Jon C. Goldsby

Glenn Research Center, Cleveland, Ohio

Micromechanical and Electrical Properties

of Monolithic Aluminum Nitride

at High Temperatures

NASA/TM—2000-210471/REV1

January 2001
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NASA/TM—2000-210471/REV1

January 2001

National Aeronautics and

Space Administration

Glenn Research Center

Jon C. Goldsby

Glenn Research Center, Cleveland, Ohio

Micromechanical and Electrical Properties

of Monolithic Aluminum Nitride

at High Temperatures
Acknowledgments

Thanks to Dr. Paul Angel and Mr. Douglass Doza for the dielectric property measurements.

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NASA/TM—2000-210471/REV1 1 

MICRO-MECHANICAL AND ELECRICAL PROPERTIES OF 

MONOLITHIC ALUMINUM NITRIDE AT HIGH TEMPERATURES 

Jon C. Goldsby 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

 

SUMMARY 

Micromechanical spectroscopy of aluminum nitride reveals it to possess extremely low background internal 

friction at less than 1×10–4 logarithmic decrement (log dec.) from 20 to 1200 °C. Two mechanical loss peaks were 

observed, the first at 350 °C approximating a single Debye peak with a peak height of 60×10–4 log dec. The second 

peak was seen at 950 °C with a peak height of 20×10–4 log dec. and extended from 200 to over 1200 °C. These 

micromechanical observations manifested themselves in the electrical behavior of these materials. Electrical 

conduction processes were predominately intrinsic. Both mechanical and electrical relaxations appear to be thermally 

activated processes, with activation energies of 0.78 and 1.32 eV respectively.

INTRODUCTION 

Aluminum nitride has been shown to have great potential as a high temperature electronic packaging 

material (refs. 1 to 3). However in extreme environment applications, low amplitude (<10–6 strain) vibrations can be 

a source of mechanical fatigue and failure. In addition, elevated temperatures encountered during device fabrication 

and during subsequent operation demand knowledge of the material’s temperature dependent mechanical and 

electrical response. To obtain this information, resonance frequency and internal friction measurements were 

performed on aluminum nitride as a function of temperature. The temperature and frequency dependent dielectric 

properties were also measured and compared with the micro-mechanical measurements to elucidate a probable 

mechanism for the observed behavior. 

EXPERIMENTAL 

Samples of a commercially available aluminum nitride (density 3.27 g/cm3

), with small additions of Y2O3

(~1 wt.%) as a consolidation aid, were sectioned into specimens with dimensions 114×9×3 mm. Temperature 

dependent elastic and anelastic properties were determined by establishing continuous flexural vibrations in the 

specimen at its lowest resonance frequency and allowing the vibrations to freely decay after the mechanical 

excitation was removed. The details of the apparatus are given elsewhere (4). At a constant frequency ( f ) the unit 

for anelastic behavior is the logarithmic decrement ψ and it is calculated as, 

 ( )( )

ln

2 1

2

1

f t t

a

a

−





 





ψ = (1) 

where amplitudes a1 and a2 are measured at times t1 and t2, respectively (11). The geometry and material dependent 

resonance frequency can be calculated from, 

2

1

2 















ρ





 



 = E

l

B f (2) 
NASA/TM—2000-210471/REV1 2 

where B is a combination of geometric and numeric constants, l is the sample length, and E and ρ are the material’s 

elastic modulus and density, respectively. From equation (2), a ratio of the resonance frequencies, at a given 

temperature with respect to the 25 °C value, gives a measure of the temperature dependent elastic response. To 

obtain the electrical measurements, specimens of aluminum nitride were plated with platinum electrodes. The electric 

permittivity as a function of temperature and frequency was measured in air using a Solatron 1260 impedance 

analyzer as previously reported (5). Micrographs of etched and polished samples were recording using scanning 

electron microscopy. Energy dispersion spectroscopy EDS was used to identify the various major phases of this 

material. 

RESULTS AND DISCUSSION 

A typical SEM micrograph is seen in figure 1, where three types of features with various amounts of 

yttrium, alumina, and carbon are clearly revealed. The observed carbon may be only an artifact from the microscope 

probe analysis. Micrographs of a cut and polished surface reveal grains free of excessive boundary phases. X-ray 

diffraction also indicates a highly crystalline material. Yttrium-containing compounds appear at grain boundary triple 

points. In addition, EDS reveals the presence of alumina among the AlN grains. 

