Applications of microwave spectroscopy pdf

Like each resonant frequency was calculated and added together turntables, mode stirring creates time-averaged uniformity. As expected In addition, adding multiple microwave inputs within a from the increase in possible resonant modes, the uniformity multi-mode cavity can further enhance the uniformity [33].

Although the variable frequency microwave fying the electromagnetic field within the microwave cavity. Hybrid heating can be achieved through to the amount of power generated. Variations industrial applications due to the robustness of the technol- of this method have been used successfully by researchers at ogy and the much lower relative cost of equipment.

Thus, multi-mode electromagnetic fields at the molecular level, and the dielec- systems are by far the most common processing systems tric properties ultimately determine the effect of the electro- used in industrial applications.

Microwave processing magnetic field on the material. Readers who are interested in more detailed models should consult the references. Dielectric properties For heat to be generated within the material, the micro- waves must be able to enter the material and transmit energy.

The dielectric constant 1 0 and the dielectric loss factor 1 00 quantify the capacitive and conductive compo- nents of the dielectric response. Electrical circuits used to model dielectric materials after Ref. The interaction of microwaves There exist a number of properties that contribute to the with molecular dipoles results in rotation of the dipoles, dielectric response of materials.

These properties include and energy is dissipated as heat from internal resistance to electronic polarization, atomic polarization, ionic conduc- the rotation. In the following section, the principles behind tion, dipole orientation polarization, and Maxwell— Wagner polarization mechanisms.

At microwave frequen- cies, dipole polarization is thought to be the most important mechanism for energy transfer at the molecular level [39,40]. In addition, in composite materials, Maxwell— Wagner polarization, which results from the accumulation of charge at the material interface, is also an important heating mechanism [39]. In dielectric materials, the local charge moves in response to an applied electric field.

Within materials, there exists bound charge and free charge, and motion of the bound charge results in polarization. Polarization of electric charge where the translational motion is restricted or polarization of molecules where the rotational motion is restricted results in a lag between the electric field and the polarization.

This time lag, known as the relaxation time, is due to dissipation of energy as heat within the material. Microwave heating is a result of this dielectric relaxation.

Dielectric relaxation Relaxation phenomena are often encountered in a variety of chemical, mechanical or electrical systems. Phazor diagrams for: a current, voltage and charge b power in an gous to relaxation in electrical circuits. Models that describe electrical circuit after Ref. The lag between the charge and the voltage is directly proportional to d when the power dissi- pation is small. Also, in Eq.

An important circuit used as a model for dielectric mate- rials is a series resistor and capacitor in parallel with a capacitor Fig. Model of a dipole used in the Debye description of an ideal liquid RivC1 1 1 after Ref. The relationship between the charge and the voltage is electrical circuits that are composed of resistors and capa- defined as the following [41]: citors in series or parallel.

The classical Debye description of an ideal dielectric liquid is useful to is the relaxation time. The solution to this differential equa- understand the physics behind the interaction of micro- tion, where the applied voltage is sinusoidal and assumed to waves with materials at the molecular level. In the Debye description, a single molecule with a small electric dipole is assumed to be at the center of a spherical volume. When there is no electromagnetic field present, the dipoles are randomly oriented throughout the material.

When the elec- tromagnetic field is applied, the dipoles tend to orient in the direction of the electric field Fig. The a Maxwell and b Voigt models for viscoelasticity after Ref. Relationship between the dielectric loss factor and ability to absorb 2 v where m 00 represents the imaginary component of the microwave power for some common materials. From the foregoing equation of motion, a statistical magnetic permeability and s is the conductance.

In dielec- analysis of the dipole orientations could be considered, tric materials, the magnetic permeability is usually small and the following relation for the complex dielectric and the first term can be neglected.

In addition, v1 00 can constant can be obtained [42], be considered as an equivalent conductance [30]. This is identical to the form of Eq. Therefore, Eq. The penetration depth is defined as the and is often not applicable to many materials. Beyond als exhibit more than one relaxation time. As a result, more this depth, volumetric heating due to microwave energy is complicated models have been developed to describe the negligible.

Assuming the dielectric constant of free space is dielectric behavior of different types of materials [40]. The phenomenon of relaxa- field decreases from the surface are particularly important in tion in dielectric materials is analogous to viscoelasticity processing thick materials. If the penetration depth of the because the governing equations are of the same form microwave is much less than the thickness of the material [43].

Although the Debye model is simplified, it shows only the surface is heated. The rest of the sample is heated that the relaxation time is affected by the structure of the through conduction. The ability of materials to heat is related to the penetration depth on the frequency of operation.

