COMPARATIVE STUDY OF DIFFUSE REFLECTANCE AND PHOTOACOUSTIC SPECTROSCOPY FOR POWDER SAMPLES

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COMPARATIVE STUDY OF DIFFUSE REFLECTANCE AND PHOTOACOUSTIC SPECTROSCOPY FOR POWDER SAMPLES

D. Meichenin, F. Auzel

To cite this version:

D. Meichenin, F. Auzel. COMPARATIVE STUDY OF DIFFUSE REFLECTANCE AND PHOTOA-

COUSTIC SPECTROSCOPY FOR POWDER SAMPLES. Journal de Physique Colloques, 1983, 44

(C6), pp.C6-151-C6-158. �10.1051/jphyscol:1983623�. �jpa-00223182�

JOURNAL DE PHYSIQUE

Colloque

C6,

suppl6ment

au nO1O,

Tome

44,

octobre

1983.

page

C6- 151

COMPARATIVE STUDY OF DIFFUSE REFLECTANCE AND PHOTOACOUSTIC SPECTROSCOPY FOR POWDER SAMPLES

D. Meichenin and F. Auzel

CIVET, 196, r u e de Paris, 92220 Bagneus, France

Rksume : On presente une etude comparative de l a r e f l e x i o n d i f f u s e e t de l a spectroscopie photoacoustique sur poudre : l a mesure q u a n t i t a t i v e de l ' a b - s o r p t i o n a i n s i que l a s e n s i b i l i t e sont analysees en f o n c t i o n de l a longueur d'onde e t de l a g r a n u l a r i t e .

A b s t r a c t : A comparative study o f d i f f u s e r e f l e c t a n c e and photoacoustic spectroscopy on powder i s presented : q u a n t i t a t i v e determination o f b u l k a b s o r p t i o n and s e n s i t i v i t y versus wavelength domain and g r a i n diameter a r e discussed.

I - INTRODUCTION

Photoacoustic spectroscopy (PAS) has o f t e n been p u t forward as a good method t o deal w i t h powder samples. However one o f t h e questions sometime asked about t h i s r e l a t i v e l y new method i s : how does i t compare w i t h t h e o l d e r d i f f u s e r e f l e c t a n c e spectroscopy (DRS) ? I n t h i s paper we i n t e n d t o present f o r t h e f i r s t time q u a n t i t a t i v e s t u d i e s and comparison between both methods a p p l i e d t o i d e n t i c a l samples.

S t a r t i n g from Melamed's theory [I] on d i f f u s e absorbance o f powddrs as a common approach t o b o t h technics, q u a n t i t a t i v e values o f b u l k a b s o r p t i o n c o e f f i c i e n t a r e shown t o be obtained on powders by both methods, provided a number o f c o n d i t i o n s on a b s o r p t i o n c o e f f i c i e n t , g r a i n s diameter and index o f r e f r a c t i o n be f u l l f i l l e d .

A comparative study between PAS and

DRS

has a l r e a d y been performed by Freeman e t a1 [2] using Kubelka-Munk's theory (K.M.) t o deal w i t h d i f f u s i o n b o t h i n photoacoustics and diffuse r e f l e c t a n c e . However, t h i s theory, as shown by the Kubelka-Munk's formula given i n Kortum [3] :

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983623

JOURNAL DE PHYSIQUE

i n v o l v e s phenomenological parameters proper f o r r e l a t i v e s s t u d i e s and n o t f o r absolute b u l k c h a r a c t e r i z a t i o n :

Rm i s t h e d i f f u s e r e f l e c t a n c e o f a sample o f i n f i n i t e thickness ; K i s a phenomenological a b s o r p t i o n parameter f o r the powder ; S i s a

phenomenological s c a t t e r i n g c o e f f i c i e n t .

Though t h e r e l a t i v e s p e c t r a l behaviour has been shown t o be c o r r e c t l y described [2] by PAS and DRS, o f course no a b s o l u t e value can be obtained f o r the bulk.

I n another comparison between PAS and DRS i n t h e case o f opaque samples, T i l g n e r [4] has stressed t h e r o l e played by d i f f u s i o n when the c o n d i t i o n f o r s a t u r a t i o n i s met ( u s > p k ; u s thermal d i f f u s i o n l e n g t h o f sample ; uk o p t i c a l absorption l e n g t h o f sample)

.

