Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Smart materials and structures for energy harvesters   Vibrational energy harvesters capture mechanical energy from ambient
vibrations and convert the mechanical energy into electrical energy to power
wireless electronic systems. Challenges exist in the process of capturing
mechanical energy from ambient vibrations. For example, resonant harvesters may
be used to improve power output near their resonance, but their narrow
bandwidth makes them less suitable for applications with varying vibrational
frequencies. Higher operating frequencies can increase harvesters power output,
but many vibrational sources are characterized by lower frequencies, such as
human motions. This paper provides a thorough review of state of the art energy
harvesters based on various energy sources such as solar, thermal,
electromagnetic and mechanical energy, as well as smart materials including
piezoelectric materials and carbon nanotubes. The paper will then focus on
vibrational energy harvesters to review harvesters using typical transduction
mechanisms and various techniques to address the challenges in capturing
mechanical energy and delivering it to the transducers.

1
The categories of this paper are: cs
5
The categories of this paper are: gr-qc
12
The categories of this paper are: nucl-th
23
The categories of this paper are: solv-int
2
The categories of this paper are: math
8
The categories of this paper are: hep-ex
16
The categories of this paper are: stat
27
The categories of this paper are: comp-gas
20
The categories of this paper are: acc-phys
14
The categories of this paper are: math-ph
0
The categories of this paper are: hep-ph
22
The categories of this paper are: chao-dyn
37
The categories of this paper are: supr-con
9
The categories of this paper are: nlin
10
The categories of this paper are: q-bio
11
The categories of this paper are: hep-lat