As ability of the dipoles to orient in the electromagnetic field, mentioned earlier, greater uniformity achieved in multi- and this ability to orient defines the dielectric properties. Energy conversion materials are suitable for microwave processing.

Materials with a high conductance and low capacitance such as The dielectric properties of materials in combination with metals have high dielectric loss factors. As the dielectric the applied electromagnetic fields result in the conversion of loss factor gets very large, the penetration depth approaches electromagnetic energy to heat. The power that is trans- zero. Materials with this dielectric behavior are considered mitted to an object can be determined by the use of the reflectors.

Materials with low dielectric loss factors have a Poynting Vector Theorem [30], which can be derived very large penetration depth. As a result, very little of the from the Maxwell equations. The power that is transmitted energy is absorbed in the material, and the material is trans- across the surface, S, of a volume, V, is given by the real parent to microwave energy.

In the resonant cavity method, the ratio of energy stored in the cavity to the energy lost often referred to as the quality or Q-factor is measured in an empty cavity and a cavity with a small sample in it. Because microwave systems are often designed by replacing system Fig. Parallel RLC circuit. The changes in the Q-factor tion, material processing is much more complex. The and resonant frequency of the cavity can be related to the dielectric properties are dependent on the mobility of the dielectric constant the loss tangent of the sample.

In the dipoles within the structure, and therefore the dielectric transmission and reflection method, a sample is placed in properties are functions of temperature, frequency, and, a waveguide and the phase and amplitude of the transmitted for reacting systems, degree of reaction.

Therefore, the abil- and reflected waves are examined. The differences in these ity of the material to absorb energy changes during proces- waves give information on the dielectric properties. For sing. For example, at room temperature silicon carbide high-temperature dielectric property measurements on cera- SiC has a loss factor of 1. The loss factor mics, factors, such as accurate temperature measurement, at C for the same frequency is As mentioned before, dipole polarization lags behind the elec- 5.

Ceramics and ceramic matrix composites tric field due to internal forces in the material. The phase shift, d , between the dipole displacement and the electric An area of microwave processing that has received a lot field result in dielectric losses. The in-phase component of of attention is ceramic processing. Because ceramics have the dipole displacement with the electric field is power low thermal conductivities and are processed at high absorbed by the dielectric material as heat.

In alternating temperatures, many researchers have attempted to take current electrical circuits, the current is out of phase with the advantage of volumetric heating for sintering, chemical voltage. The complex power is the product of the complex vapor infiltration CVI , and pyrolysis of polymeric precur- current and voltage [45].

If the preceding equation is 5. Other materials, such as where the real portion is the power dissipated in the elec- silicon nitride Si3N4 and alumina Al2O3 , are poor absor- trical circuit Fig. This is analogous to the Poynting bers of microwaves up to a critical temperature. Above this power theorem in dielectrics. Cera- 4. Measurement of dielectric properties mics that must reach a critical temperature before they couple with microwaves present some processing difficul- Because the dielectric properties govern the ability of ties.

Before reaching the critical temperature, these materi- materials to heat in microwave fields, the measurement of als have very low loss factors, and, therefore, they get these properties as a function of temperature, frequency, or heated very slowly in the microwave field. When ceramics other relevant parameters is important. Many authors [46— are processed in non-uniform electromagnetic fields, the 48] have reviewed different techniques for dielectric prop- local temperature can vary within the material.

If a local erty measurements at microwave frequencies. More recently, a number of researchers have attempted to develop models that can account for these non-thermal microwave effects. In order to explain enhanced sintering, Willert-Porada [56] developed a model based on the ther- Fig. Density versus sintering temperature for alumina sintered in a modynamic stability of pores surrounded by grains in an microwave and a conventional furnace after Ref.

The dielectric inhomogeneity of the ceramic results in enhancement of the electric field at the material, that area begins to heat more rapidly, and the convex surfaces within the pores. The local enhancement temperature begins to increase even more. This can result in of the electric field improves material flux at the convex localized thermal runaway that can cause stresses that are surfaces and affects the local driving force for densification.

Hybrid heating Calame et al. Before the critical temperature is reached, the ceramic reported that the peak intensities of the electric field in the is heated through traditional heat transfer. As the material microstructure can be much higher than the applied field. Due to the reduced thermal gradients in microwave sintering, there 5.

Processing is lower process-induced stress. In addition, the uniformity of the ceramic microstructure can be improved with micro- Much of the research in microwave processing of cera- wave heating. Because of the shorter sintering cycles and mics has been performed on sintering. In conventional cera- reduced thermal gradients, finer grain sizes can be obtained mics processing, ceramic power is compacted under high and there is more uniformity in the grain size and shape pressure into the desired shape.

High temperature sintering factor [55]. The greater uniformity in the microstructure follows the compaction. During sintering, the ceramic parti- results in reduced scatter in the mechanical properties.

