Role of water during AlN-hydrolysis-assisted solidification in ceramic suspensions studied by proton nuclear magnetic re
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ng proton nuclear magnetic resonance (1H-NMR), we have studied the thermally activated hydrolysis of aluminum nitride admixed into Al2O3 aqueous suspensions. We paid special attention to the formation of aluminum hydroxide and its role in binding the host ceramic particles into a stiff solid matrix. The water–proton NMR spin-spin relaxation time (T2) was continuously measured as a function of the time of the AlN hydrolysis in the host Al2O3 ceramic suspension. T2 was found to correlate with an increasing fraction of bound water at the surface of the formed hydrogel and so provided us with information about the gel-surface growth during the hydrolysis process. These results are in good agreement with the observed time- and composition-related increases in high-frequency impedance for the analyzed suspensions.
I. INTRODUCTION
The hydrolysis of AlN has intensively been studied, with the main aim being to avoid the reaction that detrimentally affects the properties of sintered AlN in electronic components.1,2 On the other hand, the reactivity of AlN can be put to good use: It can be used as an efficient agent for the solidification of aqueous ceramic suspensions when combined with a moderate temperature increase.3–5 The reaction product serves as a binder phase for the host ceramic particles, which results in the formation of a solid green part. The process based on this principle, known as hydrolysis-assisted solidification (HAS), has successfully been used for forming ceramics from initially low-viscosity aqueous suspensions. The process has a lot of advantages over many conventional forming techniques. According to literature data,1,2 during the hydrolysis of AlN, amorphous aluminum monohydroxide (AlOOH) is formed; however, this may react further to form Al(OH)3, depending on the aging conditions [Eqs. (1)–(3)]: AlN + 2H2O ⇒ AlOOH + NH3 ,
(1)
NH3 + H2O ⇒ NH4+ + OH− ,
(2)
AlOOH + H2O ⇒ Al(OH)3 .
(3)
The hydrolysis is accompanied by the evolution of ammonia, which results in a pH increase, the consumption of a fraction of the water from the suspension by the hydrolysis, and the formation of a solid reaction product in the liquid. As a consequence, a host ceramic suspension 1968
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J. Mater. Res., Vol. 18, No. 8, Aug 2003 Downloaded: 13 Mar 2015
containing at least approximately 25 vol% solids, which is characterized by an initially low viscosity, is transformed by aging at a moderate temperature (RT to 80 °C) into a wet, solid part. In a previous transmission electron microscopy (TEM) analysis of the dried parts, we found that the reaction product tends to be amorphous aluminum monohydroxide tightly connected to the host ceramic particles, which serves as a strong binder phase in the green ceramic part.6 Unlike with the hydrolysis of AlN in distilled water, where well-developed thin platelets form, the Al-hydroxide formed in ceramic suspensions was found to be in the form of an amorphous gel-like phase. Based on the TEM examinations, we concluded that under the conditions used for t
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