al.: Novel fabrication process of AIN ceramic matrix composites
The aluminum needed to react with magnesium nitride is supplied from the magnesium alloy and as a product of the reaction between magnesium and alumina: Al2O3 (s)+3Mg (g) → 2Al (1)+3MgO (s)
Mg3N2+2A1→2A1N+3Mg
3Mg+N2→Mg3N2
Substitutional reaction
(2)
In Figure 5, the proposed mechanism of magnesium nitride formation is illustrated. In this work, the infiltration process has been attempted with argon atmosphere at the same temperatures and for the same holding times, but products were fragile and infiltration was not successful. The formation of the magnesium nitride (Mg3N2) layer on the porous surface of silicon nitride, quartz sand and boron carbide in the process of making spinel phase by infiltration of molten magnesium alloy, has been reported before [22–24]. The effects of process conditions, such as holding temperature and time on the phase contents of the products, are investigated by the peak ratio technique and results are summarized in Figure 6. In samples infiltrated at 650°C (Figure 6A), as holding time increases, the quantity of MgO slightly increases and the amount of gamma phase also increases. However, the quantities of aluminum nitride and aluminum phases are almost constant.
A
Vaporization Mg (gas)
Mg (liquid)
Figure 5 Mechanism of magnesium nitride formation (adapted from [22]).
The magnesium content decreases due to evaporation and oxidation. Figure 6B and C depict the effects of process time on the phase contents at 800°C and 950°C, respectively. They indicate that as holding time increases, the amount of AlN is reduced and the magnesium oxide content increases. In the samples processed at 950°C, formation of spinel occurs at shorter holding times (90 min) compared to 135 min for the case of 800°C. Also, the rates of magnesia formation and
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Figure 6
Effect of holding time on the phase contents of samples processed at: A) 650°C, B) 800°C, C) 950°C.
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aluminum nitride consumptions at 950°C are higher than those at 800°C. In samples infiltrated at temperatures higher than the oxidation temperature of the AlN (>700°C) [2], alumina forms according to the following reaction: 4AlN (s)+3O2 → 2Al2O3+2N2
(3)
Hence, the formation of more magnesium oxide is attributed to the reaction of the alumina film with magnesium, reaction (2). Consequently, the rate of MgO formation at 800°C and 950°C is higher than that observed for the samples processed at 650°C. No residual metals are observed in the samples processed at 800°C and 950°C, suggesting that liquid aluminum reacts with nitrogen to form aluminum nitride according to: 2Al (l)+N2 (g) → 2AlN (s)
(4)
Formation of the MgAl2O4 occurs only in the samples processed at 800°C with 135 min holding time and 950°C with 90 and 135 min holding time, because of the reaction of alumina with magnesia according to: MgO (s)+Al2O3 (s) → MgAl2O4
(5)
The driving force for the reaction is the difference in the Gibbs free energy between the reactants and the products. To study the performance of possible reactions, the theoretical thermodynamic equilibrium conditions are calculated by minimization of the Gibbs free energy using Factsage software (Montreal, Canada) (Figure 7). According to this figure, the formation of aluminum nitride and magnesium nitride is more favorable than MgO or spinel. Hence, in the experiments performed at 800°C and 950°C, no un-reacted metal has been observed due to complete conversion of aluminum and magnesium to aluminum nitride and magnesium nitride, respectively. Magnesium oxide is provided through magnesium reaction with the residual oxygen in the atmosphere and with alumina. Therefore, the rate of MgO formation in the samples processed at 800°C and 950°C is higher than those processed at 650°C due to the oxidation of AlN.
Gibbs energy (kJ/mol)
3Mg+Al2O3 → 3MgO+2Al 3Mg+N2 → Mg3N2
-400 -500 450
Figure 7
Mg+0.5O2 → MgO
-100
-300
The average porosity and relative density of all samples were determined using Archimedes’ principle (ASTM standard C20-97). The composites processed at 950°C and 135 min holding time have the lowest measured porosity (11.6 vol%) and the highest bulk density values (2.45 g/cm3). For holding times longer than 60 min, the relative bulk density increases with increasing temperature. Samples with