Then e f f e c t i v e absorption i s reduced by s c a t t e r i n g as shown by t h e occurence o f t h e r a t i o K/S i n KM's formula.

I n our study, i n order t o recover q u a n t i t a t i v e b u l k values from powder sample o f g r a i n diameter d we s h a l l use Melamed's d i f f u s i o n theory which i n v o l v e s b u l k a b s o r p t i o n parameter. However, as we s h a l l see, i t p r a c t i c a l l y r e q u i r e s t h a t kd 6 1 where k i s bulk a b s o r p t i o n and d average g r a i n diameter.

I 1

-

MELAMED's DIFFUSION THEORY

The main d i f f e r e n c e o f Melamed's theory w i t h previous ones such as K.M. i s t h a t absolute d i f f u s e r e f l e c t a n c e R i s obtained from p r o p e r t i e s o f i n d i v i d u a l p a r t i c l e s and n o t from l a y e r s o f p a r t i c l e s .

Then R can be r e l a t e d t o b u l k o p t i c a l a b s o r p t i o n f o r considered p a r t i c l e s . I t assumes, as f o r K.M., Lambertian s c a t t e r i n g on p a r t i c l e surface however w i t h a combination o f Fresnel specular r e f l e c t i o n which was neglected i n K.M.

F o r m u l a t i o n i s t h e f o l l o w i n g :

w i t h X = 0.28510.57 T T = (1-mi) N (l-mi N)

where Ze and

mi

a r e r e s p e c t i v e l y average v a l u e f o r r e f l e c t i o n c o e f f i c i e n t a t e x t e r n a l and i n t e r n a l s u r f a c e s o f i n d i v i d u a l p a r t i c l e s ;

X i s t h e p r o b a b i l i t y t h a t l i g h t emerging f r o m a p a r t i c l e i s s c a t t e r e d towards a p a r t i c l e one p a r t i c l e c l o s e r t o t h e s u r f a c e ; T i s t r a n s m i s s i o n o f each p a r t i c l e and average t r a n s m i s s i o n o f a c o l l e c t i o n o f p a r t i c l e s ; N i s t o t a l r a d i a t i o n r e a c h i n g s u r f a c e a f t e r one p a r s

.

P l o t o f R versus k d w i t h r e f r a c t i o n i n d e x n a s parameter i s p r e s e n t e d on F i g . 1 a c c o r d i n g t o Melamed's t h e o r e t i c a l f o r m u l a n e n t e r s R t h r o u g h

fii,

and

mi

which a r e g i v e n by F r e s n e l ' s law i n t e g r a t e d on a1 1 p o s s i b l e angles. From these curves, i t can be noted t h a t d i f f u s e r e f l e c t a n c e R

becomes l e s s s e n s i t i v e t o kd f o r k d > 1 and r e l a t i v e l y more s e n s i t i v e t o n.

111

-

BULK ABSORPTION MEASUREMENT FROM POWDER SAMPLES BY DRS

Measurements a r e performed w i t h a CARY 17 s p e c t r o p h o t o m e t e r w i t h t h e double i n t e g r a t i n g sphere attachment. The two beams a r e f i r s t e q u i l i b r a t e d w i t h Magnesium Carbonate samples.

The s t u d i e d samples a r e S c h o t t n e u t r a l , BG 18 and RG 780 f i l t e r s reduced t o powder ; t h e i r t r a n s m i s s i o n h a v i n g been p r e v i o u s l y v e r i f i e d on t h e b u l k .

The powder i s passed t h r o u g h sieves o f mesh dimensions : 20, 40, 60, 75, 100, 150, 200, 500 pm. I n each c l a s s o f g r a i n t h e average diameter i s used t o compute kd. The S c h o t t g l a s s f i l t e r used have an average r e f r a c t i o n

i n d e x o f 1.5.

JOURNAL DE PHYSIQUE

Diffuse r e f l e c t i o n s c o e f f i c i e n t s ( R ) obtained a t various wavelength ( A ) on the three types of f i l t e r s have been measured. Results a r e plotted on Fig. 1 versus kd where k i s the bulk measure a t given A .

FIGURE 1

Fig. 1 shows t h a t Melamed's theory i s rather well verified a t small kd and t h a t the discrepancies increased f o r kd a 1 . So absolute measurement of bulk absorptiori property can be obtained on powder by DRS.

IV

-

ABSOLUTE ABSORPTION MEASUREMENTS BY PAS.