The mechanical spectrum in figure 2 illustrates the anelastic and elastic responses of this material up to 

1200 °C, at a fundamental vibration of 1464 Hz. This mechanical loss spectrum contains two peaks. The salient 

features of the first peak include its location at 350 °C, with a peak height of 60×10–4 log dec. The peak commences 

at about 200 °C and terminates at approximately 500 °C. This energy loss peak closely approximates a Debye peak 

with a narrow distribution of thermally activated relaxation times. The second mechanical energy loss peak was 

observed at 950 °C, and is designated the high temperature peak. The high temperature peak is a low amplitude, 

broad peak starting at approximately 200 °C and extending beyond 1200 °C. In addition, the mechanical loss 

spectrum was also obtained at the first overtone to the fundamental frequency, which for this material, vibration 

mode and geometry was 4087 Hz. This spectrum exhibits the same features as the fundamental tone with the 

exception of the first overtone peak, which is spectrum-shifted to the right with respect to the temperature. This 

thermal and mechanical coupling indicates a thermally activated mechanism as the source of the elastic and anelastic 

dispersion. Noticeably absent is the exponential increase in background internal friction characteristic of the grain 

boundary sliding observed in silicon nitride (6). In addition, figure 2 also illustrates a decrease or relaxation in the 

elastic modulus, which occurs at the same temperature as the anelastic relaxation absorption peak at 350 °C. The 

typical elastic response as a function of temperature is shown in the upper curve in figure 2. This figure illustrates 

that the relative stiffness of the material decreases 8 percent from 25 to 1200 °C, which is not unusual for monolithic 

ceramic materials (4). In addition features of the elastic response, illustrated in figure 2, also shifts to higher 

temperatures when the sample is mechanical excited at its first overtone. This behavior is indicative of thermally 

activated mechanism. Because such a low level of internal friction exist around the peaks and the corresponding 

temperature dependent elasticity remains relatively constant, no evidence of grain boundary sliding induced microcreep is evident, under the conditions of the micro-strains imposed on the aluminum nitride samples in this 

investigation. 

Given the reported piezoelectric nature of bulk and thin film AlN (7), temperature dependent dielectric 

measurements were performed to determine if these micromechanical observations manifested themselves in the 

electrical properties of this material. The complex electric modulus (M) formalism was used to characterize the 

material’s temperature dependent dielectric properties (refs. 8 and 9). The complex electric modulus is the reciprocal 

of the complex permittivity ε. The relationship between the permittivity and the electric modulus is given in 

equation (3) 

2

Imaginary

2

Real

Imaginary

2

Imaginary

2

Real

1 Real

ε + ε

ε

+

ε + ε

ε = ε

M = j (3) 
NASA/TM—2000-210471/REV1 3 

The complex electric modulus plot gives a single arc, which is representative of the bulk properties of the 

material (fig. 3). From this data, the magnitude of the imposed alternating electric field and sample dimensions, the 

direct current resistivity was determined to be 108 Ω/meters. Therefore this AlN sample, with predominately covalent 

bonding, behaves like an insulator and hence contains minimal concentrations of those agents, which could function 

as extrinsic charge carrying dopants. 

Results shown in figure 4 illustrate a dispersion of the real part of the electric modulus. The midpoints of 

these dispersion curves occur at 10, 20, 30, and 70 kHz for these temperatures 466, 490, 515, and 550 °C 

respectively. The peaks in figures 5(a) and 5(b) appear thermally activated and are located near the temperature of 

the mechanical internal friction peak found at 400 °C and 4087 Hz. Each peak extends over three decades of 

frequency, indicating a wide range of relaxation times due to the non-degenerate reorientation of elastic strain energy 

states of the relaxing entities. At higher temperatures (560 to 734°C), the complex component of the electric modulus 

becomes narrower and higher as compared to the lower temperature results (figs. 5(a) and 5(b)). In addition it is at 

these temperatures that the two mechanical loss peaks overlap in figure 2. The smaller peak at 560 °C in figure 5(b) 

appears to be a continuation of the low temperature peaks seen in figure 5(a). The higher and narrower absorption 

curves in figure 5(b), suggest a different source of origin, possibly one that is related to the high temperature loss 

peak in figure 2. 

Both the mechanical and electrical energy absorption peaks can be treated as thermally activated Arrhenius 

processes to obtain their respective activation energies. The energy equation is classically expressed as 

 ( )

( ) kT

Q

o f T f e

−

= (4)

were f (T) is the temperature dependent resonance frequency, f o is the characteristic frequency of the relaxation 

phenomenon, k and T are the Boltzmann constant and absolute temperature, respectively, and Q represents the 

activation energy. A plot of the peak frequency as a function of reciprocal temperature is given in figure 6. From the 

slope of these lines the activation energy was found to be 0.78 eV for the T= 350 °C mechanical anelastic peak. 

