Alpha Animation in Aframe for a-object-model

I have one 3d object with its obj and mtl file which is displayed using in Aframe. I want to apply animation on it which change its Alpha value gradually for Fade out effect.

I went through AFrame doc. but couldn't find anything related to 3d object alpha animation.

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));
  },
  update: function () {
    var mesh = this.el.getObject3D('mesh');
    var data = this.data;
    if (!mesh) { return; }
    mesh.traverse(function (node) {
      if (node.isMesh) {
        node.material.opacity = data;
        node.material.transparent = data < 1.0;
        node.material.needsUpdate = true;
      }
    });
  }
});

You can then use, and animate, the component like this:

<a-entity obj-model="obj: model.obj; mtl: model.mtl;" model-opacity="1">
  <a-animation attribute="model-opacity"
               dur="10000"
               from="1"
               to="0"
               repeat="indefinite"></a-animation>
</a-entity>

For more on how this works, see the docs on THREE.Material and writing a component.