In the case of Photoacoustic spectroscopy Melamed's theory gives r e l a t i v e values 153 with PAS = (1-Rs) which can be transformed a s we shall see to absolute ones by Melamed's theory and a calibration of the photo- acoustic cell already described elsewhere

[6] .

V) ^{I I}

8

+BG 18 filter

Y

8

03:

E - C -

0

-

2

^{- FIGURE 2}

B

^-

3i -

d

QI -

-

_Q

m

E,

0.01,

10 100

ktcm-I)

The c a l i b r a t i o n c u r v e ( F i g . 2) i s o b t a i n e d w i t h t h e same powder samples as above. A l l measurements were performed a t 20 Hz. F o r a l l k, t h e o p t i c a l l e n g t h pk i s l a r g e r o r equal t o t h e g r a i n diameter d and t h e thermal d i f f u s i o n l e n g t h u s i s e s t i m a t e d f o r a g l a s s i n b u l k t o be = 110 p Then, measurements correspond t o t h e t h e r m a l l y and o p t i c a l l y t h i n case o f Roscencwaig-Gersho's t h e o r y where t h e r e i s no s a t u r a t i o n . As seen f r o m t h e c u r v e a l i n e a r r e l a t i o n s h i p i s found on a Log-Log p l o t ; t h e s l o p e b e i n g 0.55.

V

-

DISCUSSION

The t r u e r e f l e c t a n c e value Rs of t h e powder l a y e r i s g i v e n b y i n t e g r a t i n g sphere t h e o r y by [7] :

R i s r e f l e c t i v i t y o f sphere w a l l s .

A l p A2, A3 a r e areas o f sample, entrance, and e x i t p o r t s . S i s n e t sphere w a l l area l e s s p o r t s .

T i s r e a d i n g o f i n s t r u m e n t

Knowledge o f c o e f f i c i e n t R i s needed t o o b t a i n RS and R has t o be as near u n i t y as p o s s i b l e i n o r d e r t o have a good s i g n a l and t o a v o i d f a s t i d i o u s c o r r e c t i o n s .

-

FIGURE 3

C6-156 JOURNAL DE PHYSIQUE

I n f a c t as seen from F i g . 3 R i s f a r from being even constant w i t h wavelength. P r a c t i c a l l y MgO, BaS04 c o a t i n g s uses a r e l i m i t e d t o v i s i b l e and near i n f r a - r e d domain. On t h e o t h e r hand, d e t e c t o r s have t o be changed : a p h o t o m u l t i p l i e r and a PbS c e l l a r e r e s p e c t i v e l y used.

V.2. Spectral domain f o r PAS.

--- ...

I n t h e case o f PAS s p e c t r a l domain i s r e s t r i c t e d by source e m i s s i v i t y , and a b s o r p t i o n o f t h e w a l l and window o f PAS c e l l . P r a c t i c a l l y these

r e s t r i c t i o n s can be a l l e v i a t e d by a c a r e f u l choice o f the c e l l m a t e r i a l s . Also s a t u r a t i o n o f PAS s i g n a l f o r h i g h k a l l o w s t o f i n d more e a s i l y a b l a c k reference f o r PAS than a w h i t e reference f o r DRS. Consequences a r e t h a t u s e f u l s p e c t r a l domain o f PAS i s much l a r g e r and n o t l i m i t e d a t a l l by d e t e c t o r s than t h e DRS one.

~,?l-~e_nsi_tlvi~~-~em~a_rI~~!!-_be_tweenPA~-~nd-_DRS-~n-two-~~_e~_tra,!-ran9eS

PAS and DRS spectra have been obtained around 0.65 urn and 1.5 pm f o r two types o f glass m a t e r i a l s : a germanate glass doped w i t h Er203 and an aluminoborate g l a s s doped w i t h 1.6 % o f Er203, Both were ground t o powder o f average g r a i n diameter

a

^{= 20 urn.}

Figures 4,5 and 6,7 g i v e spectra f o r d i f f e r e n t Er203 c o n c e n t r a t i o n

Alurninoborote Bo .x% Er203

FIGURE 4 FIGURE 5

Germanate BaK

FIGURE 6 FIGURE 7

f o r both glasses which d i f f e r e s s e n t i a l l y by t h e i r quantum y i e l d : For example they a r e f o r t h e 1.5 p t r a n s i t i o n 2.8 % and 76.5 % f o r r e s p e c t i v e l y

20 -3 t h e aluminoborate and germanate g l a s s [9] a t a doping l e v e l o f 1.6 10 cm

.

