<p>Thermodynamic, viscous and structural properties of melts and glasses in the AlCl<sub>3</sub>-ZnCl<sub>2</sub> system have been investigated. The two pure components are fundamentally different in the molten state. Both metal atoms are four coordinated, however, ZnCl<sub>2</sub> forms a corner sharing tetrahedral network whereas AlCl<sub>3</sub> is a dimer, Al<sub>2</sub>Cl<sub>6</sub>. ZnCl<sub>2</sub> is one of two known single component halide glass formers and additions of AlCl<sub>3</sub> have shown to form stable glasses. The glass forming ability remains far into the binary AlCl<sub>3</sub>-ZnCl<sub>2</sub> system.</p><p>The phase diagram of the AlCl<sub>3</sub>-ZnCl<sub>2</sub> system has been investigated by differential thermal analysis. The system is a simple eutectic system, and the eutectic point was observed at 116±2°C and 0.52±0.05 mole fraction ZnCl<sub>2</sub>. The system is highly glass forming in the range from pure ZnCl<sub>2</sub> up to about 0.46 mole fraction ZnCl<sub>2</sub>. Glass transition temperatures measured by differential scanning calorimetry (heating rate 10 K/min) were recorded from 115°C for pure ZnCl<sub>2</sub> to -5.5°C for 0.46 mole fraction ZnCl<sub>2</sub>. The reduced width of the glass transition ( T<sub>g</sub>'-T<sub>g</sub> )/T<sub>g</sub> showed a maximum at 0.80 mole fraction ZnCl<sub>2</sub> giving a minimum in fragility at this composition.</p><p>The density of AlCl<sub>3</sub>-ZnCl<sub>2</sub> melts has been determined using volume measurements in sealed quartz tubes. The molar volumes showed a negative deviation from ideality with a minimum at ~0.33 mole fraction ZnCl2 (ZnAl<sub>2</sub>Cl<sub>8</sub>). The volume expansion coefficient is strongly reduced from pure AlCl<sub>3</sub> to ZnCl<sub>2</sub>, and the excess volume of mixing is strongly increasing with temperature. A simple model using molecular Al<sub>2</sub>Cl<sub>6</sub>, ZnCl<sub>2</sub> and ZnAl<sub>2</sub>Cl<sub>8</sub> units was used to calculate equilibrium constants for the reaction of pure components ZnCl<sub>2</sub> and Al<sub>2</sub>Cl<sub>6</sub> to ZnAl<sub>2</sub>Cl<sub>8</sub>. The density data showed that molecular ZnAl<sub>2</sub>Cl<sub>8</sub> became more stable with increasing temperature. The viscosity of molten AlCl<sub>3</sub>-ZnCl<sub>2</sub> in the range from pure ZnCl<sub>2</sub> to 0.40 mole fraction ZnCl<sub>2</sub> has been measured as a function of temperature by an oscillation cup method. The melts with ZnCl<sub>2</sub> content higher than 0.60 mole fraction ZnCl<sub>2 </sub>exhibited non-Arrhenius behavior. The viscosity clearly decreases with increasing AlCl<sub>3</sub> content. However, the viscosity of ZnCl<sub>2 </sub>is affected less by addition of AlCl<sub>3</sub> compared to addition of ionic chlorides which form terminal chloride bonds. The fragility of the melts obtained from a reduced Arrhenius plot, were observed to decrease with addition of AlCl<sub>3</sub> up to 0.80 mole fraction ZnCl<sub>2</sub>. At higher AlCl<sub>3</sub> content the fragility increases in line with the more molecular nature of the melt. The present study has also demonstrated that the oscillation cup viscometer can be applied to record viscosities as high as 3 Pa·s.</p><p>IR spectroscopy has been performed on melts in the whole compositional range of the AlCl<sub>3</sub>-ZnCl<sub>2</sub> system. The spectra were recorded using an IR reflection method of thin films (>10 µm). Compensation for any splitting of strong bands was performed by defining a specular reflectance, r*, where a thick melt (2-3 mm)\ was used as reference. For all the mixtures a splitting of the anti-symmetric stretching frequency ν3(F2) for the AlCl<sub>4</sub> tetrahedron into three bands was observed. This indicated a C2v perturbation of the T<sub>d</sub> symmetry. From this observation it was proposed that the AlCl<sub>4</sub> units were always terminal consisting of two bridging and two terminal chlorines. The melts with compositions from 0.33 mole fraction ZnCl2 to pure Al2Cl6 are proposed to consist of a mixture of Al<sub>2</sub>Cl<sub>6</sub> and ZnAl<sub>2</sub>Cl<sub>8</sub>. The temperature dependence of the 0.20 mole fraction ZnCl<sub>2</sub> spectra showed that the amount of ZnAl<sub>2</sub>Cl<sub>8</sub> increased relative to the amount of Al<sub>2</sub>Cl<sub>6 </sub>with increasing temperature. At 0.40 - 0.60 mole fraction ZnCl<sub>2</sub>, the melts were proposed to remain molecular in nature, where the ZnCl<sub>4</sub> tetrahedra were enclosed by edge sharing AlCl<sub>4</sub> terminal units. At higher ZnCl<sub>2</sub> content or higher temperatures the connectivity of the molten structure is shifted from edge sharing AlCl<sub>4</sub> units to corner sharing AlCl<sub>4</sub> units bonded to two ZnCl<sub>4</sub> tetrahedra.</p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:ntnu-88 |
| Date | January 2001 |
| Creators | Pedersen, Ståle |
| Publisher | Norwegian University of Science and Technology, Department of Chemistry, Fakultet for naturvitenskap og teknologi |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Doctoral thesis, monograph, text |
| Relation | Dr. ingeniøravhandling, 0809-103X ; 2001:36 |
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