Activation energies derived from the electric modulus were 1.32 and 1.18 eV for the low temperature and high 

temperature peaks, respectively. Nakayama et al. (10) have observed the loss tangent as a function of grain size, and 

attributed the energy absorption peak to the piezoelectric effect in AlN crystal grains at 830 MHz and 30 °C. Hence 

in this study the magnitude of the thermal activation energies, temperatures, frequencies and height of the mechanical 

absorption peak indicate that the piezoelectric effect is not the source of the observed mechanical or electrical 

dispersion in the loss factors. The results of this investigation are more indicative of point defects or a cluster of 

point defects as the likely cause of the electrical and mechanical relaxation (refs. 10 and 11). 

Slack et al. (12) have identified oxygen as a major impurity in aluminum nitride. One possible source of 

oxygen clearly would be the consolidation additives, examples of which are Al2O3 and Y2O3 as seen in figure 1. 

Equation (5) is the proposed mechanism for the incorporation of M2O3, where M is a metal in the (+3) state, into 

AlN expressed in Kroger-Vink notation. In this scenario lattice misfit strain occurs due to the smaller ionic radii of 

the incorporated oxygen, as compared with the nitrogen ion of the host crystal. In addition, to compensate for the 

difference in valance, charged (+2) vacancies on the aluminum sites are created. 

Al N Al

AlN

M O ↔ M + O +V′′′ * * 2 3 2 (5) 3

The resultant physical entity, which is described by equation (5), is a charged defect cluster within the AlN 

crystal lattice. This defect or electro-elastic dipole would respond to both external mechanical and electrical 

alternating fields. The probable cause for the different magnitudes between the mechanical and electrical activation 

energies at the lower temperatures may be due to the difference in the elastic strain energy barrier to reorientation. 

The externally applied alternating mechanical stress field may provide sufficient elastic strain energy to reduce the 

reorientation barrier and hence lower the activation energy needed for the defect’s reorientation. A similar effect may 

be functioning at the higher temperature electric relaxation. At higher temperatures a more isotropic stress state may 

exist due to thermal expansion of the crystal lattice and thereby lowering the activation energy for defect 

reorientation. 
NASA/TM—2000-210471/REV1 4 

CONCLUSIONS 

Temperature dependent elastic, anelastic, and electrical properties of AlN were characterized by micromechanical and dielectric measurements. Intrinsic damping in aluminum nitride was below that needed to suppress 

vibrations at temperatures below 1200 °C in vacuum. In addition, alternating stress and electrical fields induced 

dipole reorientation of vacancy defects. The elastic modulus only decreased by 8 percent under the conditions of this 

investigation (20 to 1200 °C), and hence aluminum nitride has potential of retaining stiffness and dimensional 

tolerances at elevated temperatures. 

REFERENCES 

 

1. A.F. Belyanin, L.L. Bouilov, V.V. Zhirnov, A.I. Kamenev, K.A. Kovalskij and B.V. Spitsyn, “Application of 

aluminum nitride films for electronic devices,” Diamond and Related Materials, vol. 8, nos. 2–5, pp. 369–372, 

1999. 

 2. S.P. McGeoch, F. Placido, Z. Gou, C.J.H. Wort, and J.A. Savage, “Coatings for the protection of diamond in 

high-temperature environments,” Diamond and Related Materials, vol. 8, nos. 2–5, pp. 916–919, 1999. 

 3. J.H. Harris, “Sintered aluminum nitride ceramics for high-power electronic applications,” JOM, vol. 50, no. 6, 

pp. 56–60, 1998. 

 4. J.C. Goldsby, “Temperature-dependent elastic and anelastic behavior of silicon-based fiber reinforced silicon 

carbide ceramic matrix composites.” Materials Science and Engineering A, vol. A279, pp. 266–74, 2000. 

 5. P.W. Angel, M.R. De Guire, and A.R. Cooper, “ Electrical conductivity, relaxation and the glass transition: A 

new look at a familiar phenomenon,” Journal of Non-Crystalline Solids, vol. 203, pp. 286–292, 1996. 

 6. R. Raj, and M.F. Ashby, “On Grain Boundary Sliding and Diffusion Creep,” Metallurgical Transactions, vol. 2, 

pp. 1113–1127, (1971). 

 7. A. Nakayanna, S. Nambu, M. Inagaki, M. Miyauchi, and N. Itoh “Dielectric Dispersion of Polycrystalline 

Aluminum nitride at microwave frequencies.” Journal of the American Ceramic Society, Iss 6, vol. 79, 

pp. 1453–56, 1996. 

 8. I.M. Hodge, M.D. Ingram, and A.R. West, “Impedance and modulus spectroscopy of polycrystalline solid 

electrolytes.” Journal of Electroanalytical Chemistry and Interfacial Electrochemistry. Vol. 74, pp. 125–43, 

1976. 