In thermal processing, the matrix composites, a fibrous preform is infiltrated with a ceramics are sintered in a high temperature furnace. The composite is then pressed Because the material is heated from the surface, steep ther- or cast to form the desired shape followed by densification.

In microwave processing, where These processes can damage the preform and, thus, lower the material is directly heated, the thermal gradients are the quality of the final product.

Chemical vapor infiltration reduced. To avoid steep reverse thermal gradients, the surface phases on the fibers to form the ceramic matrix. Because of the ceramic is often insulated from the unheated micro- the decomposition reaction of the vapor occurs more rapidly wave cavity. Ceramics are particularly susceptible to ther- at higher temperatures, the solid phase deposits preferen- mal gradients because thermal stresses can be high enough tially in hotter areas when thermal gradients are present.

This can result in non-uniform density, high porosity, and Many researchers have reported significant reductions in long processing times when materials have traditional ther- processing times of microwave over conventional sintering mal gradients. As mentioned before, reverse thermal gradi- [3,52—55].

Janney and Kimrey [3] conducted research on ents are often present in microwave processing due to heat the microwave sintering of alumina. In Fig. This results in densification of held for 1 h is shown as a function of sintering temperature. The dielectric properties of thermosetting resins under- Morell et al. As thermosets pulsing the microwave power. Because the volume heating undergo crosslinking, the dielectric properties change as a is instantaneous when the microwave field is turned on, the result of changes in the network structure.

These changes in preform can be rapidly heated up to the reaction tempera- the dielectric properties correspond directly with changes in ture.

When the microwave power is removed, the preform the resin viscosity. Initially, the liquid resin couples well temperature is reduced and more reactant gas is allowed to with microwaves.

As crosslinking proceeds and viscosity diffuse into the porous preform. Additionally, Ting et al. These changes in dielectric properties are of greater than 2. Currier and Devlin [61] have reviewed particular interest for in-situ monitoring of the cure process, the key processing aspects of microwave-assisted CVI as and a knowledge of these changes can be used to optimize compared with traditional techniques.

In addition, they the cure cycle in microwave processing [67,68]. Ther- Other novel techniques for manufacturing ceramic matrix moplastics are difficult to heat until they reach a critical composites show promise for future research in microwave temperature [40]. In addition, the crystallinity affects the processing. Pyrolysis of polymeric precursors has been used dielectric properties. As the degree of crystallinity mic polymers for the processing of ceramic matrix compo- increases, the loss factor decreases [40].

Although thermo- sites. WWhhaatt iiss iissoottooppiicc mmaassss ooff CC?? HH-FF aattoommss aarree ppuusshheedd aappaarrtt aatt hhiigghheerr rroottaattiioonnaall ssppeeeedd bbyy cceennttrriiffuuggaall ffoorrccee..

SSmmaalllleerr kk,, lleessss rriiggiidd bboonndd.. NNoottee aallssoo tthhaatt rr aanndd BB vvaarryy dduurriinngg aa vviibbrraattiioonn.. We can refine the theory bbyy aaddddiinngg aa ccoorrrreeccttiioonn tteerrmm,, ccoonnttaaiinniinngg tthhee cceennttrriiffuuggaall ddiissttoorrttiioonn ccoonnssttaanntt,, DD,, wwhhiicchh ccoorrrreeccttss ffoorr tthhee ffaacctt tthhaatt tthhee bboonndd iiss nnoott rriiggiidd..

PPoollyyaattoommiicc mmoolleeccuulleess TThhiinnggss ggeett mmuucchh mmoorree ccoommpplliiccaatteedd,, bbuutt tthhee ggeenneerraall pprriinncciipplleess aarree tthhee ssaammee..

TThhiiss iiss mmoonnoocchhrroommaattiicc,, bbuutt ccaann bbee ttuunneedd mmeecchhaanniiccaallllyy oorr eelleeccttrroonniiccaallllyy.. TThhee wwaavveegguuiiddeess aarree hhoollllooww mmeettaalllliicc ccoonndduuccttoorrss tthhrroouugghh wwhhiicchh tthhee eenneerrggyy pprrooppooggaatteess.. WWMM:: wwaavveemmeetteerr mmeeaassuurreess l oorr n.. SSaammppllee - DD:: DDeetteeccttoorr.. RRaaddiioo rreecceeiivveerr oorr ccrryyssttaall ddeetteeccttoorr..