The s e n s i t i v i t y l i m i t f o r PAS and DRS as w e l l i s taken as a p o i n t where s i g n a l / n o i se (S/N) r a t i o i s u n i t y when c o n c e n t r a t i o n o f ~ ri s reduced. ~ + Measured values a r e given i n Table I :

SENSITIVITY COMPARISON FOR ~ rDOPED GERMANATE GLASS ~ +

Experimental c o n d i t i o n s f o r s e n s i t i v i t y l i m i t s measurements were the f o l l o w i n g :

-

s e n s i t i v i t y o f PAS c e l l w i t h carbon b l a c k sample = 0.5 mV/mW

-

Minimum detected power w i t h t h i s sample i s < 16.5 UW t h a t i s

3.5 pvr.m.s. a t the o u t p u t o f the microphone ( t y p e EM35 BF from L.E.M. France) [8]

-

Quantum e f f i c i e n c y v a r i a t i o n s w i t h ~ rc o n c e n t r a t i o n i s taken ~ + i n t o account (9)

.

Spectral Domain

1.5 pm

0.65 llm

DRS C Er 3+ ( t l ) S

0.33 %

<0.33 % PAS

kmin

0.26cm-I

<0.09cm-~

P i n

9 mW

6 mW

kd

0.5

c0.2 kd

<0.5cm-l

0.7cm-I cEr3+(&l)

2.6 % (.cl=O. 5)

2.6 % ( 1 7 4 )

kdxPinW

9

8:4

JOURNAL DE PHYSIQUE

V I

-

CONCLUSION

This s t u d y has shown t h a t b o t h PAS and DRS can p r o v i d e a b s o l u t e k v a l u e s o f b u l k from powder sample p r o v i d e d Melamed's t h e o r y can be a p p l i e d .

Advantages o f PAS over DRS can be summerized as f o l l o w : - u s e f u l s p e c t r a l domain o f PAS i s l a r g e r

-

s e n s i t i v i t y l i m i t s i s o f same o r d e r as f o r DRS i n t h e near I.R.

w i t h o u r experimental c o n d i t i o n s . However, i t s h o u l d be noted t h a t s e n s i t i v i t y o f o u r c e l l was n o t a t i t s b e s t w i t h 0.5 mV/mW : v a l u e s quoted i n t h e 1 it e r a t u r e a r e around 2 mV/mW

[lo]

and r e a c h 6 mV/mW [ll]

.

Then, w i t h PAS a b e t t e r s e n s i t i v i t y t h a n w i t h DRS can be expected i n t h e near I.R. p r o v i d e d a s l i g h t l y b e t t e r c e l l as w e l l as a l a r g e r i n p u t power be used, c o n d i t i o n s which c o u l d be r e a d i l y o b t a i n e d .

-

q u a n t i t y o f sample m a t e r i a l r e q u i r e d i s much s m a l l e r f o r PAS t h a n f o r DRS.

REFERENCES

N.T. Melamed, J. Appl. Phys. (1963)

2,

⁵⁶⁰

J.J. Freeman, R.M. Friedman, and H.S. Reichard, J. Phys. Chem. (1980), - 84, 315

6 . Kortiim, i n " R e f l e c t a n c e spectroscopy", ( S p r i n g e r 1969) p . 110.

R. T i l g n e r , Appl

.

O p t i c s (1981) - 20, 3780

E.M. Monahan and A.W. N o l l e , J. Appl

.

Phys. (1977),

-

48, 3519 D. Meichenin, Memoire CNAM, P a r i s 1980, unpublished.

I n s t r u c t i o n Manual f o r c e l l - s p a c e t o t a l d i f f u s e r e f l e c t a n c e Accessory, V a r i a n I n s t r u m e n t d i v i s i o n , no 01-017 R170

f o r microphone EM35BF, s e n s i t i v i t y i s 7,5 mV/Pa and NEP = 3ldBSL ( LEM-France, 127 Av Repub1 i q u e

-

92320 C h a t i l l o n

-

France).

F. Auzel, Ann. Telecom (1969),

-

24, 199 P. Poulet, thPse Strasbourg 1980

D.M. Munroe, M.S. Reichard, A p p l i c a t i o n n o t e 147 PAR C o r p o r a t i o n .