@source: https://stackoverflow.com/questions/43914818/alpha-animation-in-aframe-for-a-object-model
The built-in material component primarily works with primitives, so material="opacity: 0.5" and similarly opacity="0.5" will not work here. You'll need to modify the THREE.js materials created by your model, using a custom component. Example:
AFRAME.registerComponent('model-opacity', {
schema: {default: 1.0},
init: function () {
this.el.addEventListener('model-loaded', this.update.bind(this));
},