and Engineering A201 ( 1995) Ll -L4
Thermal stability of nanostructured and Mo,N
materials Ti, Ti,N,
Yong Zhu, Ming Tan* Institute
of Solid State
in final form
Republic, of China 1995
Abstract Nanostructred materials Ti, Ti,N, MO and Mo,N were synthesized by d.c. magnetron (reaction) sputtering and in-situ compaction. Their thermal stability has been studied by differential scanning calorimetry (DSC). On the DSC curves there are one exothermal peak and one endothermal peak for nanostructured Ti, Ti,N and MO; and there are two exothermal peaks for Mo,N. The temperature ranges in which these exothermal peaks appear are related to melting points of these materials. The exothermic peak can be explained by the elimination of distorted or broken bonds existing at interface layers of as-compacted samples. The activation energy for such processes can be associated with melting points. The endothermal peak can be attributed to thermal desorption of various gaseous molecules which were adsorbed at room temperature by the samples with pores. Keywords: Thermal stability; Nanostructure;
sputtering and in-situ thermal stabilities.
Nanostructured materials possess an extremely small grain size (1~ 15 nm) and a large volume fraction of the interface (20-50%); consequently they have interesting properties. For example, the melting point of n-Au is decreased to a temperature much lower than that of the conventional Au [l]. Studies on the preparation and properties of such materials have been active subjects [2,3]. Nanostructured materials are generally synthesized first by inert-gas condensation for the preparation of an ultrafine powder and subsequently in-situ collection and compaction of this powder . In this processing, materials to be prepared are evaporated from crucibles by resistance heating. This method cannot be used to prepare materials with high melting points such as MO. Compared with this technique, sputtering has no crucible contamination problem and can be used to prepare a wide variety of materials including MO. We investigate here the preparation of nanostructured Ti, Ti,N, MO and Mo,N by d.c. (reaction) magnetron
* Corresponding author. 0921-5093/95:X09.50 SSDI
S.A. All rights
Sample preparation was performed in a d.c. magnetron sputtering apparatus. The operating pressure of the sputtering source was chosen to be 0.5 Pa. The vacuum chamber was first evacuated to 4 x lop4 Pa. The chamber was subsequently filled with high-purity argon of 99.9997’0 to 0.5 Pa when preparing n-Ti and n-MO; with argon to 0.2 Pa and then nitrogen to 0.5 Pa when preparing their nitrides. (The inert gas was continuously introduced after the vacuum reached lop4 Pa, and impurities in the background vacuum of 10d4 Pa can mostly be pumped out together with the introducing argon. As a result, contamination during preparation of samples arises mainly from impurities in the argon; and this is negligible.) Ti and MO targets with a purity of 99.9% were sputtered at 1 A, 280 V and 2 A, 400 V, respectively. The sputtered particles are transported via convection to a liquid-notrogen cooled trap where they loosely adhere. When a sufficient quantity
Y. Zhu, M. Tan 1 Materials Science and Engineering A201 (1995) Ll -L4
Fig. 1. Bright TEM image of a compacted n-Ti sample
b n-T&N of powder has been accumulated, the chamber is pumped to 4 x lo-4 Pa again. Then the powder was scraped from the trap into a compaction unit, and compated under 1.5 GPa at room temperature into pellets with a diameter of 8 mm. Differential scanning calorimetry (DSC) measurements were conducted in a Perkin-Elmer DSC-2 differential scanning calorimeter. DSC curves were obtained at a scanning rate of 40 K min- ’ from room temperature to 1000 K. 410
Fig. 1 shows a bright transmission electron microscopy (TEM) image of an n-Ti sample compacted, its particle size being about 8 nm. Particle sizes of Ti,N, MO and Mo,N are also roughly the same as this one. X-ray diffraction indicates that Ti-nitride samples consist of a mixture of TIN and T&N. Fig. 2 shows DSC curves of the Ti, Ti,N, MO and Mo,N samples. There is one exothermal peak in the temperature range 368-464 K with the peak temperature Tp of 426 K for n-Ti, 390-648 K (T,, 550 K) for n-Ti,N, 385-666 K (T,,, 549 K) for n-MO and one endothermal peak in the range 507-962 K (T,, 689 K) for n-Ti, 674-980 K (T,, 828 K) for n-Ti,N, 695901 K (T,,, 763 K) for n-MO. There are two exothermal peaks for n-Mo,N, the first being in the range 406655 K (r,, 498 K) and the second in 655-988 K (T,,, 877 K). Table 1 shows the data of these DSC measurements. Fig. 3 shows a DSC curve of sponge Ti. There is one endothermal peak in the temperature range 459-901 K with the peak temperature of 749 K. Density measurements using an Archimedes method indicate that the density of n-Ti and n-MO are
Fig. 2. DSC curves of n-Ti (a), n-Ti,N (b), n-MO (c) and n-Mo,N (d). Heating rate, 40 “C min- ’
3.2 g cmp3 and 7 g cme3 respectively, about 70% of the density of the corresponding pure metal, and that the density of n-Ti,N is 2.8 gcmp3.
