I agree with the colleagues above they explained how it comes that the melting temperature is lowered for very fine grained solid.

In order to fuse a substance, one has to break all the bonds in the material. So, the energy supplied to the material to fuse it will be smaller as already broken bonds increase. Since in the fine grained materials contains an appreciable fraction of already broken bonds it will need only in principle a fusion temperature which is lower than the crystallographic form.

Best wishes

Dear Mokhalad Ali

The simplest (crude) answer is: as particle size decreases in nanoscale, surface-to-volume ratio increases i.e., there are more atoms/molecules on the surface than to the bulk.

Now, melting point is all about how much thermal agitation is required to break the crystal structure of the solid and melt it into liquid. If less number of atoms/molecules are there to construct the bulk crystal structure, it should be easier to break the crystal structure by applying lesser thermal agitation. So, for most nanomaterials melting point decreases with smaller size.

Note: This concept is crude and there are exceptions. I'm only talking about common inorganic compounds.

Changes in melting point occur because nanoscale materials have a much larger surface-to-volume ratio than bulk materials, drastically altering their thermodynamic and thermal properties. The decrease in melting temperature can be on the order of tens to hundreds of degrees for metals with nanometer dimension

Yours

Dr.ashraf

Melting temperature is one of the fundamental properties of materials. In principle, the melting temperature of a bulk material is not dependent on its size. However, as the size of a material decreases toward the nanometer size and approaches atomic scale, the melting temperature scales with the material dimensions. The melting temperature of a nanomaterial such as nanoparticles (isotropic) and nanorods/nanowires (anisotropic) is related to other fundamental physical properties for nanomaterial applications, including catalysts, thermal management materials, electronics materials, and energy materials.

try to get this book it will help you more

Chapter Melting Temperature of Metallic Nanoparticles

The simplest (crude) answer is: as particle size decreases in nanoscale, surface-to-volume ratio increases i.e., there are more atoms/molecules on the surface than to the bulk.

Now, melting point is all about how much thermal agitation is required to break the crystal structure of the solid and melt it into liquid. If less number of atoms/molecules are there to construct the bulk crystal structure, it should be easier to break the crystal structure by applying lesser thermal agitation. So, for most nanomaterials melting point decreases with smaller size.

Note: This concept is crude and there are exceptions. I'm only talking about common inorganic compounds.

In addition to Dr. Ashraf's answer, I will add that in nanoscale materials, surface atoms make up a significant part of the total number of them in a nanoparticles and an essential part of the substance as a whole. But they have fewer bonds with neighboring atoms than inside bulk ones, so it is easier for thermal vibrations to disrupt the regular order of the crystal lattice.

Dear Mokhalad Ali

The properties of nanoparticles, including the melting point depression and the size-dependent heat of fusion, have more interest. Small particles have lower melting points than bulk material due to an increased proportion of surface atoms as the size of particles decreases. The size-dependent melting point depression of nanoparticles has been experimentally observed using various techniques, such as scanning electron-diffraction, field emission, transmission electron microscopy, X-ray diffraction, calorimetry. The melting point depression for small crystals can be described in a classical thermodynamic approach by the so-called Gibbs–Thomson equation.

I am attaching J. Willard Gibbs book in the following

As the size decreases beyond a critical value due to the increase in the surface-to volume ratio, the melting point deviates from the bulk value and becomes a size-dependent property. As the size decreases below a critical value, the increased surface-to-volume ratio and associated higher surface energy enhanced vibrational instability. This interface induced disorder is responsible for the size dependence of particles melting at nano scales.

Changes in melting point occur because nanoscale materials have a much larger surface-to-volume ratio than bulk materials, drastically altering their thermodynamic and thermal properties. The decrease in melting temperature can be on the order of tens to hundreds of degrees for metals with nanometer dimensions

I agree with the colleagues above they explained how it comes that the melting temperature is lowered for very fine grained solid.

In order to fuse a substance, one has to break all the bonds in the material. So, the energy supplied to the material to fuse it will be smaller as already broken bonds increase. Since in the fine grained materials contains an appreciable fraction of already broken bonds it will need only in principle a fusion temperature which is lower than the crystallographic form.

Best wishes

Melting-point depression is most evident in nanowires, nanotubes, and nanoparticles, which all melt at lower temperatures than bulk amounts of the same material. Changes in melting point occur because nanoscale materials have a much larger surface-to-volume ratio than bulk materials, drastically altering their thermodynamic and thermal properties. The decrease in melting temperature can be on the order of tens to hundreds of degrees for metals with nanometer dimensions . Surface atoms bind in the solid phase with less cohesive energy because they have fewer neighboring atoms in close proximity compared to atoms in the bulk of the solid. Each chemical bond an atom shares with a neighboring atom provides cohesive energy, so atoms with fewer bonds and neighboring atoms have lower cohesive energy. The average cohesive energy per atom of a nanoparticle has been theoretically calculated as a function of particle size .

I dare to continue the thought of Dr Azeez Barzinjy:

…A gradual increase in temperature will cause breaking bonds and destroying the material. This process will start by melting the near-surface region of the particle and will spread towards deeper layers, similar to its melting in a solvent.

See also a recent paper on this topic: melting point and binding energy of metallic nanoparticles: size relationships, their relationship, and some correlations with the structural stability of nanoclusters \ August 2020 Journal of nanoparticle Research 22 (8) doi: 10.1007 / s11051-020-04923-6