 9. I.M. Hodge, M.D. Ingram, and A.R. West, “A new method for analyzing the a. c. behaviour of polycrystalline 

electrolytes.” Electroanalytical Chemistry and Interfacial Electrochemistry. Vol. 58, pp. 429–32, 1975. 

10. L. Murawski, R.J. Barczynski, D. Samatowicz, and O. Gzowski, “ Correlation between mechanical and 

electrical losses in transition metal oxide glasses.” Journal of Alloys and Compounds. Vol. 211/212, 

pp. 344–48. 1994. 

11. A.S. Nowick and B.S. Berry, Anelastic Relaxation in Crystalline Solids, Academic Press New York and London 

(1972). 

12. G.A. Slack, R.A. Tanzilli, R.O. Pohl, and J.W. Vandersande, “The intrinsic thermal conductivity of AlN.”

Journal of physics and Chemistry of Solids. Vol. 48, no. 78, pp. 641–47, 1987. 
NASA/TM—2000-210471/REV1 5

Figure 1.—Secondary electron image of aluminum nitride sample with EDS results illustrating composition.

0.6 1.2 1.8 2.0 3.0 3.6 4.2 4.8 5.4

N

Al

Al

C

O

0.6 1.2 1.8 2.0 3.0 3.6 4.2 4.8 5.4

N

C

O

0.6 1.2 1.8 2.0 3.0 3.6 4.2 4.8 5.4

N

Al

Al

C

O

0.6 1.8 2.0 3.0 3.6 4.2 4.8 5.4 1.2

Y

Y

Y

C

O

Figure 2.—Temperature dependent elastic and anelastic

 behavior of aluminum nitride at the fundamental and 

 first overtone flexural vibration frequencies.

0

Temperature, °C

Internal friction

200 400 600 800 1000 1200

100x10–4

80

60

40

20

0

Resonance frequency ratio squared

1.00

0.98

0.96

0.94

0.92

0.90

0.88

0.86

Fundamental 

tone = 1464 Hz

First overtone

 = 4087 Hz
NASA/TM—2000-210471/REV1 6

Figure 3.—Complex electric modulus plot of aluminum

 nitride.

M'

M"

2510

0

0 2.510 5.010

Figure 4.—Frequency and temperature dependent real

 part of the electric modulus.

Frequency, Hz

M'

1011

1010

109

108

102 103 104 105 106

466 °C

490 °C

515 °C

540 °C
NASA/TM—2000-210471/REV1 7

102

Figure 5.—Frequency and temperature dependence of

 the imaginary part of the electric modulus (a) from 

 466 °C to 540 °C and (b) from 560 °C to 638 °C.

Frequency, Hz

M"

410

210

310

110

0

103 104 105 106

560 °C

588 °C

613 °C

638 °C

(b)

Frequency, Hz

M"

410

210

310

110

0

103 102 104 105 106

(a)

466 °C

490 °C

515 °C

540 °C
NASA/TM—2000-210471/REV1 8

Figure 6.—Determination of activation energies for 

 mechanical and electronic loss processes in 

 aluminum nitride.

0.0009

Reciprocal absolute temperature, K–1

Frequency, Hz

0.0011 0.0013 0.0015 0.0017

442413

162755

59874

22026

8103

2981

1097

QHigh temperature

 electronic loss

 = 1.18 eV

QHigh temperature

 electronic loss

 = 1.32 eV

QLow temperature

 mechanical loss

 = 0.78 eV
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E–12439–1

WU–523–31–13–00

14

A03

Micromechanical and Electrical Properties of Monolithic

Aluminum Nitride at High Temperatures

Jon C. Goldsby

Electronic materials; Aluminum nitride high temperature properties; Internal friction high

temperature dielectric measurements; Functional ceramics high temperatures

Unclassified -Unlimited

Subject Categories: 27 and 76 Distribution: Nonstandard

Micromechanical spectroscopy of aluminum nitride reveals it to possess extremely low background internal friction at

less than 1×10–4 logarithmic decrement (log dec.) from 20 to 1200 °C. Two mechanical loss peaks were observed, the

first at 350 °C approximating a single Debye peak with a peak height of 60×10–4 log dec. The second peak was seen at

950 °C with a peak height of 20×10–4 log dec. and extended from 200 to over 1200 °C. These micromechanical observations manifested themselves in the electrical behavior of these materials. Electrical conduction processes were predominately intrinsic. Both mechanical and electrical relaxations appear to be thermally activated processes, with activation

energies of 0.78 and 1.32 eV respectively.

Responsible person, Jon C. Goldsby, organization code 5130, 216–433–8250.