Fig. 2.3, t is normaliz the power r
The powe e displacem
rops. But, th cy heating
The wavel ar-field eff wave oven equency and
s operate at what we us magnetic w
i
eikz
E E= 0 − n power and
the power o zed by skin reduces to o
r in a wav ment is norm
he power ab can rapidly lengths of R fects and n
makes use d shorter w 2.45 GHz, se today.
waves in met
x of wave dep
depth, δ. A only 1.8%.
ve attenuate malized by sk
bsorbed is d y reach requ RF are far lo
ot electrom electromag wavelength
present as f
tals can be w
pends on th After EM wa
es with the kin depth (δ
directly asso uired temper onger than t magnetic wa gnetic wave than RF h far-field typ
written as
ex
he displacem ave enters a
e displacem δ).
ociated with rature (this the cavity u aves. Howe es with ele heaters. Typ pe EM radia
(
( ment (z) in conductor
ment (z) in
2.4 Frequency Dependent Behavior of Materials
In general, comparing with the collision frequencyγ of matter, we can separate 0 the frequencies into two parts, low-frequency(ω<<γ0)and high-frequency(ω >>γ0). For a good conductor like copper, its collision frequency is about 4×1013 s-1. If
γ0
We can neglect the effect of frequency, and then the conductivity of copper is a constant. However, for the broad frequency,
13
which has an important dependence on frequency. Therefore
10 )
The permittivity of copper divided to real part and imaginary part varies with frequency as shown in Fig. 2.4. The imaginary component has dominant influences at the low frequency region. However it is much smaller in magnitude than real component at high frequency region because in fact the real part is negative at frequencies over 2.6×1015 (Hz).
Subs calcu
cond Fig num Fig per
stituting the ulated and s
The permi ductor. It in gure 2.5
mber in cop gure 2.4 rmittivity of
e real and im shown in Fig
ittivity of a nvolves som
The real p pper.
The real f copper. Th
maginary pa g. 2.5.
a dielectric me polariz part (solid l
part (solid he real part i
arts into Eq
c material i zation mech line) and im d line) and
is negative
q. (2.7) then
is more co hanisms, in maginary p d imaginar at frequenci
n the wave
omplicated ncluding ele
art (dashed ry part (da
ies over 1.6
number ca
than that ectronic, io d line) of w ashed line) 6×1015 Hz.
n be
of a onic, wave
) of
dipol
lar and int ral frequenc wn in Fig. 2
he thermal e ared region o wo small pe are able to not general
lar momen ntation pola
gure 2.6 es) parts of
s by the var
terfacial po cy dependen
.6. The vibr energy avai of electrom eaks shown polarize alm ly contribu nts, such a arization typ
Frequency f permittivit rious polariz
olarizations nce of the d ration of ato ilable and f magnetic spe n in solid lin
most in pha ute to micr as water, m pically take
dependenc ty for a die zation mech
which are different pol
oms and ion frequencies ectrum. Sinc
nes occur in ase with the
rowave abs may have es place nea
ce of real electric mat hanisms [14
strongly f larization m ns in dielec
of these vi ce electroni n the visible e alternating
sorption. M considerab ar radio fre
(solid lines erial and th 4].
frequency d mechanisms
ctric materia ibrations co ic and ionic e and infrar g electromag Molecules w ble mass equencies a
s) and imag he contribut
dependent.
in dielectri als is depen orrespond to c polarizatio red frequen
gnetic field with perma and, theref and microw
ginary (das tions on po
The
2.5 Ferrite Materials
The ceramic-like materials, ferrites, can be classified according to the crystal configuration, the manufacturing process or the composition, for example, spinel Ni-Zn ferrite, spinel Mn-Zn ferrite, hexagonal barium ferrite, etc, or sintered ferrite, ferrite composition, soft ferrite and hard ferrite. A unique characteristic of ferrite material is that its dielectric constant is studied as a function of frequency, composition, (magnetic materials) loading and temperature [15]. Moreover, ferrites have the advantages, such as mold ability, high resistivity, lower price and greater heat resistance.
The soft ferrites are most often used as materials for ferrite wave absorbers. The typical ferrite wave absorber is a ferrite tile blacked with a conductive metal plate.
Each ferrite has two matching frequencies, fm1 and fm2, and two matching thicknesses, tm1 and tm2, respectively. The former is attributable to ferrites’ complex permeability.
Therefore, if the frequency of the wave to be absorbed is specified, a particular ferrite material can be chosen to accomplish this absorption [16, 17]. Ferrite nanoparticles are also used as the component of radar-absorbing materials which coated in stealth aircraft to avoid being detected and also used for the electromagnetic compatibility measurement to diminish the reflected wave. After World War II, The use of microwave band increases annually and, hence, the requirement of the microwave absorbers are more and more for the development of the microwave technology. Many countries invest a large amount of time and resources to study the properties of ferrite materials and try to find the novel types. The historical development and the applications of ferrite materials have been reviewed in [18].
In addition to the utility for wave absorbing materials, ferrites are also characterized by their ferromagnetic. Since the resistivity of ferrite may be in the
proxi effec nonre and c
Fig circ
imity of ins cts, which
eciprocal de circulator.
gure 2.7