4. Discussion As-prepared nanostructured materials are thermodynamically unstable and annealing can lead to structural changes. Mechanisms of atomic rearrangements occurring in the temperature range of an exothermal peak should be different from that of an endothermal one and they will be discussed separately. 4.1. Origins of the exothermal
Tschiipe and Birringer have studied enthalpy changes in n-Pt and attributed the heat release to the repair of interfaces . The exothermal peak of samples studied
Y. Zhu, M. Tan 1 Materials
Table 1 Data of DSC measurements for n-Ti, n-Ti,N,
Science and Engineering
n-MO and n-Mo,N
n-Ti n-Ti, N n-MO n-Mo,N sponge Ti T,,-peak temperature,
A201 (1995) Ll-L4
AH (.I g.‘)
AH (J g ‘)
426 550 549 498
368-464 390-648 3855666 4066655
- 23.91 - 154.5 -121 -39.6
689 828 763 877 749
507m962 674-980 6955901 6555988 4599901
1815 1336 147 -124 52
range of peak and AH-enthalpy
here can also be explained in terms of such atomic rearrangements at interfaces. The rearrangement may be regarded at an atomic level as the elimination of distorted or broken bonds at interfaces. If broken bonds at an interface are completely eliminated, two grains are combined-a grain growth process. When partly eliminated, the interface atom density increases and the interface energy decreases. The heat release value is related to the number of distorted or broken bonds which are annihilated during heating. The activiation energy for the elimination of distorted or broken bonds in an as-compacted material with a higher melting point should be larger than in a lower melting-point material because the material with a higher melting point has a larger bonding energy; thus the exothermal peak of an as-compacted sample with a higher melting point should occur in the higher temperature range. This is corroborated by DSC results. Exothermal peaks of n-Ti.,N and n-MO appear in
a higher temperature range than that of n-Ti. The exothermal peak of n-Mo,N has the highest temperature range. (The melting point is 1667 “C for Ti, 2620 “C for MO, 2820 “C for T&N and 2950 “C for TiN, respectively . The melting point of Mo,N cannot be obtained in the literatures for the authors but the melting point of Mo,N should be higher that of the corresponding metal MO.) Ti has a very high affinity for oxygen and the solubility of oxygen in Ti can be very large . Therefore adsorbed oxygen can further diffuse to more open interfaces even at room temperature and form Ti-0 bonds, thus reducing the number of distorted as well as broken bonds in an as-compacted sample. This will result in the decrease of heat release during DSC measurements. We therefore observed the smaller exotherma1 peak in n-Ti. There are density fluctuation defects in metallic glasses and they are defined as an area whose density is lower than the average density [7,8]. A low-temperature annealing results in the annihilation of such defects . The mechanism of the elimination of broken bonds at interfaces of nanostructural materials might be in some extent similar to the defect annihilation during structural relaxation in metallic glasses. 4.2. Origin of the endothermal peak
Temperature (IL) Fig. 3. DSC curve of sponge Ti. Heating rate, 40 “C min-’
A free surface with broken bonds possesses a higher energy and chemically adsorbs (or chemisorbs) gaseous molecules to reduce the energy at some appropriate temperatures. When the temperature rises. adsorbed molecules pick up enough energy from thermal fluctuation to leave again or desorb [lo]. This is an endothermal process. It is well-known that sponge Ti is strong adsorptive and adsorbs gaseous molecules in air which include moisture, oxygen and nitrogen. Heating will lead these gaseous molecules to desorb. The endothermal peak of sponge Ti, as shown in Fig. 3, is apparently attributed to this desorption.
Y. Zlru, M. Tan
: Materials Science and Engineering A201 (1995) Ll-L4
Analogous to sponge Ti, n-Ti samples here can adsorb various gaseous molecules when taken out from the preparation apparatus and exposed to air. This is because there are free internal surfaces and thus broken bonds in the samples with pores which chemisorb gaseous molecules. Therefore the endothermal peak of n-Ti can be explained by desorption of these gaseous molecules. Tschope and Birringer have also observed, using mass spectroscopy, gas desorption in n-Pt with 85% density during annealing . Because of their nano-sized grains, n-Ti samples possess a much larger specific free surface than sponge Ti. n-Ti will consequently adsorb much more gaseous molecules. The desorption of these molecules leads to a far larger endothermic value than that of sponge Ti. The endothermal peaks of n-Ti,N and n-MO can also originate from thermal desorption. The number of Ti broken bonds in n-Ti, N should be smaller than in n-Ti, leading this nitride to adsorb a smaller number of gaseous molecules. The endothermal amount of the nitride, arising from desorption during heating, is therefore smaller than that of n-Ti. We know that MO possesses a much weaker adsorptive ability than Ti. The grain size and density of the n-MO sample are roughly the same as those of n-Ti; thus its endothermal amount is much smaller than that of n-Ti and n-Ti, N. n-Mo,N could also adsorb a smaller amount of gaseous molecules and desorb them during heating. The reason why we did not observe an endothermal peak can be that the endothermic process in n-Mo,N occurs in the same temperature range as that of n-MO and the heat release in n-Mo,N can be dominant over the endothermic amount owing to the strong bonding.
5. Conclusions (1) Nanostructured materials Ti, Ti,N, MO and Mo,N were prepared by d.c. magnetron (reaction) sputtering and in-situ compaction. (2) There are one exothermic peak and one endothermic peak for the Ti, Ti,N and MO, and there are
two exothermic peaks for the Mo,N. The temperature ranges in which these exothermic peaks appear are related to their melting points. (3) The exothermic peak can result from the elimination of distorted or broken bonds at interfaces. The activation energy for this process can be related to melting points of these materials. (4) The endothermic peak can be attributed to thermal desorption of various gaseous molecules which are adsorbed at room temperature under air by samples with pores. The strong adsorption ability of n-Ti for gaseous molecules in air, originating from the very high affinity of Ti for these molecules as well as its nanosized grains and pores, leads to the very large endothermic amount.
Acknowledgements We thank Ms. Zhaoqin Chu and Mr. Yong Qin for technical assistance in the DSC and TEM experiments. We also thank Dr. Xingzhao Ding for supplying sponge Ti. This work was supported by the National Natural Science Foundation of China and the National Committee of Science and Technology.
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