Page 53 - Winter Issue 2018
P. 53

A B platinum wires or gold leaf, although this efliciency is still
1 - i  g _ ,. '_ g well below that of most conventional transducers.
   -  ' Although CNT sheets are considered to be very mechani-
, lg —__: C-,—~ V” cally robust by certain metrics (and can support droplets of
_ _: “=._  liquid 50,000 times the weight of the sheet itself), their ex-
_ .; ..;-<5. 5  treme orosi leaves them vulnerable to dama e b even
V.-.r_§. . . 200: P 3 Y
‘  ;;‘r:, ' ‘ small macroscopic natural events such as a droplet of water
J ._ . I  C falling on the sheet or a moderate gust of air. In an attempt
pg _  x.’ to remedy this problem, other thermophone heaters that
   K" have been manufactured and examined include graphene
%- i*._‘= _ ‘ ..¢ —'  , _ sponges (Fei et al., 2015), CNT sponges (Aliev et al., 2015),
 _ _L’..__"n‘  3 '  l :/-'j'’ -’-" carbonized electrospun polymers (Aliev et al., 2016), and
 ' 1, I i carbon fiber (Dzikowicz et al., 2017). These denser but also
 A .  50 pm more manageable materials highlight the tradeoff between
5; iii:  _ ' i   ‘- mechanical robustness and thermoacoustic efliciency.
§«.;l’_‘ “ , 7 -. 1  Another parameter thermophone designers must take into
' '      consideration is device scalability, which asks how quickly
" If '1 ‘~   and efficiently these materials and devices can be manufac-
  ' V ,2? ‘v ._g tured and assembled in a repeatable fashion. Since Shinoda
 ~ g ,; ll‘  et al. (1999), various other thermophone active elements
,‘ l I»; ‘._\_ ’ ~  /_ .: have been produced using VLSI technology such as multi-
"V? L   l 11:7. ’  layer (Tian et al., 2011a) and single-layer (Suk et al., 2012)
".5! V‘   .,,__ _,  g P ..  .. h h t tu t th. fil d .t d b t .
  1;: V’  in ‘L/;,__’, 1‘,-. h,  grap ene s.e_e s, ngs en in ms eposie y a omic
' ' ' ,~ - layer deposition (Brown et al., 2016), and thin gold (Dutta
.4 ‘‘ et al., 2014), silver (Tian et al., 2011b), and aluminum (Nis-
' 3 ¥"‘ kanen et al., 2009) wires.
. ’ ' L’ , ' . ‘» ‘ The group from Tsinghua University who published the first
C, .2 CNT-based thermophone element described, in a patent
' submitted shortl thereafter, sound roduced from such a
device when submerged just beneath the surface of water
Figure 3. A: 1—meter—long freestanding carbon nanotube (CNT) (Iiang et al., 2008). Aliev et al. (2010) drew the conclusion
sheet pulled from the edge of a CNT forest by Zhang et al. (2005). B that such an effect is possible underwater because carbon
and C: scanning electron microscope images the interface between nanotubes are hydrophobic and sustain a thin layer of air
the hortzontally aligned sheet and the verttcally aligned forest, re— that then thermally expands on heating as opposed to relY_
spectwely. D: layering or stacking of CNT sheets at vartous angles. E: , , , _ , ,
hydrophobic CN T sheet can support droplets from various aqueous u_1g on the thermal eX_Pansl°n of water’ whlch 15 qulte neglb
solutions. Images reprinted from the AAAS, with permission. glble below VaP°nzat1°n- Although sound can be dlscerned
from a pristine CNT thermophone that has been submerged
‘ust below the surface of the water, removin the CNT sheet
tinuously drawn from the forest and, if desired, spun into Lem the water results in Physical damage togthe element
fibers. Individual CNTs can be semiconducting or metallic,
depending on their chirality (i.e., the relative orientation of An alternative to producing more robust thermophone ele-
the 2-D lattice to the angle in which the lattice is “wrapped” ments is to shield the fragile element from the external envi-
on itself), but, statistically speaking, a random array of vari- ronment. To protect CNT sheets, Aliev et al. (2010, 2014b)
ous chirality CNTs is electrically conducting (Saito et al., and Mayo (2015) have encapsulated them between various
1998). The high porosity of CNT sheets affords them a much materials ranging from polyimide film and mica sheets to
larger interface with the surrounding gas and results in more ceramic and metal plates. Introducing the encapsulation
eflicient acoustic thermophones than those that utilized thin media results in a mechanical system with resonances that
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