OOuuttppuutt:: aabbssoorrppttiioonn vvss ffrreeqquueennccyy.. Special cases ooff mmiiccrroowwaavvee aabbssoorrppttiioonn aa IInnvveerrssiioonn ssppeeccttrruumm ooff NNHH33 PPyyrraammiiddaall mmoolleeccuulleess nnoott oonnllyy rroottaattee,, bbuutt ccaann ttuurrnn iinnssiiddee oouutt ii.. MMoorree ggeenneerraallllyy,, tthhiiss ttyyppee ooff pphheennoommeennoonn iiss uusseeffuull iinn ssttuuddyyiinngg tthhee iinntteerrccoonnvveerrssiioonn ooff ccoonnffoorrmmeerrss..

Total views 21, On Slideshare 0. From embeds 0. Number of embeds Downloads Shares 0. Comments 0. Likes You just clipped your first slide! Clipping is a handy way to collect important slides you want to go back to later. Now customize the name of a clipboard to store your clips. Visibility Others can see my Clipboard. Cancel Save. Exclusive 60 day trial to the world's largest digital library.

We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime. Next SlideShares.

You are reading a preview. Create your free account to continue reading. Sign Up. Upcoming SlideShare. Microwave Spectroscopy. Embed Size px. Start on. Show related SlideShares at end. WordPress Shortcode. Share Email. Top clipped slide.

Science , Business , Technology. Download Now Download Download to read offline. Khemendra shukla Follow. Student at Babasaheb Bhimrao Ambedkar University. IR spectroscopy. Advantages of raman over infrared spectroscopy. Raman spectroscopy. Infrared spectroscopy. Single Electron Transistor. The L-shaped antenna 19 couples the MW power Where, A, B and C are the principal rotational constants and Ja, into the cavity, and polarization occurs by the interaction Jb and Jc are the components of the rotational angular between gas molecules and MW power.

The length of the momentum operator. When the molecule rotates, the centrifugal The molecular excitation signal passes through the SPDT force affects the atom, leading to an increase in the molecular 5b switch and the diode 8 to protect the detection moment of inertia and a decrease in the rotation constant B.

After a short delay, the MW pulse Accordingly, the centrifugal distortion constant must be signal path is reset to the image-rejection mixer 9 which considered. The selection rules for molecular rotational converts the molecular MW signal down to an intermediate transitions are found to be[11] frequency. This signal is amplified and filtered This calculated on the basis of quantum mechanics theory. When the signal passes a low-pass filter 12 , amplifier 13 , and a data theoretical value is in accord with the observed value, the recorder The pulse sequencer 15 activates all the molecular structure can be confirmed.

The interaction between SPDT switches, the molecular-beam valve, and the transient the molecular dipole moment and the electric field affects the recorder. Pulse sequences are derived from a reference molecular rotational spectra, whose severe effect can be used to oscillator 2 and are amplified by the frequency amplifier determine the molecular dipole moment accurately.

The spectrometer operation is fully automated. A cylindrical vacuum chamber is made of stainless steel, Table 1 Structure types of molecules with a diameter of — mm and a length of — Molecular Types Moment of Examples mm. The vacuum COS chamber can be evacuated to about 10—4 bar. MW synthesizer; 2. SPDT diode switch; 6. The pulsed nozzle has a heating system, investigated by microwave spectroscopy, are carbohydrates, the pressure in the nozzle is — kPa produced by a which are the most abundant biomolecules on the Earth.

Their pulsed driver with computer control. Gas molecules of a dynamic structures and conformers are important in dilute sample in He or Ne expand from the nozzle and form a molecular recognition and in intracellular communication[9].

The molecular Microwave spectra of the cis- and trans-conformers of rotational temperature in the supersonic beam is only a few furfural in the gas have been assigned and analyzed by Kelvin. The gas molecules are polarized with the MW power Motiyenko et al[16], and the ground and torsional states of the signal introduced by the right antenna in the Fabry-perot rans-conformer and the ground state of the cis-conformer cavity by moving the right mirror with the help of the stepper have been assigned and analyzed.

The investigation of the motor. The left antenna couples the molecular signal back to microwave spectra of furfural provides a reliable basis for the detection circuit during the polarization pulse. The hyperfine structure of nuclear quadrupole coupling, anisotropic nuclear spin-spin interaction and spin-rotation interaction of Monodeuterated diacetylene HCCCCD and its 13 C-substituted species were investigated by Matsumura et al[17] Fig.

They resolved the D nuclear quadrupole splitting completely, and presented a rough estimate of the equilibrium structure. Rey et al[18] observed the hyperfine structures of glycolaldehyde and 1,3-dihydroxyacetone, via the model developed in their study, to describe how the hyperfine structure could further be used for any carbohydrates Cn H2O n.

Moreno et al[19,20] investigated Fig. The analysis revealed that the Z spectroscopy and theoretical calculations. The energy barrier between these two high resolution microwave spectroscopy.